How to test trailer lights

No matter what type trailer you own or operate, it is important to keep its signal lights in good working order.

Step 1

Plug your truck’s wiring harness into your trailer’s wiring harness and have an assistant get into the cab of your truck to test the lights. Test brake lights, left and right turn signals, hazards and reverse lights.

Step 2

Take note of which bulbs are not working.

Step 3

Remove the lens cover on the light or lights that are not working with a screwdriver.

Step 4

Remove the bulb or bulbs that are not working by pushing them in and twisting them to the left. Inspect the bulb at first to see if it appears to be burned out. If you have a working bulb handy you can plug it in to see if that solves your problem. If the new bulb doesn’t work, proceed to next step.

Step 5

Turn on your voltmeter and place the voltage setting at 20 volts.

Step 6

Place the negative (black) meter tester lead on the frame of your trailer to establish a ground.

Step 7

Place the positive (red) test lead against the bottom of the bulb socket. Do not allow the test lead to touch the bottom and side of the bulb socket at the same time because this can cause a short.

Note the reading on your meter. It should read 12 volts. If it does not read 12 volts it means this socket or its wiring is bad. Remove the wire from the bulb socket. Place one the negative test lead on the trailer frame and the positive lead on the end of the trailer wire. If the wire tests okay, it means the the wire works, but the socket is bad. If the wire is bad, both it and the bulb should be changed out.

Items you will need

• An assistant
• Screwdriver
• Digital meter

This article was written by the It Still Works team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. To submit your questions or ideas, or to simply learn more about It Still Works, contact us.

No matter what type of trailer you tow, trailer lights are an important component of safe and legal towing. In case when something goes wrong and your trailer lights don’t work properly or don’t work at all, you should fix them immediately. Before you would call on all of your knowledge about how to test trailer lights and begin to do it, you may want to check your trailer’s light as the answer may be as simple as a burned-out bulb.

Truck Parts & Accessories
SHOP NOW

If you haven’t found it, you have to ensure that your vehicle’s brake and backup lights as well as turn signals work properly and the problem doesn’t occur in the tow vehicle. For this purpose you will have to disconnect the wiring system of your trailer from your tow vehicle as you won’t know for certain whether the problem is in the wiring harness. Testing your tow vehicle without the trailer will allow for easier testing by separating the wiring system into two manageable sections.

If your vehicle’s lights operate as they should, you will need a trailer wiring tester that allows for easy checking of the socket on your tow vehicle. Normally, each wiring tester has one or more indicators which will alert you when detecting an electric current. Use this tool to test each function (running, right, left, and stop lights.) If you have the light illuminating for each system, then the wiring system of your trailer is likely to be the source of the problem. But when any of tests results in no response from the wiring tester, your tow vehicle’s wiring should be checked. You may also find out that the tester lights up when it shouldn’t.

For instance, if you haven’t engaged the turn signal, but any of your signal indicators lights up, you may have a short in your vehicle’s wiring. So you will have to check your tow vehicle’s wiring and ensure that all wires are properly connected to the right connection points and none of them make contact between themselves. If the electrical system of your tow vehicle doesn’t require repairs, you should concentrate all your efforts on your trailer.

Use a MultiMeter to Troubleshoot Trailer Lights

To test your trailer’s plug, you will need a jump box or battery and alligator clip test leads. Keep in mind that if your trailer has electric brakes, you’ll also need a floor jack in order to jack up one wheel at a time. Whether you are using a jump box or battery, you will have to connect one end of the black test lead to the negative terminal. The other end of that test lead should be connected to the “Ground” pin.

Then you will have to connect the second test lead to the positive terminal of the jump box or battery and test all functions on the trailer by touching each of the pins with the opposite end of the second test lead. If none of the functions work, the “Ground” pin is probably the cause of the problem. Some wiring issues can be hard to detect, but knowing where to start and how to test trailer lights is half the battle.

You’ve connected the trailer to the vehicle, but the turn signals and brake lights don’t work. You don’t know if the wiring is bad or you have bad bulbs. One way to troubleshoot the voltage on the connector plug is to use a circuit tester. On a four-prong plug, one prong connects to the right turn signal, one to the left and one to the tail lights. One is the ground. You will need a helper to operate the controls of the vehicle while you check the voltages with a circuit tester.

Step 1

Tell your helper to turn on the ignition while you go to the connector plug. Connect the alligator clip on the circuit tester to a metal part of the vehicle.

Step 2

Tell your helper to turn on the right turn signal. Place the probe of the circuit tester into the connection that has the green wire. The light on the tester should blink on and off.

Step 3

Tell your helper to turn on the left turn signal. Place the probe of the circuit tester into the connection that has the yellow wire. The light on the tester should blink on and off.

Step 4

Tell your helper to shut off the turn signal and step on the brake. Check the green and yellow connections again. The light on the tester should stay on for both.

Step 5

Tell your helper to release the brake and turn on the lights. Place the probe of the circuit tester into the brown wire connection. The light should stay on.

Tell your helper to turn off the lights and shut off the ignition.

• Green, yellow, white and brown are standard wire colors, but colors may differ depending on the manufacturer. If your connector has different colors, check the data sheet for your connector. You may also use the probe to identify the pins.

Items you will need

• 12-volt circuit tester

This article was written by the It Still Works team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. To submit your questions or ideas, or to simply learn more about It Still Works, contact us.

Connect Wires to the Trailer Lights

In order for the trailer lights to illuminate without actually being connected to a vehicle, they must be wired to a power source. To accomplish this task, connect wires to the positive and negative power leads of the trailer lights. If the trailer lights are modern and adhere to electrical standards, the power leads should be black (for the negative/ground lead) and red (for the positive lead.) Depending on whether the trailer lights were purchased as part of a kit, electrical wires may already be connected to the power leads.

Install an Optional Toggle Switch

If the lights will need to be turned on and off without physically disconnecting them from the battery, it will be necessary to install a toggle switch. A toggle switch is readily available from local automotive or electronics shops, and should offer two terminals for wire connection. To install the toggle switch, connect the red (positive) lead from the trailer lights to one terminal on the toggle switch, then solder the wire in place. Connect another length of wire to the other terminal of the switch, also soldering that wire in place.

Connect the Wires to a Battery

With two wires extending from trailer lights, the wires are ready for connection to a power source. Since the lights are designed to work on a 12-volt automotive system, the ideal power source is a 12-volt car battery available from any local automotive or chain retail store. The wires can be connected to the battery using electrical tape to secure them in place; for a more secure and professional connection, though, first connect the wires to automotive battery terminals and then secure the terminals to the battery posts. Once the wires are secured to the battery, activate the toggle switch installed in Section Two above to illuminate the trailer lights without hooking them up to a vehicle.

This article was written by the It Still Works team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. To submit your questions or ideas, or to simply learn more about It Still Works, contact us.

One of the most crucial safety features of any towing setup is the light system on the back of your trailer. Improperly functioning lights can create a serious roadway hazard, making it difficult for those behind you to know whether you are slowing, stopping or turning. Fortunately, replacing a trailer light bulb is one of the easiest vehicular maintenance tasks you can do, and you ought to be able to complete the project in less than five minutes once you have the hang of it.

Step 1

Verify which light is not functioning properly. With the trailer wires hooked up to your towing vehicle, depress the brakes and test both blinkers while a friend or family member stands behind the vehicle. They’ll be able to see which lights work and which don’t so that you don’t accidentally replace the wrong one.

Step 2

Test the trailer circuit before replacing the light bulb. Trailer lights and wiring are subject to much more rigorous conditions than vehicle lights due to the exposure, bumpy ride and frequent immersions (in the case of boat trailers). A circuit tester will connect to your trailer’s converter plug, which is the fitting that connects to your vehicle’s electrical system. A light corresponding to each wire will light up to indicate a good connection; if a light doesn’t come on then you know a repair other than light bulb replacement is necessary.

Step 3

Detach the trailer converter plug from the vehicle in order to disconnect power to the lights. If the circuit tests were normal, remove the light plate that covers the dead bulb. Unplug the bulb.

Step 4

Take the dead bulb to your local auto parts store. This will make it easy to find a replacement bulb in a sea of different colors, models and sizes.

Plug the new bulb into the light socket. Before replacing the light plate, hook the converter plug up to your vehicle and perform the test again with a friend or family member. If the light works, unhook the converter plug and reattach the light plate.

When brake lights go out, you are not only prone to danger on the road such as getting rear-ended, you could get pulled over by the police and receive a ticket. Ensuring that your brake lights are always in working condition is a good way to stay safe on the road. Are all of your brake lights operating correctly? How can you test them by yourself without anyone looking at the rear of the vehicle while you pump the brakes on and off?

While there are several methods out there, here are a few that work well.

Use a Pole to Check Brake Lights

All you need is a broomstick, mop, painting pole, or basic pole to test your brake lights. You can even use the whole broom if you are okay with the broom end being in your vehicle. Take one end of the stick and press the brake pedal, then prop the other end firmly against the seat cushion. Now walk around back and check your brake lights. There are pole-like products on the market to do this, but a simple broomstick will do.

Use a Rearview Mirror to Check Brake Lights

Another easy method to check your brake lights is to park facing a glass storefront. When you look in the rearview mirror and pump the brakes, you should be able to see in the reflection if they are working properly. You may want to check your brake lights any time you are parked opposite of a reflective surface, as it is an opportune time to make sure your lights work properly.

Brake Light Testers

There are also tools you can use to check the brake lights. These may be feasible if you often attach a trailer to a vehicle and need to ensure the lights are working properly. Circuit testers are a good way to see if all of the circuits in a vehicle are fully operational. You’ll find a wide range of inexpensive plug-in testers that can take the hassle out of knowing if you are connected correctly.

What to Do if a Tail Light Goes Out

A front headlight going out is quite simple to detect as the light won’t show up on the road when it’s dark and you can simply look in front of you. Rear brake lights are a bit trickier as we’ve discussed, but you can detect when one of them is out without help from anyone else.

When you notice that a bulb is out, replacing it is necessary. You can probably replace the lightbulb fairly easily on your own instead of going to a mechanic. Most vehicles have all the bulbs in a specific location (right and left sides) under separate lenses. It is simple to purchase a new bulb at an auto store or online automotive parts supplier.

To install it, just unscrew the colored or clear light lens with a screwdriver. Remove the housing, keeping a close eye on the screws (they are custom-fit to the housing and losing one means you’ll have to find another part to replace). Then remove the assembly that holds the old bulb, insert the new one, fit the light bulb assembly back in, and screw the housing back on. Learn more about replacing tail lights.

Tail lamps for cars, trucks or motorcycles can be easily checked for proper operation using a standard multimeter. These tools allow you to diagnose problems with vehicle electrical systems by determining where the problem with your lights may be at. Rather than just checking the terminals inside the tail lamp, it’s a good idea to remove the tail light housing and check the harness, as well. This will let you determine how far back in your electrical system the problem may be located.

Step 1

Unbolt the tail light housing from the vehicle with either a 1/4-inch drive socket set or a screwdriver. If the tail light is on a motorcycle, remove the tail light lens. The housing does not need to be removed.

Step 2

Unplug the light bulb from the light socket and inspect the filament for damage. If the coiled metal filament inside the bulb is broken, there is no further need to test the tail light. A new light bulb will be sufficient for the repair.

Step 3

Ask an assistant to hold down the brake pedal on the vehicle to activate the electrical system. If the vehicle has integrated turn signal-tail lights, you can set the emergency flashers or turn on the car’s ignition and headlights. On motorcycles, the accessory power must be turned on at the ignition key and the foot brake held down to activate the light.

Step 4

Touch the black probe for the multimeter to the metal case of the tail light. The black probe is for connecting to the ground.

Press the red probe for the multimeter against the electrical connections inside the light bulb socket. If the vehicle is a 12-volt system, the multimeter should register 12 volts. Vintage cars often ran on 6-volt systems and commercial trucks routinely work on 24-volt systems. If the multimeter does not register any power, touch the probes to the wires protruding from the back of the socket. If it reads at the back of the socket, the socket connection is bad and needs to be replaced. If it does not, move on to checking the vehicle’s fuse box, which can be visually inspected without the use of a multimeter.

Items you will need

• 1/4-inch drive socket set

This article was written by the It Still Works team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. To submit your questions or ideas, or to simply learn more about It Still Works, contact us.

If you’re installing a trailer hitch on your car or truck, you’re going to need a plug for the trailer lights. Trailer wiring can be very, very frustrating. If you’ve ever found yourself in a Walmart parking lot, in the dark, in the rain, trying to fix your trailer wiring with flashlights you know how much fun it can be. If you’ve got bad wiring, now’s the time to run some new wires, not when you find yourself in a pinch. Whether it’s a new installation or a repair job, we can help you with your trailer lights, wiring and installation.

Please note: this is a basic installation, and all jobs are a little different. If you’re installing a larger trailer with electric brakes, you’ll need a brake controller, which will involve some wiring to be done under the dash.

If you like to keep tabs on all of your trailer wiring functions like electric brakes, brake lights, turn signal flashers and running lights you might consider buying a trailer plug tester. They make these testers for small and large wiring plugs and they really make troubleshooting your trailer wiring much easier!

Removing Your Tail Light

This trailer wiring installation was performed on a Nissan Titan pickup, but your application will be similar. The first step is to get to the tail light wiring harness. This is usually done by removing the tail light assembly, but in some cases, you can pop out a single harness from the back of the tail light. You just need access to the wiring. This truck’s tail light assembly was easy to remove by taking out two bolts on the side of the truck bed then sliding the assembly out.

Test Your Wiring

Before you can get any trailer lights working, you’ll need to know what wire does what. You don’t want your left turn signal to be your right or your brake lights to be your running lights. If you have a good repair manual, and you should, you can use the wiring diagrams inside to find the correct wire for your trailer wiring. Even if you have it all figured out, it’s a good idea to test it before you make any new installations. There’s nothing worse than going back and undoing, then redoing work because you didn’t do any testing beforehand.

It helps to have a helper at this point, somebody who can turn lights on and off for you, or push the brake pedal. Get out your regular old test light and put the clamp end on a good grounding point. Now take the sharp end and pierce one of the wires going into the back of the tail light. Test it by having your helper turn on the lights, left turn signal, right turn signal, brake lights, reverse lights until the test light illuminates. When it does, you know what wire it is. Make a note and move to the next wire until you have them all figured out.

Note: If this is a new installation of trailer wiring and lights, you’ll need to remove the tail light from the other side of the vehicle so you can tap into the turn signal wire for that side, too. You’ll also need to locate the ground wire or attach a suitable ground wire to the chassis of the truck.

Tackling issues with trailer lights can be one of the most troublesome problems for boat owners. It doesn’t matter what kind of boat it is; having functional trailer lights is a safety issue that should never be undervalued.

There are some easy tips to make sure your lights work right every time.

Check the ground

The ground is a critical component of properly functioning trailer lights. Flickering lights on the trailer is usually a ground issue. Basically, a trailer wiring is grounded one of two ways; either from the truck via the trailer hitch and ball or via a dedicated ground wire from the wiring harness.

Make sure the hitch ball is clean from rust. This is very important, so lubricate it every now and again with a light grease or oil; simply put a small amount on a rag and coat the ball.

If running a separate wire for the ground, make sure it makes good contact with the trailer from the plug. Scraping the paint away from a small area where the ground wire connects will ensure it is making contact. A small shot of a light oil like Lucas Toolbox Buddy or WD-40 will also keep it free from rust. Most trailers have a hole drilled near the hitch for that wire but if not, just drill a small hole, use a small bolt and lock washer with a nut and you will be good to go. Stainless steel bolts are highly recommended.

Submersible versus regular bulbs

If you have submersible lights, it’s okay to keep your plug plugged in. But trailers that have standard 1156 or 1157 trailer lights are best unplugged as to not blow the bulbs upon water contact. These traditional lights become hot and when cooler water hits them, it can result in a failure. Once a year, take the lens off your traditional lights and use a bit of Vaseline or light grease on the bulb bases. This will prevent rust and get a better contact.

Tidy up wires

Never leave wires hanging from your trailer. They can catch on weeds and other objects and pull loose. They can be cleaned up easily by using a couple of zip ties. This is particularly important near the lights themselves and around the license plate. Check your connectors on the truck and trailer regularly for bare or broken wires. Depending on the type of connector—round or flat—check both the male and female connections too. Clean up corrosion or dirt on connections with a lightweight oil.

Carry spare bulbs and fuses

Bulbs and fuses are common reasons for malfunctioning trailer lights. Check your truck fuses if all of the lights are not working. They are usually 10-amp fuses, red in color, for trailers. If you are using traditional bulbs, always carry a couple of spares too. Most trailers use an 1156 bulb that can be found at any auto parts store.

Tools

You never know when a wire could break on your trailer so carrying a set of wire pliers or crimpers, some spare wire and some submersible butt splices is a good rule of thumb. Wires can be easily stripped, and a butt splice added and crimped will get you back in business. Extra wire can be used to increase length if necessary. A roll of electrical tape is also a good addition to this kit. We use liquid tape a lot but the connection needs to be dry for it to adhere properly.

If replacing tail lights on your trailer we recommend LED-style lights for replacements. They are brighter and last much longer. They are also submersible. Follow color codes on the wires to ensure the lights are set up correctly. If the wires run through a hole in the trailer it’s easy to thread them through a small piece of plastic tube in the hole to keep them from fraying and causing a short due to water pressure or road vibration.

Trailers can be troublesome but by following these tips it may be a simple fix and allow you to spend more time on the water instead of under the trailer.

One of the facts of life when you own a recreational vehicle is the problem of intermittent running lights. Fortunately, there are only two predominant causes for this problem: road vibrations and corrosion. This fact narrows down the troubleshooting process.

Corrosion erodes the electrical contacts on the running lights, causing the lighting malfunction. Road vibrations loosen connections, thereby disrupting the flow of current. In fact, the majority of problems with exterior lighting can easily be solved with a few spare bulbs and fuses that are stored in a handy and accessible spot.

If the running lights are not working, one of the first things to check is the presence of a blown fuse. If all of the lights are connected on the same circuit and one of the fuses is blown, then the lights will not work.

If the turn signals are not working, locate the fuse block that contains the circuit for them. Take out the fuse and inspect it visually. Look to see if the filament inside the fuse is broken. If the filament is broken, then you know you have found the problem.

Sometimes, however, the fuse may appear to be in good condition, but it really is dysfunctional. To rule that instance out, take a new fuse that has the same rating and insert it into the proper location. If you don’t have any spare fuses available, use the VOM on your Ohm scale to check for continuity through the fuse. If the problem is a short, and the next fuse that you insert shorts out as well, then you will need to use the circuit breaker test leads from your tool kit. While substituting the test leads for the fuse, search for the short.

Poor electrical contact in the wiring connector between the trailer and the tow vehicle can cause an entire circuit to fail. To prevent this from happening in the first place, simply keep both the plug and the socket covered in sturdy plastic whenever they are disconnected.

Poor electrical contact is typically caused by corrosion. Corrosion occurs after water has gotten into either the trailer plug or the tow vehicle socket. This can be remedied with a good treatment from a spray can of electrical contact cleaner. This can typically be purchased at any hardware, electronics, or recreational vehicle location.

Quite possibly the easiest solution to the problem of nonworking lights is the discovery of a single burned out bulb. Simply remove the lens cover for the bulb and look to see if the filament is broken. If it is broken, replace the bulb.

However, if the filament looks good, then the lamp may have corrosion on the inside of the fixture. Take a look at the connections to see if they are firmly attached and clean. If corrosion exists or if the connections are dirty, clean it up. Take the bulb and give it a once over with fine sandpaper on its base. If you do not have find sandpaper, do not use coarse. Instead, get some emery cloth to use.

Check also to see if the contact strip and its clip are clean and free of corrosion. If not, clean them and then use fine sandpaper to burnish them. Finally, investigate to see if the contact strip actually touches the tip of the bulb, as it should. If it doesn’t, then try bending it gently until it does. If the fixture has a great deal of corrosion, ignore all these steps and simply replace the entire unit.

If you are dealing with corrosion, you will need to take a few precautionary steps to prevent the problem from continuing in the future. Corrosion is typically caused by water seepage, in this case, from the lens. Use a bit of silicone sealant on the edges of the lens to prevent any future corrosion. It is best to do this prior to placing the lens in the fixture.

If the light flickers, especially when you tap it, then a bad ground is probably at the root of the problem. In order to check for a bad ground, you’ll need to take the lens off. Take an alligator lead, or a piece of wire if you don’t have an alligator lead. Attach one end of the lead to one side of the bulb’s base. Pick up the other end of the lead or wire and touch something that is bare metal, such as the lamp’s mounting screws or an aluminum windowsill on the trailer. If the result is a lit bulb, then a bad ground is indeed the problem.
Most trailer lamps are grounded by either a double wire set up or a single wire set up. Trailers with a nonmetallic mounting surface are required to have the two-wire set up. To check to see which is the case with your trailer, remove the cover of the lens. If you see only a single wire, then the ground path is through one of the mounting screws and into the aluminum skin on the trailer. Tightening the screw usually solves this problem. If this does not solve the grounding problem, replace the screw with one that is slightly larger, typically the next size.

If you see two separate wires when you remove the lens cover, trace the ground wire. The ground wire usually is attached to any point on the chassis of the trailer. Check to make sure that this wire is not broken and that it is securely attached, and fix it if need be.

Dim lights are indicative of the hot lead that goes to the lights shorting current into the chassis. Check this wire out. If you discover worn insulation, use electrical tape to repair it. If the wiring has been spliced together, then you will also need to check out the connectors on them to look for possible water damage and corrosion.

In some cases, the trailer may have a harness that runs along the frame rails. In-line fuses may be located within the harness. You will need to check for the fuses. One of the easiest ways to do this is to use a 12-volt test light. Once you have located the fuses, you can begin the trial and error process to check for a malfunctioning fuse.

Additionally, road vibrations can cause the wire nuts to loosen in the rear lamp assemblies. Check the wires and the nuts to make sure that they are connected and tight. Use electrical tape to wrap the nuts in the first place to prevent this from happening.

Always remember to check your lights to see that they are in good working order prior to any trip. Complete the necessary repairs in a timely fashion. Purchase spare parts and keep them in a handy and secure location. If you take care of your trailer, then it will take care of you.

Trailer lights and wiring are exposed to rain, snow, salt, road dirt and grime. The wires and bulbs are subject to chaffing, corrosion and damage. The bulb sockets and ground connections are particularly prone to corrosion.

Trailer lights must be tested often for everyone’s safety in traffic and to avoid legal and regulatory citations and fines. Testing trailer lights takes time and time costs money.

Testing trailer lights normally requires a towing vehicle connected to the trailer then walking back and forth from the vehicle to the back of the trailer to verify that all the trailer lights function properly – left signal, right signal, brake lights, tail and clearance lights. A second person could be in the vehicle to operate the lights. But that is double time consuming and very expensive for a business with a fleet of trailers.

Because of the hassle, time and expense, most people don’t test their trailer lights in advance. Instead they wait until they are hooked up and do a quick walk around to check the trailer lights then realize they have a problem. But everyone is ready and anxious to go, so off they go traveling or to the job site traveling illegely and not safely. This could result in traffic citations or possibly a much more expensive traffic accident.

What do customers say about Light Check

Light Check has made trailer light checking a lot easer. I check them alone most of the time. It’s like having a second person helping.

I do a lot of D.O.T inspections. With Light Check as a tool has made my job more efficient.

Light Check makes it easy to distinguish between vehicle or trailer wiring problems.

It has saved us a lot of time

When I rent out a trailer, sometimes the lights do not work when hooked up to the customers vehicle. I connect Light Check to the trailer and show the customer that the lights on our trailer are working fine. This has helped significantly with customer relations.

Light Check is a self-contained, portable device that connects to the wiring connector of the trailer. It includes a 12-volt internal rechargeable battery, electronic diagnostics for light circuit analysis, ground wire, battery charger and instructions.

Simply connect Light Check to your trailer wiring, turn on the power and press Select button for Cycle. Light Check will continue cycling through each of the trailer light circuits in 5 second intervals allowing one person to easily check the signal lights, brake lights, tail and clearance lights without a towing vehicle or second person to assist.

Use the Select button to operate any individual circuit continuously for over current (short circuit) and low current (open circuit). If a problem is detected, LED fault lights will indicate the type and source of the fault.

Save time and money, reduce fines and penalties by testing your trailer lights with LIght Check.

• Increase safety for your family, employees or customers with properly operating trailer lights

• Test trailer lights in 5 minutes or less.
• Reduce payroll costs with one person operation and no vehicle to hook-up
• Prevent traffic citations and fines

Simple yet comprehensive

• Self contained with internal rechargeable battery
• Electronic diagnostics indicates the type and source of faults when problems are found
• Unique Cycle mode cycles through the light circuits for simple one-man operation. Tests signal lights, brake lights, tail and clearance lights in repeating cycle.
• Individual circuits may be operated continuously for trouble shooting or repair

Simple to operate

1. Connect Light Check to trailer wiring connector
2. Turn the Light Check power switch to ON
3. Press the SELECT button once for Cycle
4. Test and observe lights for proper operation
5. Turn the power switch to OFF when finished

With no hook-up vehicle or second person to assist

trailer light test tester check checker diagnostics repair maintenance trailer wiring diagnose problem tool trailer lights troubleshooting

A test light, sometimes called a test lamp or voltage tester, is a simple but extremely useful electronic tool to check your car’s circuits—that is, the presence or absence of electricity to a certain component or piece of equipment. If you are trying to diagnose and troubleshoot an electrical problem, sometimes a test light can help you rule out possible causes much more quickly and easily than a DMM (Digital Multi Meter). It’s quick, easy, and versatile. In fact, you’d be hard-pressed to find a more handy gadget to have in your vehicle. You can use it to check any positive circuit, from your cigarette lighter to your headlights and taillights. If the fuse turns out to be good, you can use a circuit tester to trace the wiring path and find out what’s gone wrong. If the positive path is intact, you can also use the test light to check the grounding points of the circuit.

Test for Positive Voltage

It’s simple to use a test light to test a positive circuit for voltage. The basic principle is illustrated in this photo. You have a positive power source (in the case of the photo, it’s the battery) and you have a ground (any exposed metal that’s bolted to the chassis). The test light is the go-between. If you connect one end to the positive power source and the other end to a good ground, it lights up. To test for positive voltage, attach one end to a known ground, and touch the other end to the wire you want to test. If it lights up, you’re good. If not, you need to replace or clean the component you just tested.

Tips:

• Before you test a circuit for voltage, be sure your test light is in good order by testing it on the car’s battery.
• The leads of the test light are reversible. It doesn’t matter which one goes on the positive and which goes on the ground. Use whichever end makes your job the easiest.
• Most test lights have a sharp pointed end. You can use this sharp point to pierce the plastic insulation on a wire. This means that you can test the circuit without disconnecting anything.

Test a Ground Circuit

Your test light circuit tester is great for checking for voltage, but it can also be used to check a ground circuit. If you know that a certain electrical component is getting juice on the positive side, you need to check to see if it has a good grounding point.

This is easy. Since you have already established a good positive source, attach one end of the circuit tester to the positive end. Now touch the other end of the tester to the ground wire for this component. If it lights up, you have a good ground and need to check the component further. If you don’t get a light, it’s time to clean contact points and check the ground path. Luckily, grounds aren’t too difficult to re-establish. Usually, all you need to do is ensure that the ground wire is attached to a point that is free of paint, rust, plating, or anything else that might act as an insulator. You can also invest in a handy component known as an engine ground strap.

This article will show how to check a flashlight bulb or any other bulb that contains a filament to see if it’s working or not. These steps will not allow you to test the newer CFL light bulbs or the latest LED light bulbs. But virtually any incandescent filament bulb can be tested in the way that is described. This will be particularly useful for checking flashlight bulbs because the filaments are very tiny and difficult to see.

Step 1

Check the bulb by determining the resistance of the filament. The filament is the wire inside of the bulb. The resistance can be checked by a multimeter. By setting the multimeter selector switch to ohms it will read ohms for you. An ohm is a measure of resistance.

Step 2

Place on of the probes on the bottom “button” of the light bulb. Touch the other probe to the side casing of the light bulb. If the needle moves at all, preferably going almost clear over to the other side, that means the bulb is OK. If the needle does not move at all that means the bulb is burned out and needs to be replaced.

Step 3

Test any electric bulb this way. Your can test fuses this way too. Remember if the needle does not move at all the thing your are testing has malfunctioned and either needs to be replaced or thrown away. This test tells you if you have continuity in your circuit. If you have no continuity, the item being tested must be replaced.

To test your meter to be sure it’s working properly, press the two probes together when you have it set on ohm’s. The needle should move almost all the way over. If it does, your unit is working.

Testing an LED with a Multimeter

In my previous article about LEDs, I discussed the distinct details of the LED. Now, I will bring in their practical use.

Although you can easily test an LED by connecting it to a circuit and seeing if it will light up, you can also use a multimeter with a diode test function to test an LED and discover a few more things about it too.

How to Test a Diode with a Multimeter

1. Connect the black lead to the COM terminal on the multimeter.
2. Connect the red lead to the Ω terminal, unless your particular model differs.
3. Turn the dial to the diode symbol on the multimeter. This allows for electric current to travel in one direction (the arrow) and not the other.
4. Turn the multimeter on. The display window should indicate either 0L or OPEN.
5. Choose a regular red LED.
6. Connect the black probe to the cathode end of the LED, which usually is the shorter end and/or cut flat at its bottom. Connect the red probe to the anode end of the LED.

Interpreting the LED Test Results

If it occurs that the multimeter display doesn’t change from 0L or OPEN, then it may be that you connected the probes in the wrong order, or that the connections are not secure. Make sure the steps above are followed accurately. Otherwise, it may indicate that the particular LED is damaged. If the voltage in the display is below 400 mV, then it is possible that the cathode and anode are touching, or the probes are touching. This is termed a short circuit—when current passes directly from the cathode to the anode, instead of passing through the LED.

If the steps are followed properly and the LED is undamaged, however, the display should indicate a value of approximately 1600 mV.

When you are testing your LED, take notice of its brightness. If you are already in a lit room, then shade the LED with your hands. A lower efficiency LED will grow dimly, or may just gleam faintly, whereas a higher efficiency LED will glow clearly.

LED Forward Voltage Drop

The value displayed on your multimeter is called the forward voltage drop. This indicates the quantity of voltage used up by the LED, or dropped, when current is traveling in the appropriate direction, forward.

This kind of data is extremely useful when it comes to building your own robot or designing your circuit board. You will definitely need to keep track of the total voltage used by your robot, whether it is from a LED or some other component, in order to choose a battery strong enough to power it. Therefore, it is equally important for you to purchase the LEDs that your battery can sustain. Usually, you should not purchase an LED with a forward voltage exceeding 4V, because most robotic circuits can not function at such voltages.

Images Courtesy

Related Reading

Basic LED Attributes – LEDs are used extensively in robots, or any electronic device for that matter. The main reason for this is that LEDs come in a large variety of shapes, sizes, and colors. This allows for many different functions, such as simple traffic lights, to more complicated devices, such as digital clocks.

DIY Trailer Wiring Harness Diagnosis and Repair

• By Benjamin Jerew
• July 20, 2019

While towing a trailer, safety should be first and foremost on your mind. Tire pressure, trailer brakes and trailer lights are all critical to ensuring you and your cargo arrive in one piece. Trailer lighting, turn signals and brake lights inform other drivers of your intentions.

If one or more of your lights aren’t working, you may just have a blown bulb, or you could have a problem with the trailer wiring harness on the tow vehicle or on the trailer.

If you have a trailer lighting or braking problem, check the tow vehicle’s trailer wiring harness first. You should have a digital volt/ohm meter (DVOM) in your toolbox, to make basic circuit checks. Once you confirm the tow vehicle’s plug has the proper ground and voltage, you can go on to check the trailer wiring harness and bulbs for problems. To check the trailer lighting system, plug it into the tow vehicle, and use the DVOM to check for voltage across whatever bulb’s socket you are having a problem with.

Pro tip: While checking exterior wiring on a vehicle or trailer, do not pierce the insulation with your DVOM probes. You can indeed check for voltage and ground this way, but you will also compromise the waterproof insulation. Corrosion will then be in your future, and you will be fixing this issue again.

Trailer Wiring Harness Diagnosis

The typical flat, four-pin connector, for example, should have ground on the white wire, then 12 V on the brown, yellow and green wires, depending on if the taillights, left turn signal or right turn signal are illuminated. Also, brake lights and turn signals are combined, so both yellow and green should have 12 V with the brake lights illuminated. Five-, six-, or seven-pin trailer connectors will have additional connections for the electric brake, reverse lights, brake lights or auxiliary power supply.

Although one can use a DVOM for these checks, a dedicated trailer tester can be an even easier way. Testers for four-pin connectors typically plug right in. LEDs should illuminate for each circuit activated. If nothing illuminates, then you may have a ground problem. If the tow vehicle is equipped with a trailer converter box, you will need to check inputs and outputs of the box itself. Once you determine the problem, make any necessary repairs, but never by-pass a faulty converter box — always replace it.

Trailer Wiring Harness Repair

Because trailer wiring is typically exposed to the elements, wire taps and crimp connectors are asking for corrosion. So to make a long-lasting repair on exposed wiring, follow these three steps:

1. Crimp the exposed wires using an uninsulated butt connector.
2. Solder the connection to ensure strength and conductivity.
3. Use waterproof adhesive-lined shrink tubing, at least one-half inch past each end of the connection, to prevent water intrusion.

Diagnosing and repairing trailer wiring isn’t particularly complicated, but will require patience. Following this guide, you will be able to easily correct any problems with your trailer lighting or brakes and make lasting repairs.

Check out all the towing products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on trailer wiring harness diagnosis and repair, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Pump the brakes! It’s spring and that means one thing…. It’s touring time!

If you’re anything like us here at REDARC, you’re excited to drag the caravan or camper trailer out of hibernation and shoot off somewhere out of reach to enjoy some quality time with friends and family.

But before you embark on your epic memory making journey, there are a few quick and easy trailer wiring checks you can do to ensure you don’t run into any hiccups on the black top. Take the time to look over our REDARC wiring guides before making any drastic changes.

5 common issues with trailer wiring

1. Corrosion

Unless you’re a grey nomad off living the dream, chances are your trusty home away from home sits in the one spot lifeless for more than half the year.

It’s ok, we understand, but the trusty trailer plug that brings the beast to life can be susceptible to corrosion from the weather thrown at it during the year.

The pins inside the plug can have dirt and god knows what caked inside, maybe even spiders! Give those connector pins a freshen up!

Shoot on down to the local auto accessory store and grab yourself a can of WD-40 and proceed to blast the plug, this will clear out anything that’ll add resistance to the trailer connection.

Make sure you do the same to the trailer plug and socket on the tow vehicle too, just to be sure. Once it’s all washed out, connect and disconnect the plug 3-4 times to further improve the connection.

Doing this simple clean will ensure the trailer doesn’t lose connection while you drive, this is especially important as the trailer plug is also supplying power to the brakes on the trailer and you don’t want to lose those!

Whilst you’re there checking the trailer plug, grab a small flat head screwdriver and make sure the pins aren’t bent out of place or pinched on either plug. If they are carefully bend them back into position.

If the pins look like they’re rusted, have green copper corrosion or bent beyond repair, just replace the plug! Trailer plugs are cheap and extremely easy to swap out, all you need is a screwdriver and the ability to remember where the wires went. This is trailer wiring 101!

2. Damaged wiring

Ask any auto electrician, a very common trailer wiring fault is usually caused by the family pet thinking it’s a chew toy!

Any sort of tear in the wire harness presents the potential of a short to earth – that’s not good. Damaged wires can also be caused by rocks and anything else you may come across on the road, especially if you’re doing some hardcore off road driving.

The best preventative for this type of damage is to become OCD neat with your trailer wiring.

Make sure the wires under the trailer are run through good quality conduit and secured better than Alcatraz, no hanging wires anywhere!

Also very important is to never, ever use scotch locks to join wires! Just don’t do it, they’re not designed for this type of application and will cause issues on the road.

3. Unbalanced braking

There are two ways to do trailer brake wiring, the right way and the wrong way. Uneven braking occurs when the length of the wire between each of the magnet connections adds resistance and therefore creates a difference in braking performance at each wheel. This often makes the trailer feel like it’s twisting when you brake.

Electric brake wiring: trailer wiring diagram

A brake controller has only one output wire. If that wire runs to one wheel, then the other, then the other and the other as demonstrated above, it creates that twisting feeling under braking.

If your trailer is wired as above, you will notice one wheel or one side getting very hot and the other wheels staying very cool. This is because the hot wheels are literally doing all the braking and it will cause excessive wear to those brakes.

(All our Tow Pro brake controller wiring guides are online, so make sure you have a good look before setting off.)

The length of wire from the single brake controller output wire to each magnet should be equal to achieve best braking balance; this ensures each wheel brakes with the same amount of power and the trailer brakes evenly.

4. Earth

It’s a good idea to check your earth wire in the trailer plug. Most trailer plugs are wired to suit very basic trailers, once you fit a brake controller the brake power wire is upgraded but the earth is left standard.

The earth cable is just as important as the positive, think of it as a circle; the positive cable delivers the power, the earth cable completes the circuit.

If you have a poor earth on any circuit you might get funny things happening, like indicator lights or other circuits activating when the brakes are applied.

This is caused by a bad earth on the brake circuit, so the power gets earthed through other circuits on the trailer.

Upgrade the earth cable through the trailer plug and make a good chassis earth where the cable gets a good metal on metal connection, then seal the connection with a sealant spray.

5. Poor connections

Connection and quality means everything in electronics, especially when the safety of you and your family is concerned.

This is why we have developed our range of Tow-Pro Electric Trailer Brake Controllers with circuit analysis to detect and indicate any faults with trailer brakes and/or wiring (you should also check out our diagram on how to wire up your tow-pro elite.)

You don’t want to get the wiring or installation wrong when installing a brake controller; after all, safety is absolutely paramount when it comes to your brakes – it’s not something you want to mess around with yourself. This is why we recommend a circuit breaker kit which has all the gear your installer needs to adequately wire up the Tow-Pro Elite.

For optimum performance and operation of your electric brake controller, the circuit breaker kit satisfies our stringent standards. Also, it’s worth noting the 5 common brake controller installation mistakes that you can go through with your installer.

Just remember, prevention is better than cure. Look after your trailer and trailer wiring well, and it will look after you for many memory making moments to come.

And remember you can always check out our resources section for all things on electric brake controllers.

Pump the brakes! It’s spring and that means one thing…. It’s touring time!

If you’re anything like us here at REDARC, you’re excited to drag the caravan or camper trailer out of hibernation and shoot off somewhere out of reach to enjoy some quality time with friends and family.

But before you embark on your epic memory making journey, there are a few quick and easy trailer wiring checks you can do to ensure you don’t run into any hiccups on the black top. Take the time to look over our REDARC wiring guides before making any drastic changes.

5 common issues with trailer wiring

1. Corrosion

Unless you’re a grey nomad off living the dream, chances are your trusty home away from home sits in the one spot lifeless for more than half the year.

It’s ok, we understand, but the trusty trailer plug that brings the beast to life can be susceptible to corrosion from the weather thrown at it during the year.

The pins inside the plug can have dirt and god knows what caked inside, maybe even spiders! Give those connector pins a freshen up!

Shoot on down to the local auto accessory store and grab yourself a can of WD-40 and proceed to blast the plug, this will clear out anything that’ll add resistance to the trailer connection.

Make sure you do the same to the trailer plug and socket on the tow vehicle too, just to be sure. Once it’s all washed out, connect and disconnect the plug 3-4 times to further improve the connection.

Doing this simple clean will ensure the trailer doesn’t lose connection while you drive, this is especially important as the trailer plug is also supplying power to the brakes on the trailer and you don’t want to lose those!

Whilst you’re there checking the trailer plug, grab a small flat head screwdriver and make sure the pins aren’t bent out of place or pinched on either plug. If they are carefully bend them back into position.

If the pins look like they’re rusted, have green copper corrosion or bent beyond repair, just replace the plug! Trailer plugs are cheap and extremely easy to swap out, all you need is a screwdriver and the ability to remember where the wires went. This is trailer wiring 101!

2. Damaged wiring

Ask any auto electrician, a very common trailer wiring fault is usually caused by the family pet thinking it’s a chew toy!

Any sort of tear in the wire harness presents the potential of a short to earth – that’s not good. Damaged wires can also be caused by rocks and anything else you may come across on the road, especially if you’re doing some hardcore off road driving.

The best preventative for this type of damage is to become OCD neat with your trailer wiring.

Make sure the wires under the trailer are run through good quality conduit and secured better than Alcatraz, no hanging wires anywhere!

Also very important is to never, ever use scotch locks to join wires! Just don’t do it, they’re not designed for this type of application and will cause issues on the road.

3. Unbalanced braking

There are two ways to do trailer brake wiring, the right way and the wrong way. Uneven braking occurs when the length of the wire between each of the magnet connections adds resistance and therefore creates a difference in braking performance at each wheel. This often makes the trailer feel like it’s twisting when you brake.

Electric brake wiring: trailer wiring diagram

A brake controller has only one output wire. If that wire runs to one wheel, then the other, then the other and the other as demonstrated above, it creates that twisting feeling under braking.

If your trailer is wired as above, you will notice one wheel or one side getting very hot and the other wheels staying very cool. This is because the hot wheels are literally doing all the braking and it will cause excessive wear to those brakes.

(All our Tow Pro brake controller wiring guides are online, so make sure you have a good look before setting off.)

The length of wire from the single brake controller output wire to each magnet should be equal to achieve best braking balance; this ensures each wheel brakes with the same amount of power and the trailer brakes evenly.

4. Earth

It’s a good idea to check your earth wire in the trailer plug. Most trailer plugs are wired to suit very basic trailers, once you fit a brake controller the brake power wire is upgraded but the earth is left standard.

The earth cable is just as important as the positive, think of it as a circle; the positive cable delivers the power, the earth cable completes the circuit.

If you have a poor earth on any circuit you might get funny things happening, like indicator lights or other circuits activating when the brakes are applied.

This is caused by a bad earth on the brake circuit, so the power gets earthed through other circuits on the trailer.

Upgrade the earth cable through the trailer plug and make a good chassis earth where the cable gets a good metal on metal connection, then seal the connection with a sealant spray.

5. Poor connections

Connection and quality means everything in electronics, especially when the safety of you and your family is concerned.

This is why we have developed our range of Tow-Pro Electric Trailer Brake Controllers with circuit analysis to detect and indicate any faults with trailer brakes and/or wiring (you should also check out our diagram on how to wire up your tow-pro elite.)

You don’t want to get the wiring or installation wrong when installing a brake controller; after all, safety is absolutely paramount when it comes to your brakes – it’s not something you want to mess around with yourself. This is why we recommend a circuit breaker kit which has all the gear your installer needs to adequately wire up the Tow-Pro Elite.

For optimum performance and operation of your electric brake controller, the circuit breaker kit satisfies our stringent standards. Also, it’s worth noting the 5 common brake controller installation mistakes that you can go through with your installer.

Just remember, prevention is better than cure. Look after your trailer and trailer wiring well, and it will look after you for many memory making moments to come.

And remember you can always check out our resources section for all things on electric brake controllers.

We all know that gut-wrenching feeling of coming too close to another car’s tail lights.

For the safety of ourselves, our cargo, our horses, and others on the road, it’s important to prepare for those scary moments when a car veers in front of your tow vehicle or stops short. If you’re not 100% sure that your trailer brakes are functioning, you’re not ready for the road.

Blue Ridge Trailers is a Virginia trailer inspection station with years of experience testing brake function and other trailer safety components. We wanted to share our quick tip for how to test trailer brakes effectively before every drive, so you can stay safe and relaxed on the road.

Note: this tip in no way replaces comprehensive trailer inspections. It’s crucial (not to mention required by law in Virginia and many other states) to get a state certified trailer inspection every year to ensure all of your trailer’s safety systems are in good working order.

Today we’ll be focusing particularly on how to test trailer brakes in electric brake systems, which are found in your typical tandem axle utility or horse trailer. Most trailers have two brakes on each axle–so if you’ve got a tandem axle trailer, you’ll have four brakes.

With electric trailer brake systems, you’ll also have a brake controller (a.k.a. a brake box), which is mounted in your tow vehicle. The brake controller sends electricity to the brakes themselves, through the trailer plug, when you press the brake pedal in your tow vehicle.

The brake controller setting will dictate how much braking power will be transferred by the brake pedal. A digital brake controller can be set to automatically override the existing setting, giving you maximum braking power in the event of an emergency.

The thing is, it’s not always easy to tell which (if any) trailer brakes are working just by pressing the pedal during a test drive.

Instead, you’ll want to look for a slide bar on your brake controller. It should go from 0 to 10 or will have an indicator light. Check your brake controller manual if you don’t find it right away.

Once you’ve located the slide bar, start pulling on it gradually as you drive forward slowly. You should feel stronger and stronger resistance as you move from 0 to 10. If you’re not feeling much or any braking as you approach 10, you know you’ve got a problem.

Because this testing method bypasses the truck pedals, you’ll get a more accurate sense of trailer brake function. You’ll know that any resistance is coming exclusively from the trailer itself, not the tow vehicle brakes.

If your trailer doesn’t pass your brake test, it’s likely due to one or more common problems:

• Corrosion of the wiring or brake components. This sometimes occurs if a trailer sits parked for weeks or months at a time.
• Poorly adjusted brakes.
• Delaminated and/or grease-saturated brake shoes.
• Missing components such as a slack adjuster.

As always, your trailer manual and a professional trailer inspector can help you diagnose issues and get your brakes running smoothly.

Got questions about how to test trailer brakes on your own trailer or other trailer safety concerns? Blue Ridge Trailers is always happy to offer our expertise. Contact us online or call us at (434) 985-4151 to speak to our staff.

The installation schematic instructions listed below are for a basic taillight converter for a car, truck, suv, van, etc. This installation is for a vehicle with a 3 wire system. (vehicle has amber turn signals or separate turn signal and stop bulbs). It then converts the taillight harness to a 4 way flat connector. Not recommended for BMW, Land Rover and Mercedes Benz vehicles, Ford Freestar or Mercury Monterey vehicles, 99 to 03 Ford Windstar, 99 to 06 Lincoln LS and ALL vehicles with tail lamp outage modules. Consult your owners manual before splicing into your taillight harness.

Instructions

1.) Determine a suitable location for mounting the taillight converter in an out of the way spot near the left tail lights in the trunk or on the frame rail, if mounted under the towing vehicle.

2.) Ground the white ground wire by placing the ring terminal under an existing screw attached to a clean metal surface or by drilling a hole and use a self tapping screw.

3.) Disconnect the tow vehicle’s negative (-) battery cable.

4.) Using a circuit tester or volt ohm meter (VOM), determine which wire attached to the left taillight assembly is the left turn wire. Attach the units yellow left turn wire to this wire using a blue wire tap or Scotchlok. Determine which wire is the taillight circuit and attach the module’s brown taillight wire to it with a wire tap. Determine which wire is the stop circuit and attach the module red stop wire to it with a wire tap or Scotchlok. (for 3 wire systems only).

5.) Route the units green right turn wire to the right side of the vehicle. Determine which wire is the right turn circuit and attach the green wire to it with a blue wire tap or Scotchlok.

6.) Secure all loose wiring with cable ties.

Testing Procedure

With the ground wire connected and all of the other circuits attached, attach the ground lead of a circuit tester to the exposed ground terminal of the 4-flat end. Activate the tow vehicle left turn, right turn, tail and stop lights one at a time. Probe the three receptacles of the 4-flat end to confirm proper functions. If testing with a trailer, make the proper connections and do the same test as the circuit tester using the trailer lights. If a function on the trailer lights does not work properly, disconnect the trailer lights, turn functions on vehicle off and recheck function with the unit with a circuit tester. Then check the trailer wiring for potential circuit problems.

Estimating Trailer Amperage Per Circuit

NOTE: For estimating use only. Amperage is rounded to the nearest whole value. May not apply in all cases. Always check with your tow vehicles owners manual about adding additional loads to your electrical system. Some 12 volt batteries produce up to 14.5 volts. Use a VOM (volt ohm meter) to test.

If you know the wattage of the bulbs on your trailer you can use the formula P = I X E

P = watts
I = amps
E = 12 (volts)

Simple plug in Vehicle Specific Trailer Wiring Harness by make, model and year.

Automotive Accessories Connection.
Your car, truck, van, crossover, suv and auto accessories dealer on the web since 1996.

Anything on our web site is quoted in US government dollars. Currency Converter.
Quantity discounts available.

Copyright: 1996, 1997, 1998, 1999, 2000,
2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013

How can I use a voltage tester to check a light switch? The Most Common Methods for Testing a Light Switch, Testing a Switch with a Continuity Tester, Testing a Switch with a Voltage Tester.

Testing a Light Switch
Electrical Question: How can I use a voltage tester to check a light switch?

• I have an older home. The bathroom light switch to turn the light on is a little different.
• The two wires there when I measure across them gives me 120 volts.
• On the light I checked before beginning to work on it with my tick tracer and shows there is power in one of the legs.
• So I check across the two wires and nothing happens no power but when connecting the two switch wires and then measuring there are my 120 volts, this was the way this switch has worked for many years.
• I was just curious how this could be?

Any theory will help.
Thank you.

This electrical wiring question came from: Angel, a Handyman from Joliet, Illinois.

Dave’s Reply:
Thanks for your electrical wiring question Angel.

How to Test a Light Switch using Electrical Testers

Application: Testing a Light Switch.
Skill Level: Intermediate to Advanced – Best performed by a qualified electrician.
Tools Required: Basic Electricians Pouch Hand Tools and Voltage Tester or Continuity Tester.
Estimated Time: Depends on personal level experience and ability to work with tools.
Precaution: Identify the light circuit, turn it OFF and Tag it with a Note before working with the wiring.
Notice : Testing with the circuit power on can be dangerous and should only be done by qualified or experienced electricians.

The Most Common Methods for Testing a Light Switch

• Testing a Switch with a Continuity Tester
  A continuity tester can be used to test the light switch while the circuit is OFF or the switch has been removed. This type of tester has two lead wires that uses the battery voltage of the tester to test a circuit, which in this example is the circuit components of a switch. Toggle the switch into the on and off position with the test leads attached to the terminals of the switch. If the switch is good then there will be a reading or indication, otherwise no reading or indication means that the switch is bad.
• Testing a Switch with a Voltage Tester
  A voltage tester has two lead wires, one for positive and one for negative, however since a switch only has a positive power source connected to it the one test lead will need to be attached to either a ground wire or a neutral wire, while the other test lead is attached to the screw terminals of the switch, one at a time while turning the switch on and off. The tester will produce a voltage reading if the switch is good, however sometimes you can get false voltage reading, which I will explain below.

Why a Tester May Read Voltage while the Light Switch is Off

• Testing a Light Switch
  This is a question that comes up often, and here is the answer. A standard incandescent light bulb has a filament wire that makes the connection between the two wires of the circuit. Therefore when the light switch is in the off position, the connection between the neutral wire is made through the filament and enables the tester to connect back to the neutral wire of the light fixture.
• The Tester as a Light Bulb
  So in essence, your tester is becoming a light when the two tester leads are placed onto the two switch terminals.
• How to Get an Accurate Test Reading
  To prove this, simply remove any light bulbs from the light fixture and the 12o volt reading will stop because the connection to the neutral wire has been disconnected.
• Light Circuit Wiring
  Similar situations occur when one wire has become disconnected within a circuit, and a load is present, which can create a back-feed of electricity which causes lights to glow dim, because the available power is feeding back through other devices in the circuit wiring, and these devices are creating resistance on the circuit.

More about Light Switches

• Electrical Testers for Light Switches
• Using Electrical Testers

Guide to Electrical Testers

» You Can Avoid Costly Mistakes! «

Here’s How to Do It:
Wire It Right with the help of my Illustrated Wiring Book
Great for any Home Wiring Project.

Complete Guide to Home Electrical Wiring

Perfect for Homeowners, Students,
Handyman, Handywomen, and Electricians
Includes:
Wiring GFCI Outlets
Wiring Home Electric Circuits
120 Volt and 240 Volt Outlet Circuits
Wiring Light Switches
Wiring 3-Wire and 4-Wire Electric Range
Wiring 3-Wire and 4-Wire Dryer Cord and Dryer Outlet
How to Troubleshoot and Repair Electrical Wiring
Wiring Methods for Upgrading Electrical Wiring
NEC Codes for Home Electrical Wiring
. and much more.

Be Careful and Be Safe – Never Work on Energized Circuits!
Consult your Local Building Department about Permits and Inspections for all Electric Wiring Projects.

You must be logged in to post a comment.

Home Electrical Wiring and Electrical Repairs

Please Note:
Unless otherwise posted, please refer to the following Guidelines when working on Electrical Projects:

Skill Level:
Intermediate to Advanced – Electrical Repairs and Circuit Wiring is Best Performed by a Certified Electrician or Licensed Electrical Contractor.

Electrical Tools Required:
Basic Electricians Pouch of Hand Tools, a Voltage Tester and a Multi Meter.

Estimated Time:
Depends on personal level experience, ability to work with tools, work with electrical wiring, and the available access to the project area.

Precaution:
Identify the project circuit, turn it OFF and Tag it with a Note before working with the wiring.

You normally take your car and trailer driving test when you’ve got a full car driving licence. You do not need to pass another theory test.

The test is sometimes called the ‘B+E test’. Check the rules to see if you need to take it.

To pass the driving test you must be able to:

• drive safely in different road and traffic conditions
• show that you know The Highway Code by the way you drive

The national standard for driving cars tells you everything you must be able to do to pass the test. Only take your test when you can do everything without instruction.

There’s no minimum number of lessons you must have done before you book and take your test.

Car and trailer driving tests and coronavirus (COVID-19)

You cannot currently book a driving test in Scotland or Wales because of coronavirus.

If you want to take your test in England and do not already have an appointment, your driving school must book your test for you.

If you had booked a test and it was cancelled because of coronavirus, DVSA will email you a link to book a new test.

You’ll need to wear a face covering during your test. If you cannot wear a face covering, for example because you have a respiratory condition, you or your driving school must tell DVSA when booking your appointment.

You can cancel your driving test appointment and get a refund if you cannot or do not want to attend because of coronavirus. Your driving school will need to cancel your test for you if they booked it.

How to Calibrate a Metal Detector

Low-voltage wiring is mainly used to connect batteries or transformers to low-powered electrical devices. Examples include railroad or racing car sets and LED lights. Sometimes wires get disconnected or damaged, and electricity can’t get to the electrical device. Determining the location of the wiring problem can be difficult, but using a multimeter is a fairly easy method to test low-voltage wiring.

Look on the label on your battery or transformer. You need to know the voltage it produces. Make a mental note of the voltage.

Turn on the power source. This enables you to test if electricity is getting to the low-voltage wires in the first place.

Turn on the multimeter. Set it to read voltage; you do this by rotating the dial or pressing the appropriate button on the meter, depending on the meter model. Multimeters can measure amperes and ohms as well as voltage, so it’s important to set it correctly.

Insert the colored plugs on the end of the wires that connect to the multimeter into the sockets, if they are not already connected. The red plug goes into the red socket and the black plug goes into the black socket. If there are several red sockets, insert the red plug into the red socket marked for voltage; this socket usually is marked with a V.

Place the two sensors on the ends of the red and black wires from the meter onto the terminals on the power source. You must put the red sensor on the terminal labeled “+” and the black sensor onto the terminal labeled with a “-” sign.

Read the meter. It should display a similar voltage as the label on the battery or transformer. If there isn’t a reading, either the battery is dead and needs replacing, or the transformer isn’t working, in which case you must get it checked by a professional.

Check the wires where they connect to the terminals on the power source. Ensure the connections are secure so you know that electricity is flowing from the power source into the wires.

Look at the wire and follow it toward the electrical device to which the wires connect. Look for damage to the wiring. If you find damaged wire, it’s best to replace the complete length of wire. Continue to check the wire until you get to the electrical device.

Check that the wires are securely connected to the terminals on the electrical device. Use the multimeter to check if electricity is getting to the device. Place the red and black sensors on the ends of the wires from the meter onto the “+” and “-“ terminals on the electrical device respectively.

Read the meter. The meter should read a similar voltage as the label on the battery or transformer. This means electricity is getting through the wires to your device. If your device isn’t working it’s not the wiring, so it must be the device. If the meter doesn’t indicate a reading, there is an internal fault in the wiring. You have also checked the wire for visible external damage. In this case you need to replace the low-voltage wire.

It’s critical to have working brake lights on your car. They’re a safety feature and a tool of courtesy to other drivers. Illuminated brake lights indicate your car is slowing down or has stopped, giving other cars — in daylight, dim light or darkness — plenty of time to stop so they don’t crash into you. Aside from all that, if your brake lights don’t work you can get a traffic ticket.

But you can avoid all those problems by testing your brake lights once in awhile — a couple times a year is sufficient. It’s especially important to do a test before the low-light winter months set in. If your brake lights aren’t working, fix them immediately for safety’s sake.

It’s simple enough to test your brakes. Just have a friend stand behind the car while you step on the brake pedal and have him or her tell you if everything looks OK. If you’re working alone, rig up a mirror or lay a broomstick on the pedal and tuck it into the seat, then walk behind the car and look for yourself. And don’t forget to check all your brake lights. Cars built in recent years feature three brake lights — one on either side in the rear, both of which are implanted in the rear bumper, and a “cyclops” light in the back window.

Brake lights, of course, depend on a car’s electrical system, which runs on a series of switches and fuses to protect circuits. If one or more of your brake lights isn’t working properly, it could mean one of three things: The brake light system fuse is blown, the brake light bulbs are burned out or the brake light wiring switch is broken. All of these issues are easy to troubleshoot.

Read on to learn how to identify and fix the problems.

ABOUT THE AUTHOR

G.K. Bayne

All 12-volt lights require electrical power for illumination. The amount of voltage reaching the terminals on the rear of the 12-volt light is indicated by the brightness of the bulb. In most instances, the electrical power is supplied through a 12-volt battery. A light switch then turns the electrical power on and off. Checking the amount of voltage from the power source, along with the voltage delivered to the light bulb terminals will indicate whether the 12-volt light is good or bad.

Things You’ll Need

Voltmeter test leads

Step 1

Insert the red test lead into the voltmeter connector identified as “volt.” Secure the black test lead into the voltmeter connector marked as “com.” Turn the main switch of the face of the voltmeter to “DC volt.”

Step 2

Touch the red lead of the voltmeter to the positive (+) terminal on the 12-volt battery. Press the black lead of the voltmeter to the negative (-) terminal on the battery. The voltmeter must read greater than 12.0 volts on the display. If the voltage is less than 12.0 volts, the battery requires charging.

Step 3

Remove the leads from the battery. Gain access to the rear terminals on the 12-volt light you are testing. Because 12-volt lights are utilized for many applications, consult the manufacturer’s operational instructions for accessing the terminals on the 12-volt light.

Step 4

Turn the switch on for the 12-volt light. Touch one of the voltmeter leads to one of the light connector terminals. Press the other lead to the other light connector terminal.

Step 5

Read the display on the voltmeter. The voltmeter should read the same voltage that was displayed in Step 2. If the voltmeter display is the same voltage from Step 2, and the light does not illuminate, then the light is bad and requires replacing. If 0.0 volts is read on the display, there is a problem with the switch or the wires leading to the light bulb.

Manufacturers will offer a wiring schematic to the electrical wiring. The schematic will allow you to trace the circuit for identifying problems in the switching and wiring circuit.

This kit comes with everything required to install rear lighting on trailers less than 80 in. wide. It includes 2 stop, turn and tail combination lights, side markers and accessories.

In-Store

Online

Feature(s)

• Includes 2 stop, turn and tail combination lights, side markers and accessories.

Application(s)

• A complete kit of stop, turn and tail combination lights, side markers and accessories required for installation rear lighting on trailers less than 80 in. wide.

Weight & Package Dimensions

• Weight (lbs) 2.5
• Package (L x W x H) 13.8 x 9.6 x 4.6 in.

Attributes & Specifications

There are no accessories for this product.

Downloads

• 4200110_manualhb_00_01_v01_manuel_enfr.pdf
• 4200110_partshb_00_01_v01_pieces_enfr.pdf

Parts associated with this item

Recently Viewed

Trailer Light Kit

Questions or comments?

National Call Center hours

Mon – Fri: 6:00 am – 9:00 pm CDT
Sat & Sun: 8:00 am – 5:30 pm CDT
or Contact Us

*Our call center is experiencing higher than normal call volumes that result in longer wait times

For faster answers to your questions, visit our Frequently Asked Questions page.

Connect with Princess Auto

Read what our customers are saying

Email Signup

Get all the newest sales events, store news and more delivered right to your inbox!

Need to make a return but can’t make it to the store? Not to worry! For non-essential returns, you can easily return it next week, next month, or next year! Our Royal Service Promise ensures we will make it right the next time you make it into the store

How to test a Tow-Pro Elite

To test your Tow-Pro Elite brake controller, connect a 21W globe between the earth pin (3) and trailer brake pin (5) of your trailer base.

The LED should illuminate BLUE or GREEN depending in what mode it’s in.

For testing we need it to be in USER control, so if your LED is not GREEN then we need to change modes. To change modes, then turn the remote dial fully anti-clockwise, then press and hold brake pedal and double press the override button. (note, do this in a fast fashion)

Once the remote head is GREEN, have someone watch the test light. With your foot on the brake pedal, as you slowly increase the dial from 0 through to 10 you will see the test light go from no brilliance to full brilliance. This is normal operation.

Once the test is completed, change mode back to proportional mode (BLUE LED).

Now with a trailer connected you have the ability to set the nominal setting. If you feel like you need more braking dial it up, if too much dial it down accordingly.

Why test with a 21w bulb?

Tow-Pro™ Elite products with a serial number of #16060749 (June 2016) or later need to be tested with a 21w bulb rather than a 55w bulb. The Tow-Pro™ Elite will check for shorts on the trailer wiring and a cold 55W bulb has an initial “inrush” current as it first warms up that exceeds the trip point, and the Tow-Pro™ Elite will see this as an intermittent wiring short. While a hot 55W bulb is not likely to reach this trip point, it is best to use a 21W bulb. Products prior to this date have a slightly higher trip point.

For further assistance and technical support contact REDARC directly.

Easy step by step guide on how to use an automotive test light, information pertains to all 12 volt vehicles.

Difficulty Scale: 2 of 10

Step 1 – A test light is needed to perform electrical power and ground testing to facilitate automotive repairs, this tool is easy to use and is reasonable priced.

Step 2 – Before staring any testing the test light itself must be tested, connect the clip or clamp to a known ground source, in this case the battery negative terminal is being used but almost any metal surface on the vehicle will work. Gently, touch the test light probe to a known power source, we use the battery positive is this example but any known power source such as a fuse or positive battery cable will work.

Step 3 – Once the test light is properly grounded, it can be used to test not only fuse condition by touching each side of the fuse, it also confirms power at the fuse. If an accessory has failed such as brake lights and the fuse tests okay, testing can then proceed further down the circuit such as the brake light switch and so on, a wiring schematic is useful when performing these tests.

Step 4 – When power tests okay for power at the source such as a relay or fuse, yet power is not present at a particular accessory such as the cooling fan a test light can trace the problem down through a process of elimination.

Step 5 – Power must be present at the large terminal of the alternator to charge, a test light performs this task easily.

Step 6 – This electrical connector tested good while probing the wires before the connector, it wasn’t until the technician removed the connector was the problem discovered.

Step 7 – A test light can be used to test for power at the fuel injector, reversing the clamp to the negative side of the battery will check the trigger side of the injector circuit.

A test light utilizes a bulb held in a probe attached to a sharply pointed rod with a connection lead. This design is optimal for piercing a wire, testing a fuse or checking the surface charge of a battery. If power is present, the bulb will illuminate confirming the circuit has power and is operating properly. Also, reversing the test light lead to the positive battery to be able to check for system ground continuity. A voltmeter is used to check resistance and to give a voltage reading which can be used for more in-depth troubleshooting.

Most trailers that have a 7-blade RV type connector and on-board battery will power this device. To be sure, check across pins 1 & 4 of your trailer’s connector using a test light (using a voltmeter may give false results). If power is present, your trailer’s battery will power the Light Tester. See diagram below for details.

Note: Some trailers have a diode in their circuitry that prevents outward current flow through pin 4 of the trailer’s the 7-way connector.

Question 2: My trailer does not have an on-board battery, can I use the LT35 Light Tester for checking its lights?

Yes, simply connect the Light Tester’s cigarette lighter plug to any 12v external power source (battery or jump box) that is capable of providing 10 amps. For example, see picture below.

Question 3: I have a trailer that has a 4-way Flat type connector. Can the LT35 be used for testing its lights?

Yes, both the LT35 & LT45 testers can be used to test the lighting systems on most any trailer by using the appropriate adapter. Adapters (see picture below) for different trailer types can be purchased through us or at most auto parts and RV supply stores. W hen using a Trailer Connector Adapter with the LT35 tester, the LT35 will need to be powered through its auxiliary power cord.

7-way RV type plug to 4-flat/6-round adapter

Question 4: If my trailer has a circuit that is shorted, will the Light Tester get damaged?

No, the Light Tester will not get damaged, as it is protected by a Circuit Breaker. The Circuit Breaker will also protect your trailers wiring from damage. If tripped, the Circuit Breaker is easily reset by pushing down the white button.

If you have any questions regarding the Trailer Light Tester, please complete the form below. Thank you!

Ensuring your trailer has the appropriate lighting is a legal requirement (The Road Vehicles Lighting Regulations 1989). This simple guide will help you to be within the current laws for use on the UK public highway.

All trailers must have fitted:

• Two red sidelights.
• Two red stoplights.
• Two red reflective triangles.
• An illuminated number plate.
• Amber indicators (designed to flash between 60-120 times per minute).
• Some form of letting the driver know trailer indicators are working correctly such as a buzzer or light.

All trailers built after September 30th 1990 also require:

If the trailer is more than 1.3m wide they also require:

• At least one red fog lamp.

Larger trailers (1.6m and over) require additional features:

• White front reflectors.
• White front markers/lights.

Trailers Manufactured from October 2012 fitted with brakes require additional features:

Longer trailers (6m and over, excluding drawbar, but not boat trailers) require additional features:

Arranging lights and reflectors:

• Indicators should be a minimum of 350mm and a maximum of 1500mm from the ground.
• Indicators should be a maximum of 400mm from the side of the trailer.
• Indicators should be a minimum of 400mm apart.
• Reflective rear triangles should be positioned, a minimum of 250mm and a maximum of 900mm from the ground, a minimum separation of 600mm and no more than 400mm from the sides.
• A single fog lamp should be mounted on the rear of the trailer, anywhere between the centre point and offside edge.
• Two fog lamps must be separated and positioned at opposite sides of the rear of the trailer.
• Fog lamps should be a minimum of 250mm and a maximum of 1000mm from the ground.
• Fog lamps should be a minimum of 400mm apart.
• There should be at least of 100mm of separation between the fog lights and the tail lights/indicators.

The rental yard is busy. The utility trailer you want is inexpensive, and the yard help assists you in hooking up the trailer hitch and safety chains. Now to drive home and load the trailer with the entirety of a teenager’s life, then ship her off to college several states away.

But not until you hook up the lights on the trailer. The trailer dealership won’t let you leave until the lights work. And the trailer plug on the back of your truck doesn’t remotely match the one on the trailer. The friendly trailer-hitch counterman points to the display of electrical connectors and suggests that if you can’t get them working on your own, he can have a mechanic do it for you, first thing Monday morning.

Desperate Times, Desperate Measures
Trailer wiring may be easy. Many vehicles, particularly pickups and SUVs, come prewired for trailers. If there’s a preinstalled hitch, the connector might already be in place behind a convenient spring-loaded cap. Or, if you’ve bought a new vehicle, you may find the harness needed to install the connector inside the glovebox. Simply unplug the connector to the rear lights on your truck, plug in the trailer harness between the male and female ends, and you’re ready.

But wiring a trailer may not be easy. If you’re towing just for the day and don’t ever plan to tow again, you can pick up some adapters that replace the taillight bulbs in your vehicle and have pigtailed wires hanging from them. How you route the wires out of the light sockets and down to the hitch can be problematic. And what if you have a trailer with simple 1-lamp lights and a car with multiple taillights, like most European cars? At the very least, odds are the trailer and vehicle use different connectors, as the trailering industry uses at least four different styles.

Red To Green To White

Let’s start with the basics. Most trailers have three circuits–for running lights, and left and right brake lights. The brake lights will flash for the turn-signal function–it’s up to the flasher relay in the car to know when the brake lights are on and to flash the appropriate filament. So technically, you need only three wires to the trailer plus a ground wire.

Always run a ground wire from the frame of the vehicle (not from the hitch or bumper) to the trailer. The metal-to-metal contact in the hitch is not reliable enough for a consistent connection.

This means you’ll need a connector with at least four contacts. Not surprisingly, the standard flat-style connector for trailers uses four pins, with one unshrouded male pin on the vehicle end for the ground. Other common styles of connectors use five, six or seven pins, and these are the styles you’ll see installed by the manufacturers on new vehicles. Why more than four? Larger trailers sometimes use a separate circuit for running lights on the sides and front of the trailer. Some trailers use electrically actuated brakes. It’s not uncommon for enclosed trailers to have interior lights or accessories powered by the vehicle battery, so that means another circuit. Do not freak. As long as you concern yourself with only one circuit at a time, this job will not require you to read those schematic diagrams that have tiny little wires labeled with type too small for even a lawyer to read.

Got a 7-pin connector on the truck and an inline 4-pin connector on the trailer? We found an adapter at the auto parts store that makes this plug-and-play. Other permutations of connectors may be adaptable with off-the-shelf parts.

Worst case–you have a car or truck with no provision for trailer lights, and a trailer with a harness that ends with chopped-off wires. You’ll need nothing more complicated than a 12-volt test light or volt-meter, and some splicing capability. Start by turning on the vehicle’s running lamps. Now probe the vehicle wiring to find out which wire leading to the taillights is hot. Splice into this wire and bring it to the vehicle side of the trailer connector. Which pin? The connector should come with a list specifying which pin handles which circuit. Follow it–but don’t for a minute trust that someone else who wired your vehicle before you followed anything except his own whim. Tag each wire with its function as you identify it. Now you can turn off the running lamps and activate the left turn signal. Identify which circuit is the blinker, and repeat for the right turn signal. Wire all to the connector.

RV LED Lights

It’s pitch black outside and you need to conserve as much power as possible so you can stay out in the woods dry camping longer; it’s only 7 pm though and bed times no longer matter.

You switch on your RV lights and proceed to enjoy the great outdoors.

1. LED lights use less power to shine and produce more light.

12 volt incandescent bulbs consume 1.6 amps of power. A 12v RV LED light consumes 0.12 amps.If your RV has 24 incandescent bulbs that need 1.6 amps of power each, you will be using 38.4 amps to power them all. If all your bulbs were LED, you would only need 2.8 amps to power all 24 bulbs.

Dodge tow vehicles with the Factory Tow Package Brake Connector Dodge vehicles with this package are equipped with a 15 amp fused power line which is insufficient for 3 & 4 axle (6 or 8 brake) applications. For 3 & 4 axle (6 or 8 brake) applications a separate power wire should be installed directly from the battery positive to the brake control and should include an inline 30 amp auto reset circuit breaker.

Ford tow vehicles with the Factory Tow Package Brake Connector Ford vehicles with this package are equipped with a 20 amp fused power line which is insufficient for 3 & 4 axle (6 or 8 brake) applications. For 3 & 4 axle (6 or 8 brake) applications a separate power wire should be installed directly from the battery positive to the brake control and should include an inline 30 amp auto reset circuit breaker.

Notify Me

Be the first to know !

You’ll be notified as soon as this item is back IN STOCK

Please enter a valid email

UnSubscribe

Special Notification:

You’ll be notified as soon as this item is on special

Please enter a valid email

UnSubscribe

Please enter a valid email

Bulk Pricing:

Regular bulb failures in your trailer lights are not only annoying to replace, but also quite dangerous to your fellow road users. Switching to LED alternatives solves this problem completely, since LEDs are not at all affected by shock and vibration, last over 50,000 hours, use a fraction of the power, and are actually more visible to other road users. These trailer lights are an “all-in-one” solution measuring only 100mm wide by 90mm high, and providing all the legal illumination needs of a caravan, boat trailer, camping trailer etc., being; Stop, Tail, Turn and number plate illumination, along with a red reflector panel. The housing is submersible which is perfect for boat trailers, and each light is fitted with a connector to make wiring easier. The complete kit includes 2x trailer lights, with a pre-made 7m trailer cable with 7pin flat trailer connector.
Sold as a pair.

Many modern cars have Multiplexed or Can bus wiring systems. Wiring systems such as Can bus were developed in order to be a simpler and more cost effective alternative to the traditional wiring harness.

This advancement in wiring has also led to the development of various fault finding and diagnostic systems. One fault finding system sends small test pulses to all of the lights in the system to identify faulty bulbs. If a faulty bulb is detected the diagnostic system will display a symbol on the dashboard, alerting you to the fault.

This system can cause problems when trying to fit a tow bar, LED lights, or even when towing a trailer that has been fitted with LED trailer lights. Unlike regular bulbs, LED lights are so sensitive they will pick up the small test pulses through Can bus and flash constantly. As well as constant flashing, LED lights do not have sufficient ‘Load’ enough to trigger the diagnostic system and dashboard warning symbol.

This can be avoided by fitting an expensive manufacturer approved plug-in wiring kit when your tow bar is installed. This should resolve any problems with the diagnostic or fault finding systems, but it won’t be cheap.

Fitting this Universal Bypass Relay to your car between the wiring system and trailer plug socket bypasses all of the fault finding and diagnostic systems created by Can bus. This basically leaves you with a standard 12v power supply. This allows LED lights to function properly, without flashing, at a the fraction of the cost of a dedicated plug-in kit.

This relay isolates the 12v power supply and bypasses any fault finding or diagnostic systems after the relay. A fuse should be fitted to the relay’s power supply so in the event of a fault or short circuit your car remains unaffected. Please be aware that no system on your car will alert you to faults with your trailer’s lights while using this relay.

On the left hand side of the relay are 8 colored cables, a 12v power terminal, and another terminal marked TT. The 12v power terminal requires a dedicated feed (with fuse) directly from the cars battery, while the terminal marked TT is for an optional ‘tell tale’ dashboard light.

The 8 colored cables have the following functions:
Yellow – Left Indicator
Blue – Fog Light
White- Earth
Green – Right Indicator
Brown – Right Tail / Side Light
Red – Brake / Stop Lights
Black – Left Tail / Side Light
Gray – Reverse Light
On the right hand side are 7 terminals marked 1, 2, 4, 5, 6, 7 and ‘R’. There is no terminal 3 as this is an earth wire which must be secured to the vehicles bodywork.

These correspond to the pins in the trailer plug socket and are wired as shown below.
Click the image to enlarge it.