How To Test Trailer Lights With A Multimeter

Trailer Light Testing 101: How To Test Them With A Multimeter

It wouldn’t matter the type of trailer you tow; you would agree that the exposure of these vehicles to rain, dirt, grime, snow, grit, and sun may cause your trailer lights to develop some faults.

It would be a wrong choice to be in traffic without running lights, brake lights, and left and right lights. Apart from the risking accidents, you would be subject to some fining.

To test your trailer lights, you would need a multimeter. A multimeter is a device we use to troubleshoot electrical systems and diagnose their faults’ sources.

But how do you test trailer lights with a multimeter? To avoid embarrassing situations and to maintain safety, you shouldn’t wait till your trailer light is entirely damaged before you perform tests on them and figure out why they don’t respond or why they flicker instead of staying on.

Keep reading! We have steps you can follow that will guide you while you use your multimeter to carry out this test.

How To Test Trailer Lights With A Multimeter?

For this test, we use a Fluke 87-V multimeter, which we buy on

which fluke multimeter should you buy

Check the necessary things

Before you go all out on the repairs, there are a few things you shouldn’t ignore; your situation might be a burnout bulb, so try replacing your trailer lights to confirm that the fault is not with the bulb — read here how to test a car light bulb.

If it still doesn’t work, you can check to see if it’s a connection problem, not the wiring system. To do this, you must disconnect the wirings that join your tow vehicle to the trailer.

Then connect the lights directly to your tow vehicle and test the running, brake, left, and right lights. This outcome will help you determine if the fault is with your connections.

Test for grounding

The first thing you should do would be to disconnect your trailer plugs. Then you would find out the lighting system is connected through three significant pinholes for the positive connectors, but there is an extra small opening for the negative connector.

Most of the faults with trailer lights are weak ground connections.

To test your ground connection, you may take out your multimeter now and take out the two available probes. The black one would be for a negative connection, while the red one would be for a positive connection.

Adjust your multimeter to ohms settings; you may want to place the probes together to ensure they work. Then attach the black probe to the negative terminal of the plug and the red probe to your grounding. For adequate grounding, your multimeter should read about 0.3 ohms.

Test your trailer plugs

If you have confirmed the fault is not an inadequate grounding, next you have to test the trailer plug to be sure if it is receiving voltage.

Study your connecter to know the wires for each light. Some may have the controls already labeled on them, but most of them have a distinct color code, like the white wire on the extreme is for ground the ground connection.

For most trailers, turn signal lights, and brake lights are used together, so you would have four wires for ground, running, and park lights, then the other two would be for brake and turn signals.

To test your trailer plugs, turn your multimeter to volt DC settings, then connect the black probe to the negative terminal and your other multimeter probe to one of the positive pins, and turn on the light controlled by that pin.  ​

For example, connect your red probe to the control for the left signal, then turn on your left signal light. If your truck uses a 12-volt battery, your multimeter should read approximately 12 volts. If this is correct, the faults are not with your trailer plugs.

Test your lighting connector

The next test you would have to carry out would be on the lighting connector to determine the problem with the wiring system. To do this, you have to test the resistivity of the system.

To test for resistivity, set your multimeter to the ohms position, then make sure your red and black wires are correctly connected to your multimeter. Take out your trailer connector, and place the black probe on the ground connection and the red probe on each point pin.

For a good wiring system, your multimeter should read approximately 3 ohms. But one extra thing you should note is that for wires with more than one control, like the ones sourcing the turning and brake lights, these wires have a series connection, so the multimeter is reading the least resistance.

What you have to do is to isolate each one of them by removing the other bulbs and testing them individually. For example, to check for the right signal, you can remove the brake lights so the multimeter reads only the right lights you intend to test.


Simple, isn’t it? Testing for wiring problems might not be difficult, but the problem we face is how to begin; we have explained with detailed information how you can carry them out.

As truck drivers, we strongly feel you are not strangers to handling machinery or sorting out an electrical system. Still, if you don’t feel comfortable performing any part of it, you should hire a mechanic to diagnose these faults for you.

We would conclude this article with life-extending advice that immediately you find fault with your trailer lights, you should fix it quickly.

Frequently Asked Questions

How to test a trailer plug with the multimeter?

When you are having problems with your trailer light and want to check if you don’t have a faulty trailer plug, there are two things to do before the test.

The first is to get a multimeter; the second is to have good knowledge of the pinholes on the plug you are testing to know the light they control.

Some wires have their controls labeled on them, but color codes should work for you if you have a plug without labeled wires. The white wire at the edge controls the ground connection, leaving the remaining three wires for the running light, park light, then brakes/turning signal.

Set your multimeter to read volt DC, and connect the black probe to the negative and the red probe to one of the control wires. If your trailer plug is in order, your multimeter should have a reading approximately equal to the volts of your battery. Now apply the same process to the other control pinholes.

How to test trailer lights without a vehicle?

Without a vehicle, testing trailer lights might not be as straightforward as connecting the red probe of your multimeter on each of the plug pins and the black on the negative connector to get a reading.

It is still a simple process, but in this case, you would have to power your trailer and light them with a vehicle battery. So disconnect the trailer plugs, and use the wires on the pin to help you identify the specific pinholes you want to test.

For a trailer light test without a vehicle, you would need wires to attach your plugs to the battery. So connect the negative pinhole to the negative terminal on your battery and one of the pinholes to the positive terminal on your battery. The lights attached to the pinhole should turn on, then repeat the same process with the other control pinholes.

How to test a brake light switch with the multimeter?

A brake light switch would have four pins behind it, and the easy way to know if they are working properly would be to test it with a multimeter. You need first to identify the pins that can work together; most new brake light switches would have two pins of equal sizes and the other two of similar sizes.

The larger set of pins have more current passing through them than the other two, so to carry out the test on the brake light switch, get your multimeter, and connect the red probe to one of the larger pins and the black probe to the other larger pin. Do the same to the other set of pins, and then press the switch.

When you connect the probes with the larger pins, your multimeter should not have a reading until you press the switch button, but when you attach them to the other two pins with lower current passing through them, your device should not have a reading when the button is pressed. If this is correct, then your brake light switch is working correctly.

How to troubleshoot trailer lights?

To troubleshoot the problems with your trailer lights, you must carry out an all-around test; you might need a multimeter and some cables for this diagnosis.

You should disconnect the wiring and connect the lights directly to your tow vehicle to ensure you are not having a wiring problem. Test each light to know the ones that are not working right, then be sure you do not have a burnout bulb.

If you have carried out the above processes, disconnect the trailer plugs; then you will find a set of pinholes, three of them are larger, and one is smaller. Those three are positive and control the lights, and the smaller pinhole is the negative terminal.

Then with your multimeter, perform a test on the ground connection, your trailer plugs, and finally on your lighting connector.

How to fix trailer lights?

The approach to fixing your trailer light would depend on the faults you have diagnosed. If your diagnoses reveal that you have a faulty wire or a burnout bulb, we recommend replacing them.

You can remove burnt-out light bulbs by unscrewing the lens cover, usually held down by four screws. Push down the bulb and rotate it clockwise to remove it. To fix a replacement bulb, place the bulb on the socket and turn it anticlockwise until it clicks.

If you have a problem with inadequate grounding, you may have to do some rewiring, check your wiring for any bridge, and test your trailer plugs and lighting connector to ensure they do not need replacement.

How to connect trailer lights?

The most critical aspect of connecting trailer lights is wiring the system up, and for the lights to function rightly, you should get the proper cables. Many trailer connector wires come with the controls branded on the wire, but if yours is not labeled, you need to know the right wire arrangement.

Your vehicle would have a female connector to receive the trailer plugs; the trailer plug is also called the male connector. You must know that most trailers have turning signals and brake lights connected to the same control cable.

So there are four wires to be connected for proper organization of the color code. The white wire would be the ground wire and brown for the tail or running lights. Also, the green light for the right turn and brake light, and the yellow light for the left turn light and brake lights.

1 thought on “Trailer Light Testing 101: How To Test Them With A Multimeter”

  1. Gilbert Sollars

    Excellent help. Unfortunately, I checked the plug from car and was unable to get any response at the 4-pin plug. I have a good ground. But none of the other wires worked. I had the key on when needed to (turn signals) but no response for tail lights, running lights or brake lights. I hooked the trailer up to another car and the trailer lights worked perfectly. I checked the 3 fuses related to the trailer; all looked fine. Do trailer harnesses have fuses? Could the harness be bad? The system worked perfectly the first time I tested it a few days ago. I CANNOT find the problem! Any suggestions? Thanks ahead of time for the help.

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