How To Test Trailer Lights With A Multimeter?


It wouldn’t matter the type of trailer you tow; you would agree with me 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, 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 to diagnose the sources of their faults.

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 some steps you can follow that would guide you while you use your multimeter to carry out this test.

How To Test Trailer Lights With A Multimeter?

1.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 lightbulbIf it still doesn’t work, you can also check to be sure if it’s a connection problem and not the wiring system, to do this you would have to 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 the right light. The outcome of this will help you determine if the fault is with your connections.

2. 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 what to take out your multimeter now, take out the two available probes, the black one would be for negative connection while the red one for 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 on your grounding. For adequate grounding, your multimeter should read about 0.3 ohms.

3. 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 it, 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 light, then the other two would be for brake and turning signal. 

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 that is 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, then the faults are not with your trailer plugs.

4.Test your lighting connector. 

The next test you would have to carry out would be to 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. Now take out your trailer connector, place the black probe on the ground connection and the red probe on each of the point pins.

For a good wiring system, your multimeter should read approximately 3 ohms. But one extra thing you should note is that for wires that have 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 test 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.

Here’s the part 2 of the useful video:


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 on how you can carry them out.

As a truck driver, we have a strong feeling that you are not a stranger to handling machinery or sorting out an electrical system, but if in any way you don’t feel comfortable to perform any part of it, then 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.

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