Troubleshooting Running Lights on a Trailer Not Working Even After Replacing with New Lights

Question:

Lights on trailer the running lights wont work but others do. We put on new lights same thing.

asked by: Shirley S

Expert Reply:

This sounds like either a problem with the wiring or a bad ground. First, check the vehicle side connector for functionality. You will be checking for power and for a signal when your vehicle lights are on. You will need a circuit tester like # PTW2993. If you find that the connector is faulty, you will need a new harness. The brown wire is for running lights, the green is for right turn, yellow is left turn, and white is the ground. If you do not find a problem, the issue is most likely with the trailer wiring.

The first, and easiest, part to check is the ground at the taillights. On trailers, the ground is usually through the mounting studs. If there is any dirt or corrosion, it could cause a bad ground. You will also need to check the ground from the lights to the connector. I have included a video on the procedure for testing a ground below.

If the grounds check out, you will have to test the wiring at various points from the plug all the way back to the lights to make sure the lights are receiving power.

If your trailer has a lot of lights on it (marker lights on the side for example) the power draw might be too much for the wiring to handle. If this is the case, you would need a powered converter that connects directly to the battery to power the lights.

expert reply by: Michael H