Trailer Lights Turn Off When Headlights Come On

Question:

It’s a 4 wire everything works when hooked up but taillights when I turn on headlights nothing works anymore. Any thing I might check

asked by: David A

Expert Reply:

The first step is to check the ground on the 4-way harness on the vehicle and the trailer side. Make sure the ground is attached to a clean bare metal surface free of debris or paint. Usually when the trailer lighting doesn't work when the headlights are on there is a weak ground.

In some cases when the trailer lights stop functioning after the head lights are turned on the harness is overloaded due to excessive draw from lights on the trailer or a short in the wiring. It is a good idea to check wiring for frayed or damaged wire on the vehicle side and trailer side.

If you check the wiring and there is not any damaged wiring present you want to test the circuits on the 4-way with a circuit tester like part # PTW2979. Have someone run the functions while you test the 4-way. The taillight circuit is the normally the brown wire on the 4-way. If you are getting power to all of the circuits on the 4-way you know its the trailer's wiring that is the issue.

Please let me know if you are not able to pin point the issue and I can help you troubleshoot further.

expert reply by: Heather A