Troubleshooting Trailer Wiring that Stops Working When Headlights Are On

Question:

I have rewired a 4 trailer. I have retraced all wires: brown is tail and running lights, yellow is left, green is right. Everything works perfect except when the running lights are on, when you step on the brake the taillights go out. When the running lights are off everything works correctly. I put new taillights in the trailer, is it possible that I hooked them up backwards?

asked by: Dylan B

Expert Reply:

Try improving the connection of the main ground wire of the trailer wiring harness. Make sure it is attached to a clean metal surface. Or you can run all of the trailer wires back to the main ground. This is actually the ideal solution. But you can also check the current grounds for the lights to make sure they are making a solid connection to a clean metal surface.

What you described is the classic insufficient ground scenario so finding a way to improve the grounds will fix the problem.

expert reply by: Jameson C