Why Would Trailer Wiring Stop Working When Headlights Come On

Question:

Turn signals and running lights work on trailer. When truck lights are on the brake lights do not work. Any suggestions

asked by: R B

Expert Reply:

Try improving the connection of the main ground wire of the trailer wiring harness on the trailer. 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. The issue is that when your headlights are on the maximum amount of amperage draw is put on the setup which if there's an insufficient ground this would be when this would show up. This is a very common issue too so we are very familiar with it. I attached a couple trailer wiring help articles for you to check out as well.

expert reply by: Jameson C