Troubleshooting Trailer Wiring that Stops Working When Headlights are On

Updated 05/20/2019 | Published 05/18/2019

Question:

The harness works perfectly when the lights on the vehicle are off, but when I turn the lights on the only thing that works is the running lights on the trailer. When I brake all trailer lights cut off and when I give either a left or right signal both signal lights on the trailer flash as if my hazards were on. I installed the harness myself and went strictly by the installation video. Ive went back through the system several times and dont see any problem. Im not sure if there might be a solution to it whether this may be a common problem or not, or if the module might be faulty.

asked by: Calvin M

Expert Reply:

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

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.

I attached a link to a page with more trailer wiring troubleshooting info as well.

expert reply by: Jameson C