Troubleshooting Trailer Taillights that Stop Working When Running Lights Come On

Question:

I have signal coming out of my plug on my truck but no tail lights. When I turn on the tail lights my turn and brake lights stop working. Have a good ground at the front and I believe a good ground on the lights themself on the trailer. Any help would be appreciated

asked by: Larry

Expert Reply:

Sounds like your trailer lights have weak grounds. The behavior you described is textbook insufficient grounding. What you will want to do is check the grounds for the trailer taillights, make sure they are attached to a clean metal surface. Also, make sure the main ground from the trailer's wiring connector is attached to a clean metal surface on your trailer frame. I know you said your grounds looked good at the assemblies but you might run a jumper wire from the ground there back to the main ground of the harness.

I attached an FAQ article on diagnosing 4-way installs for you to check out plus an FAQ article on trailer wiring.

expert reply by: Jameson C