Trailer Lights Malfunction When Vehicle Headlights are Switched On

Question:

My trailer lights work properly until I turn on the headlights/running lights, then when I hit brakes just the right goes blank ? I discovered this also happens at the connection at the truck

asked by: Jack

Expert Reply:

What you describe is a classic symptom of a poor/insufficient ground connection. Everything works fine until you turn on the headlights, which draws maximum amperage through the system, which is when the problems shows up.

First check the main trailer ground, which is usually located on the trailer tongue. Trace the white wire from the trailer connector, it should be grounded to the trailer via a bolt or screw. Make sure this connection is secure and not corroded.

Each lamp on the trailer will ground one of two ways, either through its mounting hardware being in direct contact with the trailer frame (or to metal that's in direct contact with the frame) or via a separate ground wire attached to the frame.

You can improve the ground connections by sanding a dime sized area around them down to bare metal and installing a star washer between the ground connection's ring terminal and the trailer frame. When the ground screw/bolt is tightened, the star washer will bite into the metal for a bulletproof connection. Protect the ground connection with a quick shot of rust inhibiting spray paint.

I've linked to some articles about trailer wring that you might find helpful.

expert reply by: Mike L