Trailer Taillights Act Up When Vehicle Headlights are Turned On

Question:

my led trailer lights are really baffling me. The tail lamp of the tail lights isnt illuminating but the turn signals and brake lights work. When I turn on the headlights on the tow vehicle the turn and brake lights stop working on the trailer. The license plate light is tied into the supply for the tail lamps wire and it works just fine. How do I fix this?

asked by: Carl

Expert Reply:

It sounds as if you're experiencing a classic symptom of a poor ground connection. A ground connection problem can make a taillight system do all sorts of things, but the dead giveaway in your case is that the problems get worse when the vehicle headlights are turned on which is when the maximum amperage draw occurs.

Locate the white wire that extends rearward toward the rear of the trailer from the connector. This wire will ground to the trailer frame, usually on the trailer tongue and is your main trailer ground. Make sure this connection is secure and he metal around the connection and the screw or bolt connecting it to the trailer frame isn't loose or corroded. You can improve this connection by sanding a small area around the ground connection down to bare metal and installing a star washer between the ground connection's ring terminal and the trailer frame. When the ground connection's screw or bolt is tightened, the star washer will bite into the trailer frame providing a secure and reliable ground connection.

Each trailer lamp will ground either via a separate ground wire ran to the trailer frame or metal that's in direct contact with the trailer frame. If there is no separate ground wire, the lamp's mounting hardware provides the ground connection. Check these connections to make sure they're secure.

The trailer connector on the tow vehicle will also have its own ground connection, make sure that connection is made to the vehicle's frame. I'll link to some helpful articles about troubleshooting trailer wiring problems.

expert reply by: Mike L