Troubleshooting Brake and Running Lights Not Working on Trailer But Blinkers Work

Question:

I have a 4 way connector on a 2012 GMC. The lights worked before. We rewired a horse trailer so the wiring is new. The Running lights work. Tail lights do not work. Blinkers work. Tail lights blink when brake is applied. Harness is correct and I have tested the circuits. Im stumped. Tail lights blink when brakes applied.

asked by: Jamie

Expert Reply:

What you described sounds like a classic ground problem. You need to improve the quality of the grounds on your trailer and the brake and taillights will start working again.

When the running lights and/or brake lights are on the amperage draw of the trailer is much higher than just a blinker circuit so a weak ground will cause problems at this point.

Make sure that the trailer side connector are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces. The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

expert reply by: Jameson C