Trailer Brake and Turn Signals Not Working when 2012 GMC Sierra Running Lights are On

Question:

I have a 2012 GMC Sierra 1500. When pulling a trailer during the daytime the signal and brake lights work on the trailer When its dark the marker lights turn on but the signals and brake lights dont work

asked by: Stan

Expert Reply:

This sounds like a ground or a short. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light but can show up any time.

Make sure that the vehicle side and trailer side connectors 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: Michael H