Trailer Brakes Lock Up when Connected to 2014 GMC Sierra 2500HD with OEM Brake Controller

Question:

I have a 2014 GMC Sierra 2500 HD with the towing package brake controller already installed in dash. Im trying to use a 1999 70pi BigTex trailer with electric brakes. When I plug the trailer connector into the trucks connector....the lights work but the brakes on the trailer are activated without depressing the brake pedal. If I disconnect the trailer then it rolls freely. Is it my truck or the trailer? Do the 5 wires under the dash that were left blunt by the factory need to be installed? Is there a harness plug for them? Can you give me any advice? Thank you!!

asked by: John Y

Expert Reply:

Your 2014 GMC Sierra 2500HD has an industry standard 7-Way trailer connector. It is possible that the one on your trailer is not pinned out to match. In other words it may have the 12 volt circuit switched with the brake output circuit and that would be why the brakes lock up when you plug in the trailer.

I have included a picture that shows the way a trailer side 7-Way should be pinned out. Ignore the wire colors though and focus just on the functions. There is no industry standard for wiring color codes.

It is also possible that the trailer connectors are dirty or corroded (less likely on the new truck). Make sure they are clean and free of corrosion inside and out.

If your trailer has a 6-Way round pin connector and you are using an adapter, the 12 volt circuit and brake output pins could be switched. This happens because there are 2 ways to wire a 6-Way. Using adapter # HM47570 will allow you to switch between the 2 styles as needed.

expert reply by: Michael H