Brake Controller Does Not Activate Trailer Brakes with Brake Pedal on 2003 GMC Sierra 3500  

Question:

Expert Reply:

There are indeed 2 different brake controller wiring harnesses used between the 2002 and 2003 GMC Sierra. A 2002 would use # 3025-P. The 2003, like you have, uses a # 3015-P. The difference between these 2 harnesses is the way they are pinned out.

On # 3025-P the red wire is located next to the white ground wire, and below the black power wire.

On # 3015-P the red wire is located next to the blue wire.

You will not necessarily need a completely different harness if yours is pinned out wrong. You will just need to move the location of the red wire. The red wire is input from the brake switch that tells the brake controller when you are stepping on the brake.

If you use a circuit tester like # PTW2993 on the red wire and you do not get any signal on the red wire, then it is either in the wrong place or the stop light fuse is blown. These are the 2 most common ailments when the red wire is not functioning as it should.

If you find that the fuse is good and there is power on the red wire then there is something wrong inside the brake controller and it will need to be replaced. Since the same issue was happening on both controllers, it seems more likely it is a blown fuse, or the red wire is in the wrong place.

On a 2003 model, if you are looking at the second port from the left in the top row where the brake controller harness plugs in, the top left pin is output to the trailer brakes. The top right pin is input from the brake switch. The center left pin is 12 volt power. The bottom right pin is ground.

expert reply by: Michael H

0

click to enlarge