Troubleshooting Trailer Wiring With Wire Colors Not Being Same On All Wiring

Published 01/06/2023

Question:

I have a car hauler and I just completely rewired the trailer. The tail lights, brake/blinkers all work like they should. I was testing the lights at the junction box with a car battery. Then I tested for the electric brake wire and it arched. Online I read the blue wire is for brakes. So I hooked the brake wire to the blue wire but like I said it arched when I tested it. In the junction box the blue post says aux. The red post says stop and the black post says clear. On my gm truck Im not sure if the brakes are blue wire or not. Hopefully you can help. Thanks

asked by: Rick

Expert Reply:

I always recommend going of the position of the terminals and their function versus trusting the wire colors. Since wire colors is not a requirement to be followed by every manufacturer they can use which ever color wires they want. If you are combining products from different manufacturers you will need to use their wiring guide to determine each wires function and not rely on the color or the wiring. I have included an image showing the functions for the 7-way connectors below. Remember to check the position and not the color of the wire. If you have a 7-way harness and not sure of what color wires control each function you can connect the end of the open wire to your battery and touch the terminals with a Test Light # PTW2993 to determine where each wire goes.

expert reply by: Bud M