Brake Controller Installation on 2020 GMC Sierra 4500 Medium Duty Truck  

Question:

Expert Reply:

I spoke with several of our brake controller manufacturers, none of which had any specific information regarding how the 2020 GMC Sierra 4500 medium duty trucks are wired for a brake controller. A couple of them told me that there are bare wires or a plug under the dash, but none of them offer a harness or could tell me where exactly the wires were located. No worries though, we can figure this out.

The connector you show in the photo appears to have 4 pins. I'd recommend testing the connector with a # PTW2993 circuit tester or multimeter to determine if it's indeed the connector for a brake controller. If the connector you located is confirmed as being for the brake controller, you can simply remove the connector and connect the wires feeding it directly to the pigtail coming off your brake controller.

With the ignition on and the circuit tester alligator clip attached to a good ground test each pin. The wire that shows a constant 12V is the power wire that connects to the black wire on the brake controller pigtail. Next, test the remaining pins while pressing and releasing the brake pedal. The socket that makes the tester blink along with the brake pedal is the brake stop lamp switch wire, and would connect to the red brake controller pigtail wire. Of the two remaining pins, one is the ground, the other is the output circuit back to the trailer connector. Test both remaining pins for continuity with the brake output circuit pin in the 5:00 position in the 7-way trailer connector at the rear bumper, the wire feeding the pin that shows continuity would attach to the blue pigtail wire and the wire feeding the remaining socket would be the ground which connects to the white pigtail wire.

expert reply by: Mike L

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