I have a 2012 GMC Yukon with a trailer brake controller that has worked fine for the past 5 years. Now all of a sudden, my trailer brakes won't operate. I tested the voltage between ground and the trailer brakes at the connection plug at the rear of the truck, and even with no braking applied, there is a constant voltage that pulses between 3 and 3.5 volts. Not sure what this means, but if anyone can tell me what I should try next to diagnose this issue, it would be much appreciated. Trying to understand if their is an issue with the truck itself, or the trailer.

Update: This is a built-in GM brake controller. I will test the trailer brakes my applying 12V directly to the trailer plug. Won't get to it for a week or so. There is a manual lever. Neither the truck brakes or the manual lever are working.


  • Well what is the brake controller? Who makes it? Until you give us more info we have little to go on... is it electro-mechanical? Electro-hydraulic?
    – Solar Mike
    Jul 1, 2019 at 18:00
  • @SolarMike - It is most likely a built in GM trailer brake ... my new Silverado has one (which surprised me!). Jul 1, 2019 at 19:25

1 Answer 1


You can rule out the trailer by jacking up one wheel (that has brakes) and applying 12V directly to the disconnected trailer plug pins. If the brakes are functional, the wheel should lock solidly. I doubt it's the trailer, but it's a fairly easy check.

Does your controller have a manual lever? If so, does that work?

Testing the controller may likely require electrical load, so you might try running extension wires to the brake and ground pins to your multimeter in the cab. Drive, apply the trailer brakes (or service brakes) and then observe the voltage.

The earliest units had a mechanical pendulum which dragged across the wiper of a potentiometer. These could get out of adjustment, or the whole unit could get bumped and tilted making brake apply erratic.

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