Troubleshooting Brake Controller that Only Works with Override Lever 2008 GMC Sierra  

I have a P3 controller I hooked up in my 2008 GMC Sierra. Everything has worked fine during the first season until my last tow. The brakes will not activate when the pedal is pressed, but the manual override works. I also noticed the cruise control only works intermittently. The light on the steering wheel always works when powering on the cruise, but it does not always engage. Once set, it remains engaged until I brake or turn it off, it just doesnt always engage. I read many forums suggesting to change the stop light switch at the top of the brake pedal. I bought a new one from the dealer and installed it - all symptoms still exist. I noticed that when the pedal is not pressed, and the ignition is on, there is about 0.3v coming out of the stop light wire on the truck end light blue wire w/white stripe, even at the terminal of that switch at the brake pedal. The voltage goes away when the ignition is switched off. Unsure if thats normal or not. Upon pressing the pedal, the voltage does not climb, in fact, it s to about 0.2v with ignition switched on nothing with ignition switched off All fuses, including the CHMSL are good although upon initial diagnosis I found the CHMSL blown, no idea for how long. I looked under the door seal trim plates leading toward the back of the cab on the drivers side and didnt see any obvious worn, bare or pinched wiring. Could it be likely that the stop light switch I bought was defective, or does this sound like something else? Im at my wits end with this!

asked by: Ryan H