2009 GMC Yukon XL SLE 5.3L VIN 3 Cruise Cancels When Set

Hello, I hope someone has had the same issues as I am having with this 2009 GMC Yukon XL K1500 SLE 5.3L VIN 3. I have a code for P0573 - Brake Switch Low Voltage/Open in the PCM. When you activate cruise the light flashed on when set and goes out immediately. When I first went into the body control module and found the brake switch to be working as designed. When I go into the transmission control module and the PCM the brake switch says applied. My owner had insisted on trying a brake switch first which didn't fix the issue. When I measure the voltage on the output of the switch (Light Blue w/white tracer wire) I measure 2.5Volts. From talking with other techs and people it should be 0 volts. When I remove the brake switch signal input from the BCM I get 9Volts. I have eliminated the underhood fuse panel by depinning the connectors and tieing the wires together because on the diagrams it operates as only a pass-through. If I have all the modules disconnected to the brake switch signal wire I still have 9 Volts, and the PIDS still stay applied and intermittently released. The vehicle is equipped with an aftermarket trailer brake controller but was disconnected at the start to eliminate possible aftermarket issues. I think I have a short to power somewhere but just wanted to see if anyone thinks I missed anything. 

I need to see the diagram of the brake switch circuit to help and I'm not by my computer. Any chance of posting the diagram?
It sounds like the switch signals the BCM and then gets shared across the network?
If so, what is the circuit design of the brake switch? The side that doesn't go to to computer, does it go to a power or ground?

Thanks for providing the YMME details. Makes looking up service info a breeze.

After reading up on P0573, I tend to think you're not missing anything. :silly: It sure sounds like you're chasing a resistive short to voltage on circuit 6311 light blue/white. If you use an incandescent test light connected to B- and touch it on the light blue/white, does it light up? Or does it pull the voltage down, and does the ECM PID show released?

Since you've already bypassed the underhood fuse block, you're down to something in the exterior light circuit, the wire to the ECM or the ECM itself. Is P3 of X2 at the underhood fuse block accessible?

 

It might be useful to separate the two wires at that pin and measure voltage individually. The one that has 9V on it is your suspect. From there, you can ohm test at the ECM on the same wire to determine which light blue/white is which.

When I measure the voltage on the output of the switch (Light Blue w/white tracer wire) I measure 2.5Volts. From talking with other techs and people it should be 0 volts. When I remove the brake switch signal input from the BCM I get 9Volts.

Should be battery voltage on the Blu/wht wire, with the pedal depress. What do you have on the pink wire? (powered by 10 amp MISC IGN FUSE 43) 

 

 


EDIT: My apologies. I, now, realize that you are testing with the brake pedal released.


 

If I have all the modules disconnected to the brake switch signal wire I still have 9 Volts, and the PIDS still stay applied and intermittently released.

I agree with Tyler.

It sure sounds like you're chasing a resistive short to voltage on circuit 6311 light blue/white. If you use an incandescent test light connected to B- and touch it on the light blue/white, does it light up? Or does it pull the voltage down, and does the ECM PID show released?