Brake Switch to PCM - what's it for, and do I need it?
 

The last wire I have to identify and route on my 2006 Sierra 2500HD harness goes to Blue 33 on the PCM, and is called "Brake Sw Sig". According to the wiring diagrams, it looks like it goes from the brake pedal switch to the fuse box, and then the fuse box to the PCM. What is this for, and do I need it?

BTW, the TAC module has a separate (and very different) brake signal wire, which I think turns off the cruise control when the brake pedal is pressed. So, I don't think this has anything to do with the cruise control.

Any suggestions?! Do I need it?

One is for cruise the other is for torque converter locking/unlocking.

The PCM Brake switch signal AND the TAC Module Brake Switch signal BOTH MUST be connected properly for cruise control to function.

The PCM looks for 12v+ on the Brake Switch Signal wire when the pedal IS NOT PRESSED. When you hit the brakes, a switch OPENS and the 12v+ goes away. This signal is used to unlock torque converter when you press the brakes and is also used for cruise control.

The TAC MODULE is looking for 12v+ when brakes ARE PRESSED. The TAC Module also looks for the resistance of the brake light bulb to GROUND (when pedals is not pressed) So the TAC module brake switch signal should go on the wire that feeds to the brake lights.