This 2007 Chevrolet Tahoe came in with the complaint that the cruise control had been not working at times but now the situation had changed to not working at all. With or without a scan tool this is a very easy diagnosis. Checking the engine codes there was a code P0573 00 for a fault with the brake light switch circuit 1. The same basic test can be performed by checking to see if the stoplights are on with the engine running.
Going into engine data there is a data line that reads “Cruise Control Data”.
Within the cruise control data there will be a couple of lines starting with BPP which is short for Brake Pedal Position.
Note that BPP Circuit Signal reads applied with the ignition on and engine off.
BPP Signal also reads applied with the ignition on and the engine running.
The brake pedal is not depressed.
I removed the duct work shown in the next picture so that I could get better pictures for this article. I am holding the push pin retainer that secures it to the dash. It does not have to be removed to change the stoplight switch but it does make it easier to see.
The stoplight switch. Note that this vehicle has adjustable foot pedals.
The harness connector has a lever latch that has to be depressed in order to pull the harness loose from the switch.
The spring retainer has a latch that has to be lifted before the clip can be slid up and off of the shaft. It is pretty easy to get off but a bit of a pain to re install. I had to use a long narrow screwdriver (not shown) to depress the inner upper edge while at the same time using the small screwdriver (shown) to lift the outer clip edge (wedged) and my other hand to push the retainer down into the slot. Both hands involved and my head in the way so getting a picture was not possible. I should mention it was also 97°F and 85% humidity while doing this.
The switch can then be pulled down and off of the shaft.
The next picture shows the side of the switch that is visible while it is installed.
This shows the hidden side of the switch away from the retaining clip. Note that there is a slot in the plastic. This allows the switch to be removed without having to fully pull the master cylinder push rod off of the brake pedal shaft.
The stoplight switch has two switches built into it. One Normally Open (NO) and the other Normally Closed (NC).
The NO portion is supplied power on the Pink wire, terminal “A” with the ignition on only. The stoplights do not work unless the ignition is on. When the switch is activated by depressing the brake pedal, power leaves the switch on the Light Blue/White wire where it goes to the underhood fuse box and is then split and sent to the stoplights and to the ECM.
The NC side of the switch is supplied power on the Orange/White wire, terminal “C” from the BCM. It flows through the NC contacts when the brake pedal is at rest. It flows out of the switch on the Purple wire, terminal “D” back to the BCM. When the brake pedal is depressed the NC contacts open and break the circuit to the BCM. The BCM then transmits that information on High Speed GMLAN serial data bus lines to other modules affecting various functions.
Strangle enough today as I was putting this article together another Tahoe came in with this story. The customer was pulled over by a police officer and asked if he was riding with his foot on the brake pedal because the Tahoe’s stoplights had been on for quite a distance at highway speed. The owner informed the officer that he had not had his foot on the brake pedal for some time. The engine had been left running to keep the a/c working and they looked at the stoplights together and sure enough the stoplights were on and there was no one in the driver’s seat. The brake pedal was depressed a few times and the lights rechecked. They went out. The owner was given a warning and advised to have the vehicle fixed asap. Even though the stoplights were known to have been stuck on by at least two people there was no code P0573 stored. Instead there was a code C0161 00 stored in the EBCM. The customer had known that the ABS light had been on and there was also a message to check the Stabilitrak system. My assumption would be if the C0161 00 sets it prevents/blocks the systems from checking for conditions relevant to a P0573 00.