Cannot understand from electrical schematic how start system works starting with pushing brake pedal. Need a verbal description.

 

BCM is looking for the car in park via the front of shifter assembly micro switch/ tcm on newer vets, the brake pedal pressed down to increase the ohm reading of the brake pedal sensor, the RCDLR sending the correct signal that a programmed fob is in the car, then will send the command to the ECM after the starter button push so the ECM starts the motor.
In a manual trans car, no shifter micro switch/TCM park switch, and uses the clutch pedal bottom switch, instead of the brake switch.
If a 05 manual trans, then throw the SCLM in the mix, and the it has to send the signal that the steering column is unlock when the starter button is pushed as well.
To add, there is a lot of module security hand shakes between the BCM and other modules going on, and some modules if replaced, will need to be relearned to the BCM with a Tech II so they are hand shaking correctly with the correct security key.

As for the brake pedal push, the brake pedal sensor is not like a adjust type sensor of older cars that is just a on/off type switch to turn the brake light on and off , but is a ohm type switch that has to be learned to the BCM of it's at-rest position initial ohm reading, then pressed down position next to start the car.

Hence if you replaced the brake pedal sensor, will need a tech II so the BCM can learn the new sensor ohm readings, both with your foot off the pedal ohm reading, then with it pushed all the way down to fully apply the brakes.

 

Am always impressed by Dano's knowledge. Will keep this one filed away with his past responses.

 

Many thanks, great reply. Is there a schematic that goes with your dialogue?

 

This official GM description gives some terminal info:

When the ignition mode switch is placed in the START position, a discrete input is sent to the body control module (BCM) notifying it that engine start has been requested. The BCM then verifies that the brake pedal has been depressed with a discrete input from the brake pedal position sensor and that the key fob, with the correct code, is in the vehicle. After this information has been verified the BCM grounds the control side of the RUN/CRANK relay closing the switch side of it and allowing battery positive voltage to flow from the BATT 3 fuse through the RUN/CRANK relay switch, engine control module (ECM) fuse to the battery positive voltage side of the CRANK relay coil. The BCM also sends a high speed GMLAN message to the ECM requesting engine start. The ECM will then verify that the internal mode switch (IMS) is in Park / Neutral or the clutch pedal is fully depressed. After this has been verified the ECM the supplies ground to the control circuit of the CRANK relay closing the CRANK relay switch circuit and allowing battery positive voltage to flow through the STARTER fuse, the CRANK relay switch to the S-terminal of the Starter solenoid cranking the engine. Ground is supplied through the engine block.

 

Many thanks C6 To Go