The Transfer Case Control Module (TCCM) is the brain behind the electronically shifted GM 4WD systems.
The TCCM processes the shift request from the dash mounted mode selector buttons, executes the shift, and verifies the shift was completed. If the TCCM detects a fault with the 4WD system, it will turn on a warning light or illuminate a warning message on the Driver Information Center (DIC).
The TCCM uses vehicle speed and the current transfer case mode to determine if the requested shift is possible. If the TCCM determines that the shift requested is not possible, the mode lights on the selector switch will flash for about 30 seconds and then go off with no shift completed.
The TCCM will complete requested 4WD shifts by activating the transfer case encoder motor and the front differential locking actuator as necessary. Shifts are deemed complete when the position sensor in the encoder motor indicates the correct position for the shift requested and the front differential switch indicates the locked position.