Circuit/System Description
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2012 GMC Yukon XL, 2012 GMC Yukon, 2012 Chevrolet Tahoe, 2012 Chevrolet Suburban, and 2012 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
The Transmission Control Module continuously monitors the voltage on the Ignition Accessory/Run circuit from the body control module (BCM). When the ignition switch is in the Accessory or Run position, the BCM sends a 12 volt wake up signal to the Transmission Control Module. This wake up signal allows the Transmission Control Module to power up and start functioning before the engine is started.