Circuit/System Description
WARNING: This page is about a different car, the 2009 GMC Sierra and 2009 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The Run and Crank/On and Start Power signal connects the Serial Data Gateway Module (SDGM) along with numerous other modules to the vehicle 12-volt system through a fused relay in the fuse block - underhood. This relay is closed by the Body Control Module (BCM) based on the key being in either the Run or Crank position.