Circuit Description
WARNING: This page is about a different car, the 2008 Pontiac G8. However, it is still accessible from the selected car via links, so may be relevant.
When the ignition switch is placed in the Start position a discrete signal is supplied to the body control module (BCM) notifying it that the ignition is in the START position. The BCM then sends a message to the engine control module (ECM) notifying it that CRANK has been requested. The ECM verifies that the transmission is in Park/Neutral. If it is, the ECM then supplies 12 volts to the control circuit of the starter relay. When this occurs battery positive voltage is supplied through the switch side of the starter relay to the starter solenoid.