System Operation: Start
WARNING: This page is about a different car, the 2013 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.
When the BCM-B receives the START voltage input from the ignition switch it activates the RUN/START bus and the RUN/START relay, providing ignition power to the various vehicle systems and modules. The BCM-B also sends out the ignition mode message on the MS-CAN and the HS-CAN .
The ignition switch START circuit is also hard wired to the PCM . The PCM uses the ignition switch START input to determine when to activate the starter motor relay. REFER to STARTING SYSTEM - WITHOUT INTELLIGENT ACCESS , Starting System.