Serial Data Power Mode

WARNING: This page is about a different car, the 2003 Buick Park Avenue. However, it is still accessible from the selected car via links, so may be relevant.

On vehicles that have several control modules connected by serial data circuits, one module is the power mode master (PMM). On this vehicle the PMM is the body control module (BCM). The PMM receives three signals/circuits from the ignition switch. These are the Unlock IGNO, Run/Crank IGN1 and Crank ignition switch signals/circuits.

To determine the correct power mode the PMM uses:

The state of these signals/circuits, either switch closed B+ = 1 or switch open open = 0.
The sequence of switch closures received by the PMM.
The status of the engine run flag.
The chart below indicates the modes detected and transmitted by the PMM:

Fig 1: 3-Wire Ignition Switch Reference Table
Courtesy of GENERAL MOTORS CORP.