Serial Data Power Mode: Notes

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 Accessory (ACC) ignition switch signals/circuits.

To determine the correct power mode the PMM uses:

• The state of these signals/circuits, either switch closed, battery positive voltage = 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 Table

Courtesy of GENERAL MOTORS CORP.