Serial Data Power Mode

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 BCM receives three signals/circuits from the ignition switch: Acc/Run/Crank (Ignition 0), Run/Crank (Ignition 1) and Acc/Run (Accessory).

To determine the correct power mode the BCM 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 BCM
• The status of the engine run flag

The chart below indicates the modes detected and transmitted by the BCM:

Fig 1: Body Control System Description & Operation

Courtesy of GENERAL MOTORS CORP.