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 monitors Ignition 1 (Run/Crank), Ignition 2 (Acc/Run), and Ignition 3 (Run) signals from the ignition switch, and the engine run flag (ERF) message from the engine control module (ECM) in order to determine the present power mode.
To determine the correct power mode the BCM uses:
- The state of these signals
- The sequence of switch closures received by the BCM
- The status of the engine run flag
The chart indicates the power modes detected and transmitted by the BCM:
| Ignition Key Position | Power Mode Parameter | Engine Running | Sampled Ignition Signal State | ||
|---|---|---|---|---|---|
| Ignition 1 (Run/Crank) | Ignition 2 (Acc/Run) | Ignition 3 (Run) | |||
| OFF | Off-Awake or RAP | No | Low | Low | Inactive |
| ACC | Accessory | No | Low | High | Inactive |
| ON | Run | No | High | High | Active |
| START | Crank | No | High | Low | Inactive |
| ON | Run | Yes | High | High | Active |
The power mode message is a periodic with event message type and is transmitted by the PMM at every 2 seconds on class 2 link and at every 250 ms on high speed GMLAN link. The power modes are identical transmitted on both class 2 and high speed GMLAN serial data circuits with the exception of retained accessory power (RAP). RAP is transmitted only on class 2 and it is mapped on high speed GMLAN as Off Awake.