For 1990-2009 cars

### 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 uses 3 signals from the ignition switch:

• | Ignition Off/Run/Crank

• | Ignition 1 Run/Crank

• | Ignition Accessory

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 (ERF)

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

#### Correct Ignition Switch Inputs and Power Modes

Ignition Switch Position | ERF | BCM Input and Scan Tool Display
--- | --- | ---
Ignition Accessory | Ignition 1 Run/Crank | Ignition Off/Run/Crank | Power Mode
OFF | 0 | Inactive | Inactive | Active | Off-Awake or RAP
ACC | 0 | Active | Inactive | Inactive | Accessory
ON | 0 | Active | Active | Active | Run
START | 0 | Inactive | Active | Active | Crank
ON | 1 | Active | Active | Active | Run

### Fail-safe Operation

Since the operation of the vehicle systems depends on the power mode, there is a fail-safe plan in place should the body control module (BCM) fail to send a power mode message. The fail-safe plan covers modules with discrete ignition signal inputs as well as those modules using exclusively serial data control of power mode.

#### Serial Data Messages

The modules that depend exclusively on serial data messages for power modes stay in the state dictated by the last valid BCM message until they receive the engine status from the powertrain control module (PCM). If the BCM fails, the modules monitor the serial data circuit for the engine run flag serial data. If the engine run flag serial data is true, indicating that the engine is running, the modules fail-safe to Run. In this state the modules and their subsystems can support all operator requirements. If the engine run flag serial data is false, indicating that the engine is not running, the modules fail-safe to OFF-AWAKE. In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.

#### Discrete Ignition Signals

Those modules that have discrete ignition signal inputs also remain in the state dictated by the last valid BCM message received on the serial data circuits. They then check the state of their discrete ignition input to determine the current valid state. If the discrete ignition input is active, B+, the modules will fail-safe to the RUN power mode. If the discrete ignition input is not active, open or 0 voltage, the modules will fail-safe to OFF-AWAKE. In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.