Power to many of this vehicle’s circuits are controlled by the module that is designated the power mode master (PMM). This vehicle’s PMM is the body control module (BCM). The PMM controls which power mode (Run, Accessory, Crank, Retained Accessory Power, or Off) is active.

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 3 discrete ignition switch signals to differentiate which power mode will be sent over the Serial Data circuits. The 3-wire Ignition Switch table below illustrates the state of these inputs in correspondence to the ignition switch position:

The body control module (BCM) uses the discrete ignition switch inputs Off/Run/Crank, Accessory, and ignition 1, to distinguish the correct power mode. The ignition 1 circuit is also routed to the Run/Crank relay for relay controlled power feeds during the appropriate power mode. The BCM, after determining the desired power mode, will activate the appropriate relays for that power mode.

Run/Crank Relay

The run/crank relay control circuit is connected to a constant ground and is switched by the ignition 1 circuit or the BCM. The BCM, will in lieu of the ignition switch position, activate the Run/Crank relay during a remote start event. The Run/Crank relay supplies a power signal to the following circuits, when the a Run or Crank power mode is selected:

Accessory (ACC) Relay

The Accessory (ACC) relay is energized when the Run or accessory power mode has been selected. The relay uses a Hot At All Times B+ power source derived from the underhood electrical center. The ACC relay is also energized by the BCM to supply power during the retained accessory power (RAP) mode. The following devices are controlled by this relay:

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

Serial Data Messages

The modules that depend exclusively on serial data messages for power modes stay in the state dictated by the last valid PMM message until they can check for the engine run flag status on the serial data circuits. If the PMM 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 PMM 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, battery positive voltage, the modules will fail-safe to the RUN power mode. If the discrete ignition input is not active, open or 0 volts, 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.

The body control module (BCM) is able to control or perform all of the BCM functions in the wake-up state. The BCM enters the sleep state when active control or monitoring of system functions has stopped, and the BCM has become idle again. The BCM must detect certain wake-up inputs before entering the wake-up state. The BCM monitors for these inputs during the sleep state, where the BCM is able to detect switch transitions that cause the BCM to wake-up when activated or deactivated. Multiple switch inputs are needed in order to sense both the insertion of the ignition key and the power mode requested.

The BCM will enter a wake-up state, if any of the following wake-up inputs are detected:

The BCM will enter a sleep state when all of the following conditions exist:

If all these conditions are met, the BCM will enter a low power or sleep condition. This condition indicates that the BCM, which is the power mode master (PMM) of the vehicle, has sent an OFF or OFF-ASLEEP message to the other systems on the serial data line.