Impala V8-5.3L VIN C (2006)
Body Control Module: Description and Operation
Power Mode
POWER MODE DESCRIPTION AND OPERATION
SERIAL DATA POWER MODE MASTER
Power to many of this vehicles circuits is controlled by the module that is designated the power mode master (PMM). This vehicles PMM is the body
control module (BCM). The ignition switch is a low current switch with multiple discrete ignition switch signals to the PMM for determination of the
power mode that will be sent over the serial data circuits to the other modules that need this information, and so the PMM will activate relays and
other direct outputs of the PMM as needed. The PMM determines which power mode (Run, Accessory, Crank, Retained Accessory Power, or Off) is
required, and reports this information to other modules via serial data. Modules which have switched voltage inputs may operate in a default mode if
the PMM serial data message does not match what the individual module can see from its own connections.
The PMM receives 3 ignition switch signals to identify the operators desired power mode. The 3-wire Ignition Switch table illustrates the state of
these inputs in correspondence to the ignition switch position:
Relay Controlled Power Mode
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 BCM, after determining the desired power mode, will activate the appropriate relays for that power mode.
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 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 discrete signal inputs and serial data inputs from other modules on the vehicle.
Discrete Ignition Signals
Those modules that have discrete ignition signal inputs from the body control module (BCM), 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.
BCM Awake/Sleep States
The body control module (BCM) is able to control or perform all of the BCM functions in the awake up state. The BCM enters the sleep state when
active control or monitoring of system functions has stopped and a time limit has passed. The BCM must detect certain wake-up inputs before entering
the awake state. The BCM monitors for these inputs during the sleep state.
The BCM will enter the awake state if any of the following wake-up inputs are detected:
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Activity on the serial data line
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Detection of a battery disconnect and reconnect condition
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Any door is opened.
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Headlamps are ON.
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Key-in-ignition switch
