The body control module (BCM) is an integrated module that combines functions and features from a normal control module and a body electrical center. There are four electrical interfaces that exist between the BCM and the rest of the vehicle. There are two connections from the BCM to both the body wiring harness and the instrument panel wiring harness. The BCM contains fuses and relays that are serviceable.

The main features of the BCM are:

The BCM is the gateway between class 2 serial data circuit and high speed GMLAN serial data circuit. The purpose of the gateway is to transfer serial data messages from one subnet to another.

The BCM performs multiple body control functions. The BCM can control devices directly connected to its outputs based on input information. The BCM evaluates this information and controls certain body control systems by commanding outputs on or off. The BCM inputs can be:

The BCM is also capable of controlling other vehicle systems that are not directly wired to the BCM. The BCM does this by sending specific messages on the class 2 serial data link or high speed GMLAN serial data link. The system capable of performing the required function will respond to the BCM messages.

Some of the other systems' functions that BCM controls or contributes to are:

The BCM controls electric power management following an idle boost and a load shed algorithm. The load shed control algorithm will deactivate electrical loads if the idle boost algorithm is unable to maintain system voltage. If idle boost is active for a certain amount of time and the system voltage is still below a set value, then load shed will command the heated seats, HVAC blower, and rear defog to deactivate periodically. The deactivations will be cycled such that the customer may not observe limited operation. The duty cycle will be controlled independently by each of heated seats, HVAC blower, and rear defog. If idle boost continues to be active, the engine RPM low, and the system voltage remains low, then load shed will command the heated seats, HVAC blower, and rear defog to deactivate completely. When this occurs, the DIC will display "Battery Saver Active". Once the idle boost becomes unnecessary or the engine RPM is above a set value, and the system voltage is above a set value, the load shed algorithm will command the heated seats, HVAC blower, and rear defog to resume normal operation.

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 BCM. The BCM will monitor the mode switch only with battery voltage above 7 volts to prevent invalid input readings during low voltages encountered during vehicle cranking. If the mode switch input or the battery voltage input changes, a ratiometric calculation of mode switch input value is performed. The BCM discerns four thresholds:

To determine the correct power mode the BCM uses:

The following power mode states are transmitted on the class 2 network:

The following power mode states are transmitted on the GMLAN network:

The power mode message is a periodic with event message type and is transmitted by the PMM or BPMM at every 2 seconds on class 2 link and at every 250 ms on GMLAN link.

The Accessory power mode times out after 20 minutes and will transition to Off/Awake or RAP power mode.

Fail-safe Operation

The remote control door lock receiver (RCDLR) is the back up power mode master (BPMM). The both BCM and RCDLR receive signals from the ignition mode switch, representing the mode switch data, through 2 different circuits. Therefore if BPMM becomes enabled, it can determine and transmit all the system power modes per the lists above, except RAP. There is no degradation in system operation if the BPMM is the RCDLR. Since the operation of the vehicle systems depends on the power mode, there is a fail-safe plan in place if the power mode message is not received from the BCM or RCDLR. The fail-safe plan covers modules with discrete ignition signal inputs as well as those modules using exclusively serial data control of power mode. The engine control module (ECM) behaves differently.

Serial Data Messages

If no power mode message can be received, the modules remain in the last power mode received and check for the ERF message. If the ERF serial data is true, indicating that the engine is running, the modules fail-safe to "Run" power mode. 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" power mode.

Discrete Ignition Signals

Those modules that have Run/Crank discrete ignition signal input also remain in the state dictated by the last valid power mode message from the BCM or the RCDLR. They then check the state of their Run/Crank discrete ignition signal input to determine the current valid state. If the discrete ignition input is active, the modules will fail-safe to the "Run" power mode. If the discrete ignition input is inactive, the modules will fail-safe to "Off" power mode.

PCM and TCM Power Mode Backup Strategy

The PCM and TCM remain in the last known power mode state until it can determine its backup system power mode based on their discrete Accessory/Wake up and Run/Crank signal inputs.