GM Service Manual Online
For 1990-2009 cars only

BCM Functions

The body control module (BCM) is capable of accomplishing multiple body control functions. Components directly connected to the BCM are controlled by the BCM outputs. The BCM can control its outputs based on input information it obtains from sensors and switches that are directly connected to the BCM, or by receiving information from other vehicle systems connected to the class 2 serial data link. The BCM evaluates this information and controls certain body systems by commanding an output on or off.

The BCM is also capable of commanding other vehicle systems to control functions that are not directly wired to and/or controlled by the BCM. The BCM accomplishes this task by sending specific messages on the class 2 serial data link. The vehicle system capable of performing such function will respond to the BCM message.

The BCM controls/monitors the following functions:

    • Audible warnings
    • Indicators control
    • Interior lighting
    • Exterior lighting
    • Power/automatic door locks
    • Keyless entry
    • Content theft deterrent
    • Personalization
    • Serial Power Modes
    • Electrical Power Management

Audible Warning

The body control module (BCM) provides the following audible control functions. Some audible control functions may be controlled by other systems through the class 2 serial data link:

    • Headlamp on reminder
    • Key in ignition reminder
    • Personalization mode
    • Delay locking indication
    • Keyless entry programming/diagnostic mode indication

Refer to Audible Warnings Description and Operation in Instrument Panel, Gages, and Console for more information.

Indicator Control

The body control module (BCM) provides the following indicator control functions through the class 2 serial data circuits. Some indicator control functions may be controlled by other systems through the class 2 serial data link:

    • Door ajar
    • Liftglass/endgate ajar
    • Park brake
    • High beam
    • Park lamp malfunction
    • Service vehicle soon
    • Personalization mode
    • Security LED
    • Battery

Refer to Instrument Cluster Description and Operation and Indicator/Warning Message Description and Operation in Instrument Panel, Gages, and Console for more information.

Interior Lighting

The body control module (BCM) provides the following interior lighting functions. Some interior control functions may be controlled by other systems through the class 2 serial data link:

    • Interior illumination control
    • Backlight/Illumination
    • Delayed illumination
    • Illumination on key cylinder unlock
    • Exit illumination
    • Illumination after air bag deployment
    • Keyless entry unlock illumination
    • Interior lighting personalization

Refer to Interior Lighting Systems Description and Operation in Lighting Systems for more information.

Exterior Lighting

The body control module (BCM) provides the following exterior lighting control functions. Some exterior lighting control functions may be controlled by other systems through the class 2 serial data link:

    • Headlamps
    • Daytime running lights (DRL)
    • Park lamps
    • Perimeter lighting
    • Front fog lamps
    • Exterior lighting personalization

Refer to Exterior Lighting Systems Description and Operation in Lighting Systems for more information.

Power/Automatic Door Locks

The body control module (BCM) provides the following power/automatic door lock control functions. Some door lock control functions may be controlled and/or sent by other systems through the class 2 serial data link:

    • All door unlock
    • All door lock
    • Last door closed locking
    • Lockout prevention
    • Lockout prevention override
    • Shift into PARK unlock
    • Shift out of PARK lock
    • Unlock after air bag deployment
    • Delayed locking
    • Delayed locking override
    • Power/automatic door lock personalization

Refer to Power Door Locks Description and Operation in Doors for more information.

Keyless Entry

The body control module (BCM) provides the following keyless entry control functions. Some keyless entry control functions may be controlled and/or sent by other systems through the class 2 serial data link:

    • Remote driver door unlock
    • Remote all door unlock
    • Remote all door lock
    • Remote activation verification
    • Remote alarm
    • Keyless entry personalization

Refer to Keyless Entry System Description and Operation in Keyless Entry for more information.

Content Theft Deterrent

The body control module (BCM) provides the following content theft deterrent control functions. Some content theft deterrent control functions may be controlled and/or sent by other systems through the class 2 serial data link:

    • Intrusion detection
       - Status of the doors and rear endgate
       - Lock cylinders
       - Rear liftglass
       - Power door locks
       - The keyless entry system
    • Intrusion notification
       - Security indicator LED
       - Flashes the headlamps
       - Sounds the horns
       - Disables fuel delivery to the engine
    • Content theft deterrent personalization

Refer to Theft Systems Description and Operation in Theft Deterrent for more information.

Personalization

The body control module (BCM) provides personalization of vehicle functions which allow BCM features to be personalized based on driver preferences. The following vehicle functions may be personalized:

    • Headlamp exit delay
    • Interior lighting delay
    • Interior lighting on
    • Auto door lock/unlock
    • Delayed locking
    • Remote driver door unlock
    • Remote lock/unlock confirmation
    • Unlock feedback
    • Lock feedback

Refer to the following Personalization Description and Operation in Personalization for more information on how to set personalized options.

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 PMM receives 3 signals/circuits from the ignition switch. These are the Unlock (IGN0), Run/Crank (IGN1) and Accessory (ACC) ignition switch signals/circuits.

To determine the correct power mode the PMM uses:

    • The state of these signals/circuits, either switch closed (B+ = 1) or switch open (open = 0)
    • The sequence of switch closures received by the PMM
    • The status of the engine run flag

The table below indicates the modes detected and transmitted by the PMM:

Power Mode Selected/Expected

Engine Run Flag Serial Data

Sampled Ignition Signal State

Look Up Table Contents

Key-In Sense

Accessory

Run/Crank (IGN1)

Unlock (IGN0)

DTC Set

Power Mode State

OFF

0

0

0

0

0

OFF-AWAKE

NA

OFF

1

0

0

0

0

OFF-AWAKE

NA

RAP

1

0

0

1 to 0

0

RAP

NA

RAP

0

0

0

1 to 0

0

RAP

NA

RAP-UNLOCK

0

1 to 0

0

1

0

RAP-UNLOCK

NA

RAP-UNLOCK

1

0

0

1

0

RAP-UNLOCK

NA

UNLOCK

0

0

0

0 to 1

0

UNLOCK

NA

UNLOCK

1

0

0

1

0

UNLOCK

NA

ACCESSORY

0

1

0

1

0

ACCESSORY

NA

RUN

1

1

1

1

0

RUN

NA

RUN

0

1

1

1

0

RUN

NA

CRANK

0

0

1

1

0

CRANK

NA

OFF

0

0

1

0

1

OFF-AWAKE

Key Not In

RAP

0

0

1

0

1

RAP

Key In

ACCESSORY

0

1

0

0

1

ACCESSORY

Key In

RUN

0

1

1

0

1

RUN

Key In

RUN

1

0

1

0

1

RUN

Key In

RUN

1

0

1

1

RUN

Key In

RUN

1

1

0

0

1

RUN

Key In

RUN

1

1

0

1

1

RUN

Key In

RUN

1

1

1

0

1

RUN

Key In

Bold text indicates a DTC setting situation.

¹ DTC sets after one minute in this situation.

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 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, 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.

Electrical Power Management

The body control module (BCM) provides the following electrical power management functions. Some power management functions are controlled by working with other systems through the class 2 serial data link:

    • Retained accessory power (RAP). Refer to Retained Accessory Power (RAP) Description and Operation in Retained Accessory Power (RAP) for further information.
    • Load shed and idle boost actions. Refer to Load Shed System Description and Operation in Engine Electrical for further information.
    • Inadvertent lighting control for battery run-down protection. Refer to Exterior Lighting Systems Description and Operation and Interior Lighting Systems Description and Operation in Lighting Systems for further information.