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Body Control System



BODY CONTROL SYSTEM DESCRIPTION AND OPERATION

BCM OVERVIEW
The main features of the body control module (BCM) are:
- The BCM is the gateway between class 2 serial data circuit and high speed GMLAN serial data circuit.
- The BCM controls multiple functions.
- The BCM is the power mode master.

GATEWAY
The body control module (BCM) communicates functionally on both class 2 and high speed GMLAN serial data buses. The BCM is the gateway between the buses and transfers serial data messages from one subnet to another. Physical communication of the BCM with the scan tool is through class 2 serial data circuit.

BCM FUNCTIONS
The body control module (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:
- Sensors and switches that are directly connected to the BCM
- Class 2 serial data messages received from other vehicle systems connected to the class 2 serial data link
- GMLAN serial data messages received from other vehicles systems connected to the high speed GMLAN serial data link

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 system functions that BCM controls or contributes to are:
- Audible warnings-Refer to Audible Warnings Description and Operation in Instrument Panel, Gages, and Console.
- Exterior lighting control-Refer to Exterior Lighting Systems Description and Operation in Lighting Systems.
- Interior lighting-Refer to Interior Lighting Systems Description and Operation in Lighting Systems.
- Automatic door locks-Refer to Power Door Locks Description and Operation in Doors.
- Keyless entry-Refer to Keyless Entry System Description and Operation in Keyless Entry.
- Theft deterrent-Refer to Content Theft Deterrent (CTD) Description and Operation in Theft Deterrent.
- Wipers/washers-Refer to Wiper/Washer System Description and Operation in Wipers/Washer Systems.
- Rear defogger-Refer to Rear Window Defogger Description and Operation in Heated Glass Element.
- Horn-Refer to Horns System Description and Operation in Horns.

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 monitors Ignition 1 (Run/Crank), Ignition 2 (Acc/Run), and Ignition 3 (Run) signals from the ignition switch, and the engine run flag (ERF) message from the engine control module (ECM) in order to determine the present power mode.

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




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

The power mode message is a periodic with event message type and is transmitted by the PMM at every 2 seconds on class 2 link and at every 250 ms on high speed GMLAN link. The power modes are identical transmitted on both class 2 and high speed GMLAN serial data circuits with the exception of retained accessory power (RAP). RAP is transmitted only on class 2 and it is mapped on high speed GMLAN as Off Awake.

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.
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