Body Control Modules: Description

WARNING: Deactivate air bag system before performing any service operation. See appropriate AIR BAG RESTRAINT SYSTEMS article. Do not apply electrical power to any component on steering column without first deactivating air bag system. Air bag may deploy.

Body Control Module (BCM) performs the following functions: A/C compressor request, A/C cooling fan, exterior and interior lighting control, battery rundown protection, chime, gauge control, instrument cluster indicator control and theft deterrent.

BCM communicates with components using class 2 data line and Serial Peripheral Interface (SPI) data line. BCM acts as an interpreter for data communication between class 2 and SPI data systems. Class 2 data line is used by BCM to communicate with the Electronic Brake Control Module (EBCM), Sensing and Diagnostic Module (SDM) and Powertrain Control Module (PCM). Class 2 serial data uses the peer-to-peer method and message arbitration to manage data line traffic.

SPI is a 3-wire synchronous serial data link through which the BCM controls the instrument cluster. BCM interprets driver warnings sent by control module on class 2 data line, then sends a SPI data message to the instrument cluster. SPI data bus messages include fuel gauge position, temperature gauge position, oil pressure, low engine coolant, check oil, volts, ABS, high beam, low fuel, high temperature, washer fluid level low, CRUISE, BRAKE, SERVICE VEHICLE SOON, door ajar, seat belt, theft system, and PRNDL.

Signals that activate the BCM are known as "wake-up" signals. BCM wake-up signals include: door ajar switches, when ignition switch is turned to RUN or ACC position, or inadvertent power.