Description & Operation

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

The Body Control Module (BCM) performs the following functions: battery guard, door lock and unlock (including keyless entry), various telltale controls in instrument cluster and headlight switch, retained accessory power, theft deterrent, exterior and interior lighting control, trunk release, fuel door release, and class 2 diagnostic protocol.

Signals that activate the BCM are known as "wake-up" signals. BCM wake-up signals include any ignition state transition, door switch activated or trunk opened with ignition switch in LOCK position, or any class 2 communication requiring BCM interaction. BCM also has a low power current sense feedback, which will wake-up BCM if current feedback sense is over 2.4 amps.

After BCM is awakened, it will enter a high power mode. If no further activity occurs within 15 minutes, it will enter a low power ("asleep") mode. After 3 days and 15 minutes, BCM will convert to a no controlled power mode. Both low and high power will be deactivated until a wake-up signal is received.

Battery guard feature provides protection from excessive battery drain caused by high current loads that are accidentally left on. BCM controls battery power regulation by using a low power driver and a high power control of battery guard relay to the following components: Radio receiver, Radio Interface Module (RIM), fuel door relay coil, trunk release relay coil and phone (dealer installed).

Retained Accessory Power (RAP) feature allows use of the radio and phone systems after ignition is turned off. If ignition switch is still in LOCK position after 10 minutes, RAP feature is disabled. If any door is opened or closed during the 10-minute period, RAP feature will be disabled. RAP feature will also disable if ignition switch is turned to START position.

BCM uses class 2 serial data line to communicate with the following modules: Cellular telephone, radio (including clock, tape player and CD player), remote control door lock receiver, and sound processor interface. Class 2 serial data uses the peer-to-peer method and message arbitration to manage data line traffic. Communication on the class 2 serial data line is performed on an "as needed" basis.

BCM acts as an interface between the optional OnStar vehicle communication system and the vehicle architecture. The Vehicle Interface Unit (VIU) will send the following class 2 messages to the BCM: Door unlock or lock activation, horn control, interior light control and exterior light control (will override daytime running light, twilight sentinel or wiper-on lighting modes). BCM is hard-wired to air bag restraint sensing module. If air bag is deployed, BCM will send a class 2 message to VIU to indicate a restraint system was activated.