Description & Operation

WARNING: Vehicles are equipped with Supplemental Inflatable Restraint (SIR) system. Before attempting ANY repairs involving steering column, instrument panel or related components, see SERVICE PRECAUTIONS and DISABLING & ACTIVATING AIR BAG SYSTEM in appropriate AIR BAG RESTRAINT SYSTEMS article.

NOTE: Class 2 device is also referred to as class 2 serial data link.

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

BCM is also capable of commanding other vehicle systems to control functions that are not directly wired and/or controlled by BCM. BCM accomplishes this task by sending specific messages on class 2 serial data link. Class 2 serial data link is used by various systems to share data information with other systems in order to execute various vehicle functions. Communication between each system is accomplished by sending digitally coded messages, which consist of specific information the system module must follow. Each system module is assigned its own recognition code, so it can respond to appropriate messages issued by BCM. This way, BCM can determine state of health of other systems.

Class 2 device (serial data link) is configured as a ring with discrete links entering and leaving each module connected to the link. This allows communication on the ring if one of the 2 discrete circuits is open. Control modules on the class 2 device are:

Body Control Module (BCM).
Electronic Brake Control Module (EBCM).
Heater-A/C Control.
Inflatable Restraint Sensing and Diagnostic Module (SDM).
Instrument Panel Cluster (IPC).
Powertrain Control Module (PCM).

BCM performs the following functions: audible warnings, interior lighting, automatic door locks, keyless entry, Pass-Key(R) II theft deterrent and Retained Accessory Power (RAP). The BCM also performs the following miscellaneous functions: BRAKE indicator light control, fog lamp enable control and day/night mode sensing.

Signals that activate the BCM are known as "wake-up" signals. BCM can enter a sleep state when active control or monitoring of systems has stopped or when BCM is idling. BCM monitors its inputs during the sleep state, which allows BCM to switch between sleep and awake states. BCM is asleep when it is not controlling or monitoring the following wake-up inputs: instrument panel dimmer switch, door lock (ajar) switch, door lock switch, a keyless entry system signal, inadvertent power (battery rundown) protection transition, or ignition is turned to LOCK, RUN or ACC position. The BCM will enter a sleep state when all of the following conditions exist: no activity on class 2 serial data link, ignition is turned to OFF position, no outputs are being commanded by BCM, no delay timers are actively counting and no wake-up inputs are present.

BCM has 3 main power feeds and 3 grounds. Power feed circuits are used to provide power for BCM's logic, courtesy lights, internal driver operation, and door locks.