Body Control Systems: Description and Operation

Body Control System Description and Operation

The body control system consists of the body control module (BCM), communications, and various input and outputs. Some inputs, outputs and messages require other modules to interact with the BCM. The BCM also has discrete input and output terminals to control the vehicle's body functions. The BCM is wired to the GMLAN high speed serial data buss and the GMLAN low speed serial data buss and acts as a gateway between them. If the BCM does not communicate the vehicle will not start due to the inability of the engine control module (ECM)/powertrain control module (PCM) and theft deterrent module (TDM) to communicate without the BCM providing the gateway function.

Power Mode Master

This vehicles body control module (BCM) functions as the power mode master (PMM). The ignition switch is a low current switch with multiple discrete ignition switch signals to the PMM for determination the power mode that will be sent over the serial data circuits to the other modules that need this information, and so the PMM will activate relays and other direct outputs of the PMM as needed. Refer to Power Mode Description and Operation Power Mode for a complete description of power mode functions.

Serial Data Gateway

The body control module (BCM) in this vehicle functions as a gateway or translator. The purpose of the gateway is to translate serial data messages between the GMLAN high speed buss and the GMLAN low speed buss for communication between the various modules. The gateway will interact with each network according to that network's transmission protocol.

One example of this necessary communication is the communication between the engine control module (ECM)/powertrain control module (PCM) which is high speed serial data and Theft Deterrent Module which is low speed serial data. If these modules can not exchange information, the vehicle will not start.

Communication between the BCM and a scan tool can be on the high speed GMLAN network or low speed GMLAN network. If one network is lost, the BCM can still communicate with the scan tool. A lost communication DTC typically is set in modules other than the module with a communication failure.

Body Control Module

The various body control module (BCM) input and output circuits are described in the corresponding functional areas indicated on the BCM electrical schematics. Some BCM functions with the subsystems may be as a gateway only or as an enable for the system. The BCM related systems/subsystems include, but are not limited to the following:

* Antilock Brake System -Hybrid (HP2) - Refer to ABS Description and Operation Description and Operation ,

* Antilock Brake System -Non-Hybrid Less than 3900 kg (8600 lb) GVW -Refer to ABS Description and Operation Description and Operation.

* Antilock Brake System -Non-Hybrid Greater than/Equal to 3900 kg (8600 lb) GVW -Refer to ABS Description and Operation (Without JL4) Description and OperationABS Description and Operation (With JL4) Description and Operation.

* Automatic Day-Night Mirror-Refer to Automatic Day-Night Mirror Description and Operation Automatic Day-Night Mirror.

* Cruise Control System-Refer to Cruise Control Description and Operation Description and Operation.

* Electrical Power Management (EPM)-Refer to Electrical Power Management Description and Operation (Diesel) Description and OperationElectrical Power Management Description and Operation (Gasoline) Description and Operation.

* Exterior Lighting-Refer to Exterior Lighting Systems Description and Operation Exterior Lighting Systems.

* Horn System-Refer to Horns System Description and Operation Description and Operation.

* HVAC-Refer to Air Delivery Description and Operation Air Delivery and Air Temperature Description and Operation Air Temperature.

* Instrument Cluster Indicator Control-Refer to Instrument Cluster Description and Operation Instrument Cluster.

* Interior Lighting-Refer to Interior Lighting Systems Description and Operation Interior Lighting Systems.

* Power Door Lock System-Refer to Power Door Locks Description and Operation Description and Operation.

* Rear Window Defogger System-Refer to Rear Window Defogger Description and Operation Description and Operation.

* Redundant Steering Wheel Controls-Refer to Steering Wheel Controls Description and Operation Steering Wheel Controls.

* Remote Function Actuation (RFA) Control-Refer to Keyless Entry System Description and Operation Description and Operation.

* Retained Accessory Power (RAP)-Refer to Retained Accessory Power Description and Operation Retained Accessory Power.

* Shift Lock Control System-Refer to Automatic Transmission Shift Lock Control Description and Operation Description and Operation.

* Starting System-Refer to Starting System Description and Operation Description and Operation.

* Supplemental Inflatable Restraint (SIR) System-Refer to SIR System Description and Operation Description and Operation.

* Theft Deterrent-Refer to Theft Systems Description and Operation Theft Systems.

* Tire Pressure Monitor (TPM) System-Refer to Tire Pressure Monitor Description and Operation Description and Operation.

* Wiper/Washer System Functions-Refer to Wiper/Washer System Description and Operation Description and Operation.

Export Body Control Module

The export body control system has an additional body control module located in the rear of the vehicle. This additional body control module is referred to as the XBCM. The XBCM has discrete input and output terminals to control additional vehicle body functions not supported by the primary body control module (BCM). The XBCM is wired to the GMLAN low speed serial data buss and the BCM and acts as a gateway between them. If the XBCM does not communicate, various rear end lighting such as rear fog lamps and trailer lighting will be impaired.

The various input and output circuits are described in the corresponding functional areas indicated on the XBCM electrical schematics. Some XBCM functions with the subsystems may be as a gateway only or as an enable for the system. The XBCM related systems/subsystems include, but are not limited to the following:

* Exterior lighting-Refer to Exterior Lighting Systems Description and Operation Exterior Lighting Systems.

* Instrument cluster indicator control-Refer to Instrument Cluster Description and Operation Instrument Cluster.

* Theft deterrent-Refer to Theft Systems Description and Operation Theft Systems.