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) and theft deterrent module (TDM) to communicate
without the BCM providing the gateway function.
Power Mode Master
This vehicles 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
for a complete description of power
mode functions.
Serial Data Gateway
The 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
ECM which is high speed serial data and TDM 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 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:
