RELAY AWD V6-3.5L VIN 8 (2005)
Body Control Module: Description and Operation
Body Control System
BODY CONTROL SYSTEM DESCRIPTION AND OPERATION
The body control system consists of the body control module (BCM) and its associated controls. Battery positive voltage is provided to the BCM from
the TURN/HAZ fuse and the CHMSL/BKUP fuse in the fuse block instrument panel (I/P). The module grounds are wired to ground G200, G201 and
G301. The BCM is wired to the class 2 serial data communication bus as well as discrete input and output terminals to control the functions of the
vehicles body.
Serial Data Power Mode
On vehicles that have several control modules connected by serial data circuits, one module is the power mode master (PMM). On this vehicle, the
PMM is the body control module (BCM). The PMM receives 3 discrete ignition switch signals to differentiate which power mode will be sent over the
Serial Data circuits. The table illustrates the state of these inputs in correspondence to the ignition switch position:
Relay Controlled Power Mode
The body control module (BCM) uses the discrete ignition switch inputs ignition 1, ignition 3 and crank to distinguish the correct power mode. Once
the BCM has determined the power mode selected by the vehicle operator it will energize the Ignition relay, Run relay and retained accessory power
(RAP) relay, depending on which power mode is selected.
Ignition 1 Relay
The relay uses a Hot At All Times B+ power source derived from the underhood electrical center. The ignition 1 relay supplies a power signal to the
following circuits when the Run or Crank power mode is selected:
-
AC clutch relay
-
ABS STG angle sensor
-
Antilock brake system (ABS) yaw sensor
-
Auxiliary power drop connector
-
Crank relay
-
Cruse control switch
-
Electronic brake control module (EBCM)
-
Engine control module (ECM)
-
HVAC module
-
Ignition control module (ICM)
-
Instrument panel cluster (IPC)
-
Sensing and diagnostic module (SDM)
-
Transmission solenoid circuit
Retained Accessory Power (RAP) Relay
The retained accessory power (RAP) relay is energized when the Run or Accessory power mode has been selected. The relay uses a Hot At All Times
B+ power source derived from the underhood electrical center. The B+ power source is protected by the 50 Amp BATT MAIN 2 fuse. The body
control module (BCM) also energizes the relay for 10 minutes after the vehicle operator transitions the ignition switch from Accessory to OFF or Run
to OFF positions. The following circuits are controlled by the RAP relay:
-
Power Sunroof
-
Power windows
Fail-Safe Operation
Since the operation of the vehicle systems depends on the power mode, there is a fail-safe plan in place should the power mode master (PMM) fail to
send a power mode message. The fail-safe plan covers those modules using exclusively serial data control of power mode as well as those modules
with discrete ignition signal inputs.
Serial Data Messages
The modules that depend exclusively on serial data messages for power modes stay in the state dictated by the last valid PMM message until they can
check for the engine run flag status on the serial data circuits. If the PMM fails, the modules monitor the serial data circuit for the engine run flag
serial data. If the engine run flag serial data is True, indicating that the engine is running, the modules fail-safe to RUN. In this state the modules and