Body Control Systems: Description and Operation
BODY CONTROL SYSTEM DESCRIPTION AND OPERATIONThe body control module (BCM) has a bussed electrical center integrated into the housing. Body control relays and fuses are installed directly on the BCM, which simplifies wiring and junction blocks on the vehicle. The body control system consists of the BCM and its associated controls. Battery positive voltage is provided to the BCM from IBCM BATT 1 fuse and the IBCM BATT 2 fuse in the underhood fuse block. The module grounds are wired to ground G201, G203 and G305. The BCM is wired to the GMLAN High speed serial data buss, and the GMLAN Low speed serial data buss, as well as discrete input and output terminals to control the functions of the vehicle's body.
Serial Data Gateway
The body control module (BCM) in this vehicle functions as a translator between the High and Low speed GMLAN busses. The 2 communication protocols use the BCM to translate communicated serial data messages between the control modules over the 2 serial data busses.
Serial Data Power Mode Master
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 below 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 Off/Run/Crank, Accessory, and Ignition 1, to distinguish the correct power mode. The ignition 1 circuit is also routed to the Run/Crank relay for relay controlled power feeds during the appropriate power mode. The BCM, after determining the desired power mode, will activate the appropriate relays for that power mode.
Run/Crank Relay
The Run/Crank relay control circuit is connected to a constant ground and is switched by the ignition 1 circuit or the body control module (BCM). The BCM, will in lieu of the ignition switch position, activate the Run/Crank relay during a remote start event. The Run/Crank relay supplies a power signal to the following circuits, when the a Run or Crank power mode is selected:
- Automatic transmission shift indicator
- Automatic transmission (A/T)
- Switched power feeds to the BCM
- Electronic brake control module (EBCM)
- Electronic ignition module (spark control)
- Electronic power steering module (EPS)
- Fuel injectors
- Powertrain control module (PCM)
- Park neutral position (PNP) switch
Accessory (ACC) Relay
The adaptive cruise control (ACC) 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 ACC relay is also energized by the body control module (BCM) to supply power during the retained accessory power (RAP) mode. The following devices are controlled by this relay:
- Inside rearview mirror
- Heated seats
- Power windows
- Sunroof
- Wiper washers switch
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 power mode master (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 their subsystems can support all operator requirements. If the engine run flag serial data is False, indicating that the engine is not running, the modules fail-safe to OFF-AWAKE. In this state, the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.
Discrete Ignition Signals
Those modules that have discrete ignition signal inputs, also remain in the state dictated by the last valid power mode master (PMM) message received on the serial data circuits. They then check the state of their discrete ignition input to determine the current valid state. If the discrete ignition input is active, battery positive voltage, the modules will fail-safe to the RUN power mode. If the discrete ignition input is not active, open or 0 volts, the modules will fail-safe to OFF-AWAKE. In this state, the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.
BCM Wake-Up/Sleep States
The body control module (BCM) is able to control or perform all of the BCM functions in the wake-up state. The BCM enters the sleep state when active control or monitoring of system functions has stopped, and the BCM has become idle again. The BCM must detect certain wake-up inputs before entering the wake-up state. The BCM monitors for these inputs during the sleep state, where the BCM is able to detect switch transitions that cause the BCM to wake-up when activated or deactivated. Multiple switch inputs are needed in order to sense both the insertion of the ignition key and the power mode requested.
The BCM will enter a wake-up state, if any of the following wake-up inputs are detected:
- Activity on the serial data line
- Detection of a battery disconnect and reconnect condition
- Headlamps are ON.
- Ignition is turned ON.
- Key-in-ignition switch
- Park lamps are ON.
- RFA message
The BCM will enter a sleep state when all of the following conditions exist:
- The ignition switch is OFF.
- No activity exists on the serial data line.
- No outputs are commanded.
- No delay timers are actively counting.
- No wake-up inputs are present.
If all these conditions are met, the BCM will enter a low power or sleep condition. This condition indicates that the BCM, which is the power mode master (PMM) of the vehicle, has sent an OFF or OFF-ASLEEP message to the other systems on the serial data line.
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. The BCM functions include the following:
- A/C compressor request-Refer to Air Temperature Description and Operation in HVAC Systems - Manual.
- A/C cooling fan-Refer to Air Delivery Description and Operation in HVAC Systems - Manual.
- Battery rundown protection (inadvertent power)-Refer to Interior Lighting Systems Description and Operation in Lighting Systems.
- Bulb check-Refer to Instrument Panel Cluster (IPC) Description and Operation in Instrument Panel, Gages, and Console.
- Chimes-Refer to Indicator/Warning Message Description and Operation in Instrument Panel, Gages, and Console.
- Check gages
- Door ajar warning
- Headlamps ON
- Key-in-ignition
- Oil reset
- Park brake warning
- Seat belt not fastened
- Turn signal ON
- Door lock interface-Refer to Power Door Locks Description and Operation in Doors.
- Electronic power steering (EPS)-Refer to Power Steering System Description and Operation in Power Steering System.
- Engine coolant level sensing-Refer to Cooling System Description and Operation in Cooling System.
- Exterior and interior lighting control-Refer to Exterior Lighting Systems Description and Operation in Lighting Systems.
- Automatic lighting control
- Daytime running lamps (DRL)
- Fog lamps
- Park brake lights
- Turn hazard lamps
- Gage control-Refer to Instrument Panel Cluster (IPC) Description and Operation in Instrument Panel, Gages, and Console.
- Horn interface-Refer to Horns System Description and Operation in Horns.
- Instrument cluster indicator control-Refer to Instrument Panel Cluster (IPC) Description and Operation in Instrument Panel, Gages, and Console.
- Interior lighting-Refer to Interior Lighting Systems Description and Operation in Lighting Systems.
- Key-in-ignition sensing-Refer to Indicator/Warning Message Description and Operation in Instrument Panel, Gages, and Console.
- Parking brake state sensing-Refer to Brake Warning System Description and Operation in Hydraulic Brakes.
- Retained accessory power (RAP)-Refer to Retained Accessory Power (RAP) Description and Operation.
- Rear compartment interface-Refer to Power Door Locks Description and Operation in Doors.
- Rear defogger-Refer to Rear Window Defogger Description and Operation in Heated Glass Element.
- Remote function control-Refer to Keyless Entry System Description and Operation in Keyless Entry.
- Seat belt use sensing-Refer to Seat Belt System Description and Operation in Seat Belts.
- Content theft deterrent-Refer to Theft Systems Description and Operation in Theft Deterrent.
- Vehicle theft deterrent-Refer to Vehicle Theft Deterrent (VTD) Description and Operation in Theft Deterrent.
- Washer solvent level sensing-Refer to Wiper/Washer System Description and Operation in Wipers/Washer Systems.
- Wiper/washer-Refer to Wiper/Washer System Description and Operation in Wipers/Washer Systems.