BCM Functions
The Body Control Module (BCM) performs multiple body control functions. The BCM can control devices directly connected to its outputs based on input information obtained from two sources:
| • | sensors and switches that are directly connected to the BCM |
| • | by receiving information from other vehicle systems connected to the class 2 serial data link. |
The BCM evaluates this information and controls certain body control systems by commanding outputs on or off.
The BCM is also capable of controlling other vehicle systems that are not directly wired to the BCM. The BCM does this by sending specific messages on the class 2 serial data link. The vehicle system capable of performing a required function will respond to the BCM message.
The BCM controls these functions:
| • | Audible warnings, refer to Audible Warnings Description and Operation in Instrument Panel, Gages and Console. |
| • | Interior lighting, refer to Interior Lighting Systems Description and Operation in Lighting Systems. |
| • | Automatic door locks, refer to Power Door Locks Description and Operation in Doors. |
| • | Automatic headlamp control, refer to Exterior Lighting Systems Description and Operation in Lighting Systems. |
| • | Keyless entry (AUO option), refer to Keyless Entry System Description and Operation in Keyless Entry. |
| • | Passlock theft deterrent, refer to Vehicle Theft Deterrent (VTD) Description and Operation in Theft Deterrent |
| • | Content theft deterrent, refer to Content Theft Deterrent (CTD) Description and Operation in Theft Deterrent |
| • | Retained Accessory Power (RAP), refer to Retained Accessory Power (RAP) Description and Operation in Retained Accessory Power. |
Simple Power Mode Example
The power mode signal may be as simple as a B+ input wired to a particular ignition switch contact. If this is also the B+ supply to the module/device, the module/device will only operate with the ignition contact closed to B+. An example of this is the starter relay when it is wired directly to the CRANK/START contact of the ignition switch. When the CRANK/START contacts are closed, the starter relay is energized and provides a current source to the starter and starter solenoid. When the ignition switch leaves the CRANK/START position, the switch contacts open and the starter relay is de-energized. This removes the current source from the starter and solenoid and the starter operation stops.
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 BCM receives three signals/circuits from the ignition switch: Acc/Run/Crank (Ignition 0), Run/Crank (Ignition 1) and Acc/Run (Accessory).
To determine the correct power mode the BCM uses:
| • | the state of these signals/circuits, either switch closed (B+ = 1) or switch open (open = 0) |
| • | the sequence of switch closures received by the BCM |
| • | the status of the engine run flag |
The chart below indicates the modes detected and transmitted by the BCM:
Power Mode | Engine Run Flag | Sampled Ignition Signal State | ||
Selected/ Expected | Serial Data | Accessory | Ignition 1 | Ignition 0 |
OFF | 0 | 0 | 0 | 0 |
RAP | 0 | 0 | 0 | 1 to 0 |
ACCESSORY | 0 | 1 | 0 | 1 |
RUN | 0 | 1 | 1 | 1 |
CRANK | 0 | 0 | 1 | 1 |
RUN | 1 | 1 | 1 | 1 |
NOTE: Any ignition switch states not covered in this table indicates a DTC setting situation.
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 BCM 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 BCM message until they can check for the engine run flag status on the serial data circuits. If the BCM 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 BCM 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, B+, the modules will fail-safe to the RUN power mode. If the discrete ignition input is not active, open or 0 voltage, 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.
