The body control module (BCM) controls a number of functional systems. The BCM is connected to the class 2 serial data line. Many control signals are implemented by class 2 messages.
The BCM functions include the following:
| • | Ajar switch and tamper switch inputs from the rear compartment lid |
| • | Auxiliary chime control |
| • | Control of headlights and exterior lamps |
| • | Heated seat controls |
| • | I/P dimmer switch input |
| • | Interior lamps incandescent dimming |
| • | Interior lamps switch input |
| • | Key in ignition switch input from the ignition switch |
| • | Low coolant sensor input |
| • | Power moding control over Class 2 serial data line |
| • | Rear compartment lid release control |
| • | Rear defogger control |
| • | Retained accessory power (RAP) control |
| • | Seatbelt reminder/warning control |
| • | Steering wheel controls |
| • | Traction control switch input |
| • | Various controls for the interior lamps |
Power Mode Defined
The power mode is the information used by the various control modules on the vehicle to determine operation. If a control module does not receive a power mode signal, either a serial data message or a hard wire input, the control module does not operate.
No operations using two or more modules is possible without a power mode decision by the power mode master (PMM) and a power mode message from the PMM. The possible power modes are:
| • | OFF-ASLEEP, no activity on the serial data circuits, the modules are asleep and in their minimum power usage state. |
| • | OFF-AWAKE, activity on the serial data circuits, the modules are awake and expecting either serial data or hardwire inputs. |
| • | RAP, those module that have functions enabled in RAP are fully operational, the rest will be OFF-AWAKE. |
| • | ACCESSORY, those module that have functions enabled in ACCESSORY are fully operational, the rest will be OFF-AWAKE. |
| • | RUN, all modules are fully functional |
| • | CRANK, those modules that have no function critical to engine starting are OFF to both provide maximum power for cranking and starting operation and to limit customer concerns, i.e. audio system noise. |
Simple Power Mode Example
The power mode signal may be as simple as a battery positive voltage input wired to a particular ignition switch contact. If this is also the battery positive voltage supply to the module/device, the module/device will only operate with the ignition contact closed to battery positive voltage. 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 PMM receives three signals/circuits from the ignition switch. These are the Unlock IGN0, Run/Crank IGN1 and Crank ignition switch signals/circuits.
To determine the correct power mode the PMM 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 PMM. |
| • | The status of the engine run flag. |
The chart below indicates the modes detected and transmitted by the PMM:
Ignition Switch Position | Engine Run Flag (Serial Data) | Accessory (Run/Accessory) | IGN 1 (Run/Crank) | IGN 0 ((Unlock/Accessory/Run/Crank) | Power Mode Transmitted |
|---|---|---|---|---|---|
Off | 0 | 0 | 0 | 0 | OFF or RAP |
Unlock | 0 | 0 | 0 | 1 | Unlock or RAP-Unlock |
Start | 0 | 0 | 1 | 1 | Crank |
Accessory | 0 | 1 | 0 | 1 | Accessory |
Run | 0 | 1 | 1 | 1 | Run |
Unknown/Error | 0 | 1 | 1 | 0 | Run |
Unknown/Error | 0 | 1 | 0 | 0 | Accessory |
Unknown/Error | 0 | 0 | 1 | 0 | OFF or RAP |
Run | 1 | 1 | 1 | 1 | Run |
Off | 1 | 0 | 0 | 0 | Off or RAP |
Unlock | 1 | 0 | 0 | 1 | Unlock or RAP-Unlock |
Unknown/Error | 1 | 0 | 1 | 0 | Run |
Unknown/Error | 1 | 0 | 1 | 1 | Run |
Unknown/Error | 1 | 1 | 0 | 0 | Run |
Unknown/Error | 1 | 1 | 0 | 1 | Run |
Unknown/Error | 1 | 1 | 1 | 0 | Run |
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 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 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 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 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.
