Power to many of this vehicle’s circuits are controlled by the module that is designated the Power Mode Master (PMM). This vehicle’s PMM is the Body Control Module (BCM). The PMM controls which power mode (Run, Accessory, Crank, Retained Accessory Power, or Off) is active.
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 signals/circuits from the ignition switch. These are the Unlock (IGN 0), Run/Crank (IGN 1) and Accessory (ACC) 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:
Power Mode Selected/Expected | Engine Run Flag Serial Data | Sampled Ignition Signal State | Look Up Table Contents | Key-In Sense | |||
|---|---|---|---|---|---|---|---|
Accessory | Run/Crank (IGN1) | Unlock (IGN0) | DTC Set | Power Mode State | |||
OFF | 0 | 0 | 0 | 0 | 0 | OFF-AWAKE | NA |
OFF | 1 | 0 | 0 | 0 | 0 | OFF-AWAKE | NA |
RAP | 1 | 0 | 0 | 1 to 0 | 0 | RAP | NA |
RAP | 0 | 0 | 0 | 1 to 0 | 0 | RAP | NA |
RAP-UNLOCK | 0 | 1 to 0 | 0 | 1 | 0 | RAP-UNLOCK | NA |
RAP-UNLOCK | 1 | 0 | 0 | 1 | 0 | RAP-UNLOCK | NA |
UNLOCK | 0 | 0 | 0 | 0 to 1 | 0 | UNLOCK | NA |
UNLOCK | 1 | 0 | 0 | 1 | 0 | UNLOCK | NA |
ACCESSORY | 0 | 1 | 0 | 1 | 0 | ACCESSORY | NA |
RUN | 1 | 1 | 1 | 1 | 0 | RUN | NA |
RUN | 0 | 1 | 1 | 1 | 0 | RUN | NA |
CRANK | 0 | 0 | 1 | 1 | 0 | CRANK | NA |
OFF | 0 | 0 | 1 | 0 | 1 | OFF-AWAKE | Key Not In |
RAP | 0 | 0 | 1 | 0 | 1 | RAP | Key In |
ACCESSORY | 0 | 1 | 0 | 0 | 1 | ACCESSORY | Key In |
RUN | 0 | 1 | 1 | 0 | 1 | RUN | Key In |
RUN | 1 | 0 | 1 | 0 | 1 | RUN | Key In |
RUN | 1 | 0 | 1 | 1 | 1¹ | RUN | Key In |
RUN | 1 | 1 | 0 | 0 | 1 | RUN | Key In |
RUN | 1 | 1 | 0 | 1 | 1 | RUN | Key In |
RUN | 1 | 1 | 1 | 0 | 1 | RUN | Key In |
Bold text indicates a DTC setting situation. ¹ DTC sets after one minute in this 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 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 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, 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.
Electrical Power Management
The body control module (BCM) provides the following electrical power management functions. Some power management functions are controlled by working with other systems through the class 2 serial data link:
| • | Retained accessory power (RAP)--Refer to Retained Accessory Power (RAP) Description and Operation for further information. |
| • | Load shed and idle boost actions--Refer to Load Shed System Description and Operation for further information. |
| • | Inadvertent lighting control for battery run-down protection--Refer to Exterior Lighting Systems Description and Operation and Interior Lighting Systems Description and Operation for further information. |
