Power Mode Defined
The power mode is the information used by the various modules/devices on the vehicle to determine operation. If a module/device does not receive a power mode signal, either a serial data message or a hard wire input, the module/device does not operate properly.
Simple Power Mode Example
The power mode signal may be as simple as a B+ input wired to the ignition switch. 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.
Vehicle Power Modes
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 two signals from the ignition switch. These are the ON/START (IGN1) and Accessory (ACC) ignition switch signals.
The state of these signals are either HIGH (B+) or LOW (open), and the sequence received by the BCM determines the power mode detected by the BCM. The chart below indicates the modes detected by the BCM for the ignition switches currently used.
Ignition Switch Signal | ACCESSORY Mode | RUN Mode | CRANK Mode | RAP Mode | OFF-ASLEEP OFF-AWAKE Modes |
|---|---|---|---|---|---|
ON/START (IGN1) | LOW | HIGH | HIGH | LOW | LOW |
ACC | HIGH | HIGH | LOW | HIGH-LOW | LOW |
The chart indicates that regardless of the ignition switch type, if the BCM detect the ON/START (IGN1) signal is low and the ACC signal is transitioning from HIGH to LOW the BCM determines that the vehicle should be in the RAP mode
Relay Control of Retained Accessory Power (RAP)
The body control module (BCM) monitors the ignition switch position, battery condition and passenger compartment door status to determine whether retained accessory power (RAP) should be initiated. If the ignition switch transitions from RUN/ON to OFF, the battery level is within the acceptable range and the passenger compartment doors are closed, the BCM closes the RAP relay. This provides power for the following:
| • | Power windows |
| • | Rear quarter windows |
The BCM grounds the RAP relay control circuit to close the relay. This also grounds a line to the instrument cluster (IPC) module. The IPC then sends out a serial data message to the radio to tell the radio that the RAP condition exists.
The BCM turns OFF the RAP function and opens the RAP relay when one of the following conditions are met:
| • | The opening of a door in the passenger compartment |
| • | The BCM receives a message from its internal timer indicating the end of the RAP period |
| • | The transition from OFF to ON/START or ACC of the ignition switch |
