Body Control Systems: Description and Operation

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 BCM. The PMM receives 2 signals from the ignition switch.

The PMM uses the following circuits to determine the correct power mode:
^ Battery voltage
^ Ignition 1 voltage
^ Ignition 3 voltage

When the ignition is turned to the OFF position, the vehicle enters Retained Accessory Power (RAP) mode. RAP power mode enables selected vehicle functions to be operated for 20 minutes. Selected customer convenience devices will operate. Refer to Retained Accessory Power (RAP) Description and Operation in retained accessory for a detailed description and operation.

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 - if equipped, those modules 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.

BCM CONTROLLED/AIDED FUNCTIONS
The BCM controls/aids the following functions. For additional circuit information and descriptions, refer to the appropriate sub-system.
^ Class 2 serial data
^ RKE remote keyless entry/panic alarm/LOCKING and UNLOCKING of doors/lockout prevention
^ DRL daytime running lamps/auto headlamps/exterior lighting
^ VTD vehicle theft deterrent/passlock(TM) system
^ Inadvertent power relay/battery rundown protection
^ RAP retained accessory power
^ Interior/Exterior lighting
^ Audible warnings

Detailed descriptions of functions reside within the respective sub-subsystem circuit description and operation.

BCM POWER DOWN PROCEDURE
The following procedure stops the BCM from drawing current to allow parasitic load battery testing.
1. Turn ignition OFF
2. Select the special functions option for the BCM on the scan tool
3. Select output control
4. Select the power down now option
5. Follow any further instructions listed in the scan tool

This procedure ONLY removes the BCM load from the battery.