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The body control system consists of the body control module (BCM), communications, and various input and outputs. Some inputs, outputs and messages require other modules to interact with the BCM. The BCM also has discrete input and output terminals to control the vehicle's body functions. Battery positive voltage is provided to the BCM from the BCM fuse in the engine fuse block. The module has two grounds at GX201 as well as one directly to the vehicle battery. The BCM is wired to the GMLAN High speed serial data buss and the GMLAN Low speed serial data buss and acts as a gateway between them. If the BCM does not communicate the vehicle will not start due to the inability of the engine/powertrain control module (ECM/PCM) and vehicle theft deterrent (VTD) Control Module to communicate without the BCM providing the gateway function.
This vehicle's BCM functions as the power mode master (PMM). The ignition switch is a low current switch with multiple discrete ignition switch signals to the PMM for determination the power mode that will be sent over the serial data circuits to the other modules that need this information, and so the PMM will activate relays and other direct outputs of the PMM as needed.
Power to many of this vehicle's circuits is controlled by the module that is designated the power mode master (PMM). This vehicle's PMM is the body control module (BCM). The ignition switch is a low current switch with multiple discrete ignition switch signals to the PMM for determination of the power mode that will be sent over the serial data circuits to the other modules that need this information. The PMM will also activate relays and other direct outputs of the PMM as needed. The PMM determines which power mode (Off, Accessory, Run, Crank Request) is required, and reports this information to other modules via serial data. Modules which have switched voltage inputs may operate in a default mode if the PMM serial data message does not match what the individual module can see from its own connections.
The PMM receives ignition switch signals to identify the operators desired power mode. The PMM Power Mode Parameters table below illustrates the correct state of these input parameters (circuits) in correspondence to the ignition switch position:
Ignition Switch Position | Power Mode Transmitted | Ign. Off/Run/Crank (Off/Run Crank voltage circuit) | Ignition Accessory/Run (Accessory voltage circuit) | Ignition Run/Crank (Ignition 1 Voltage circuit) |
|---|---|---|---|---|
Off Key Out | Off | Key Out/ACC | Inactive | Inactive |
Off Key IN (LOCK) | Off | Key In/OFF | Inactive | Inactive |
ACC | Accessory | Key In/ACC | Active | Inactive |
ON | Run | Run | Active | Active |
START | Crank Request | Crank | Inactive | Active |
The body control module (BCM) is able to control or perform all of the BCM functions in the awake state. The BCM enters the sleep state when active control or normal monitoring of system functions has stopped and a time limit has passed. The BCM must detect certain wake-up inputs before entering the awake state. The BCM monitors for these inputs during the sleep state.
The BCM will enter the awake state if any of the following wake-up inputs are detected:
| • | Activity on the serial data line |
| • | Detection of a battery reconnect |
| • | Any door open signal |
| • | Headlamps ON |
| • | Key-in-ignition |
| • | Ignition ON |
| • | Park lamps ON |
| • | Keyless entry or remote start message |
The BCM will enter a sleep state when all of the following conditions exist:
| • | The ignition switch is OFF, key out. |
| • | No activity exists on the serial data line. |
| • | No outputs are commanded. |
| • | No delay timers are actively counting. |
| • | No wake-up inputs are present. |
If all these conditions are met, the BCM will enter a low power or sleep condition.
The BCM in this vehicle functions as a gateway or translator. The purpose of the gateway is to translate serial data messages between the GMLAN high speed buss and the GMLAN low speed buss for communication between the various modules. The gateway will interact with each network according to that network's transmission protocol.
One example of this necessary communication is the communication between the engine control module (ECM) which is high speed serial data and vehicle theft deterrent (VTD) control module which is low speed serial data. If these modules cannot exchange information, the vehicle will not start.
All communication between the BCM and a scan tool is on the High speed GMLAN serial data circuits. A lost communication DTC typically is set in modules other than the module with a communication failure.
The modules receive the power mode message from the body control module (BCM) over the serial data circuits, indicating when the retained accessory power (RAP) power mode is current. The BCM monitors the ignition switch position, battery condition and passenger compartment doors status to determine whether RAP should be initiated. The modules then support the operation of the systems under their control as required by their RAP power mode operation. Components and systems that are active in RAP are also activated anytime the ignition is any position other than OFF.
RAP will end when one of the following conditions is met:
| • | The BCM receives an input indicating the opening of any passenger compartment door after the ignition key is out of the ignition. |
| Important: The only door that will turn off the radio during RAP is the driver door. This is a function of the radio and will still turn off after the time limit. |
| • | The BCM internal timer for the RAP expires after approximately 10 minutes. |
| • | The BCM detects a decrease in battery capacity below a prescribed limit. |
The BCM then sends a serial data power mode message ending the RAP function.
The BCM keeps the RAP relay energised during all power modes, except Off-Awake and Crank. The relay is also energised for approximately 10 minutes after shutting the ignition OFF and removing the key, providing no door is opened.
The devices powered by the accessory relay during the retained accessory power (RAP) power mode are the sunroof, power window switches and BTSI/Park Lock.
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