General description and operation of the body control system (Chevrolet Captiva 1)

Contents: Power mode master device ↧ Serial Link Power Mode Master ↧ PMM Power Mode Parameters ↧ BCM states awake/asleep ↧ Gateway ↧ Serial communication control of… ↧ Relay control of auxiliary power… ↧ Body systems management ↧

The body control system consists of the body control module (BCM), communication lines, and various input and output circuits. Some input and output circuits and commands require communication with the BCM. The BCM also provides discrete input and output terminals to control body functions. Battery positive voltage is supplied to the BCM from the BCM fuse in the engine fuse block. The module is connected to ground at two points on the GX201 and also has one wire directly connected to the vehicle battery. The BCM is connected to and acts as a gateway between the high-speed GMLAN serial data bus and the low-speed GMLAN serial data bus. If the BCM does not communicate, the engine will not start because the engine/powertrain controller (ECM/PCM) and the vehicle theft deterrent (VTD) module cannot communicate without the BCM acting as a gateway.

Power mode master device

The BCM on these vehicles operates as a Power Mode Master (PMM). The ignition switch is a low current switch that provides several discrete ignition switch signals to the PMM to determine the power mode, which are transmitted over the serial communication circuits to other controllers that require this information, and the PMM thus activates relays and other direct outputs of the PMM as needed.

Serial Link Power Mode Master

The power supply to many of the circuits in this vehicle is controlled by this controller, which functions as the power mode master (PMM). On these vehicles, the PMM function is performed by the body control module (BCM). The ignition switch is a low-current switch that provides several discrete ignition switch signals to the PMM to determine the power mode, which are transmitted over the serial communication circuits to other controllers that require this information. The PMM activates relays and other direct outputs of the PMM as needed. The PMM determines which power mode is required (OFF, AUXILIARY, RUNNING, START REQUEST), and transmits this information to the other controllers via a serial link. Controllers that have switched voltage inputs may operate in the original mode if the serial link commands from the PMM do not match the information that the individual controllers can receive via their connections.

The PMM receives signals from the ignition switch to determine the driver's desired power mode. The following PMM Power Mode Parameter Table illustrates the correct state of these input parameters (circuits) according to the ignition switch position.

PMM Power Mode Parameters

Ignition switch position	Transmitted power mode	Ignition Off/Run/Start (off/run/start voltage circuits)	Auxiliary ignition/Start (auxiliary voltage circuit)	Ignition run/start (voltage circuit "Ignition 1")
Off, key removed	Off	Key removed/auxiliary	Inactive	Inactive
Off, key inserted (LOCK - locked)	Off	Key in/out	Inactive	Inactive
ACC	Auxiliary	Key inserted/ACC (auxiliary)	Active	Inactive
ON	Job	Job	Active	Active
START	Launch request	Start (cranking)	Inactive	Active

BCM states awake/asleep

The Body Control Module (BCM) can control all BCM functions or perform them while in the wake mode. The BCM enters the sleep mode when active control or normal monitoring of system functions has ceased and a set time has elapsed. The BCM must detect certain wake-up inputs before entering the wake state. The BCM monitors these inputs while in the sleep mode.

The BCM enters the wake mode when one of the following wake-up signals is detected:

Commands via serial line.
Battery connection detection.
Signal for opening any door.
Turning on the headlights.
The key is in the ignition.
Turn on the ignition.
Turning on the parking lights.
Activation of remote control of locks or remote start command.

The BCM enters sleep mode when one of the following conditions exists:

The ignition switch is off, the key is removed.
No commands are received over the serial line.
There are no commands for output chains.
There are no delay timers in active countdown mode.
There are no wake-up input signals.

If all of these conditions are met, the BCM goes into a low power mode, or sleep mode.

Gateway

The BCM on this vehicle functions as a gateway or converter. The gateway is designed to transmit serial line commands between the high-speed GMLAN bus and the low-speed GMLAN bus to exchange data between the different controllers. The gateway communicates with each network according to each network's transmission protocol.

One example of such necessary data exchange is the communication between the engine controller (ECM), connected to a high-speed serial communication line, and the vehicle theft deterrent (VTD) controller, connected to a low-speed serial communication line. If these controllers cannot exchange data, the vehicle's engine will not start.

All communication between the BCM and the scan tool is done over the high speed GMLAN serial data link. The Communication Lost DTC is usually set in a different module than the one that is experiencing the communication failure.

Serial communication control of Auxiliary power hold mode (RAP)

The controllers receive a power mode message from the body control module (BCM) via serial communications, indicating that the hold auxiliary power (RAP) mode has been established. The BCM monitors the ignition switch position, battery status, and passenger door position to determine whether RAP mode is necessary. At the same time, the controllers maintain the operation of the systems they control in accordance with the RAP power mode requirements. Components and systems active in RAP mode are also active any time the ignition switch is turned to any position other than the OFF position.

RAP mode is terminated when one of the following conditions is met:

The BCM receives an input signal indicating that any passenger door has been opened after the key has been removed from the ignition switch.

Important: The only door that turns off the radio in RAP mode is the driver's door. This is a function of the radio and it turns off after a set time.

The BCM's internal timer for the RAP mode will expire in approximately 10 minutes.
The BCM detects that the battery capacity has dropped below a set limit.

The BCM then transmits a RAP end message over the serial line.

Relay control of auxiliary power (RAP) hold mode

The BCM maintains voltage at the RAP relay at all times during all power modes except the off-wake and start states. This relay is also energized for approximately 10 minutes after the ignition is turned off and the key is removed if no doors are opened.

Devices supplied with power from the auxiliary relay in the Hold Auxiliary Power (RAP) mode include the sunroof, window switches and BTSI/parking lock.

Body systems management

The various input and output circuits of the BCM are described in the corresponding functional sections indicated on the BCM electrical diagrams. Systems associated with the BCM include, but are not limited to, the following: