Body Control Modules: Overview Chevrolet Blazer S15 facelift

DESCRIPTION

Body Control Module (BCM) performs the following functions: exterior and interior lighting control, battery rundown protection, retained accessory power, chime, instrument cluster indicator control, power door locks, remote keyless entry and theft deterrent.

BCM communicates with components using class 2 data line. Class 2 data line is used by BCM to communicate with the Driver Information Display Module, Driver Seat Adjuster Memory Module, Electronic Brake Control Module (EBCM), inflatable restraint Sensing and Diagnostic Module (SDM), instrument cluster, Powertrain Control Module (PCM), Transfer Case Shift Control Module and Vehicle Interface Unit (VIU). Class 2 serial data uses the peer-to-peer method and message arbitration to manage data line traffic.

Signals that activate the BCM are known as "wake-up" signals. BCM wake-up signals include: door ajar switches, door handle switches, dome light, parking lights, key in ignition, key removed from ignition, key in door lock, when ignition switch is turned to RUN position, or inadvertent power.

OPERATION

Battery rundown protection (or battery guard) feature provides protection from excessive battery drain caused by high current loads that are accidentally left on. When ignition is turned off, a time-out period is started. If odometer reads less than 15 miles, time-out period will last for 3 minutes. If odometer reads more than 15 miles, time-out period will last for 20 minutes. When time-out expires, inadvertent power output is turned off.

Each time ignition switch is turned from ACC to RUN position, BCM controls an instrument cluster bulb check for a calibrated amount of time. During this bulb check, instrument cluster will perform a WOW (gauge needle sweep from low to high to low positions) check.

When BCM activates an indicator in the instrument cluster, it may also produce a chime. The following chimes are provided: key-in-ignition, headlights on, unfastened seat belt, turn signal on, parking brake warning, door ajar warning and check gauges.

BCM runs self-check program to determine an internal fault when power up is commanded. When DTC is set, module refuses all inputs. DTC is cleared when microprocessor makes a successful write to EEPROM and check sum.

BCM provides and monitors 5 volt reference voltage to ambient light sensor from connector C3 terminal A4 (Gray wire). DTC B1397 will set when system voltage is 9-16 volts, ignition is on, parking brake is released, headlight switch is in AUTO position and output is not 4.75-5.25 volts. When DTC is set, Daytime Running Lights (DRL) will not operate. DTC B1397 can be cleared using scan tool, or history DTC will disappear after 100 ignition cycles if fault conditions do not reoccur.

BCM provides and monitors 5 volt reference voltage to ambient light sensor from connector C3 terminal A4 (Gray wire). DTC B1398 will set when system voltage is 9-16 volts, ignition is on, parking brake is released, headlight switch is in AUTO position and output is not 4.75-5.25 volts. When DTC is set, Daytime Running Lights (DRL) will not operate. DTC B1398 can be cleared using scan tool, or history DTC will disappear after 100 ignition cycles if fault conditions do not reoccur.

Retained Accessory Power (RAP) function allows radio, power windows and sunroof to operate with doors closed and ignition off for up to 20 minutes. DTC will set when there is no voltage on Black wire at BCM connector C1 terminal A7 with RAP relay commanded off. DTC B1477 can be cleared using scan tool, or history DTC will disappear after 100 ignition cycles if fault conditions do not reoccur.

Retained Accessory Power (RAP) function allows radio, power windows and sunroof to operate with doors closed and ignition off for up to 20 minutes. DTC will set when there is battery voltage on Black wire at BCM connector C1 terminal A7 with ignition switch in RUN position. DTC B1478 can be cleared using scan tool, or history DTC will disappear after 100 ignition cycles if fault conditions do not reoccur.

Battery rundown protection function allows BCM to disable courtesy lights, cargo lights and underhood light 40 minutes after ignition is turned off. DTC will set when BCM commands the inadvertent power relay on but voltage does not exist at relay output to BCM. DTC B1480 can be cleared using scan tool, or history DTC will disappear after 100 ignition cycles if fault conditions do not reoccur.

During normal vehicle operation, each module communicating on class 2 serial data line sends a State Of Health (SOH) message every 2 seconds to ensure system is operating properly.

DTC U1000 will set if any class 2 module does not receive a SOH message from another class 2 device for 5 seconds. No driver warning message will be displayed. DTC U1000 will only set if DTCs U1300 or U1301 are not current. Under all fault conditions, ignition must be cycled before DTC can change from current to history. DTC U1000 can be cleared using scan tool.

Serial data line is used to communicate information and commands between modules. When a module receives a message for a critical operating parameter (Node Alive message), module records ID number of module which sent message for State Of Health (SOH) monitoring. Once an ID number is learned by a module, it will monitor for that module's Node Alive message. Each module on serial data line is required to send a Node Alive message every 2 seconds. When no message is detected from a learned ID number for 5 seconds, DTC U1XXX (XXX is equal to 3-digit identification number) will set. Use DTC SUFFIX IDENTIFICATION table to determine which system is not communicating. For example, if DTC U1016 is set, PCM is not communicating. A module with a class 2 serial data line malfunction or which loses power during a current ignition cycle will have a loss of communication DTC set by other modules that depend on information from that failed module. Modules that can communicate, will set a DTC indicating module can not communicate.

Current DTC will clear when Node Alive message from failed identification number is detected on class 2 serial data line or at end of current ignition cycle. history DTC can only be cleared using scan tool. If more than one loss of communication DTC is set in either one or multiple modules, perform diagnosis in the following order

1. Current DTCs before history DTCs, unless otherwise indicated in testing procedure.
2. DTC which is reported most times.
3. From lowest number DTC to highest number DTC.

DTC SUFFIX IDENTIFICATION

Modules connected to class 2 serial data circuit monitor for serial data communications during vehicle operation. A State Of Health (SOH) message is communicated from each module to all other modules every 2 seconds. If a module detects low voltage and no SOH messages along class 2 serial data line for approximately 3 seconds, DTC U1300 will set. When DTC U1300 is set, no additional communication DTCs (communication DTCs begin with "U") can set.

Current DTC will clear when valid class 2 message is detected on class 2 serial data line or when ignition is turned off. History DTC will clear only with scan tool.

Modules connected to class 2 serial data circuit monitor for serial data communications during vehicle operation. A State Of Health (SOH) message is communicated from each module to all other modules every 2 seconds. If a module detects high voltage and no SOH messages along class 2 serial data line for approximately 3 seconds, DTC U1301 will set. When DTC U1301 is set, no additional communication DTCs (communication DTCs begin with "U") can set and will use default values for all class 2 communications.

Current DTC will clear when valid class 2 message is detected on class 2 serial data line or when ignition is turned off. History DTC will clear only with scan tool.