Body Control Modules - Old Style: Overview GMC Yukon Denali

DESCRIPTION

Note. This article applies to "old style" "C" & "K" Series vehicles equipped with 5.7L, 6.5L and 7.4L engines. Old style pickups are not equipped with a Body Control Module (BCM). For "new style" "C" & "K" Series vehicles equipped with 4.3L, 4.8L, 5.3L and 6.0L engines, see BODY CONTROL MODULES -- SIERRA, SILVERADO, SUBURBAN, TAHOE, YUKON & YUKON XL article.

Body Control Module (BCM) is designed to process various input information it receives from sensors and switches or borrows from other vehicle systems connected to class 2 serial data line. BCM evaluates this information and controls certain body systems by commanding an output on or off.

BCM can also command other vehicle systems to control functions by sending specific messages over class 2 serial data line. BCM controls operation of audible warnings, indicator warnings, power door locks, remote keyless entry, Passlock(tm) theft deterrent, content theft deterrent, interior and exterior illumination.

BCM goes into low power mode (asleep state) when ignition has been turned off and 20 minutes have elapsed without any wake up signals. BCM goes into high power mode (wake up state) after receiving any wake up signals.

The class 2 serial data line allows communication between modules and scan tool. Modules on the class 2 serial data line are

1. Body Control Module (BCM).
2. Instrument Panel Cluster (IPC).
3. Transfer Case Control Module (TCCM).
4. Powertrain Control Module (PCM)/Vehicle Control Module (VCM).
5. Supplemental Inflatable Restraint (SIR) Module.
6. Electronic Brake Control Module (EBCM).

Modules connected to class 2 serial data line monitor serial data communications during normal vehicle operation. Operating information and commands are exchanged between modules. Connecting scan tool to Data Link Connector (DLC) allows communication with modules for diagnostics. DTCs may be set due to this symptom and during testing procedure. Complete testing procedure to ensure that all DTCs are diagnosed and cleared.

DTC 12 is always displayed as the first code, indicating diagnostic display is in progress. If DTCs do not display, go to Testing . If DTCs display, go to appropriate diagnostic procedure. See BCM DTC DEFINITIONS table under SELF-DIAGNOSTIC SYSTEM.

Body Control Module (BCM) provides a 12-volt output to courtesy lights feed circuit when commanded by instrument panel dimmer switch, door jamb switch or rear compartment lid ajar indicator switch inputs.

DTC 21 will set when battery circuit is open due to a problem with COURTESY fuse No. 8, faulty harness or connector, courtesy lights feed circuit is shorted to ground for more than 50 milliseconds while BCM is providing input on circuit, or BCM is defective.

When DTC 21 is set, courtesy lights will not come on and instrument panel dimmer switch will not operate. Glove box and center console lights will also be inoperative. DTC 21 will clear when fault is corrected and memory is cleared.

Body Control Module (BCM) provides a ground path on courtesy lights return circuit when commanded by instrument panel dimmer switch, door jamb switch or rear compartment lid ajar indicator switch inputs.

DTC 22 will set when courtesy lights return circuit is shorted to battery for more than 50 milliseconds while BCM is providing ground for circuit.

When DTC 22 is set, courtesy lights will not come on and instrument panel dimmer switch will not operate. Glove box and center console lights will operate normally. DTC 22 will clear when fault is corrected and memory is cleared.

Retained Accessory Power (RAP) is enabled when ignition switch is turned from RUN to OFF position. RAP will continue to supply power to power windows, radio and front and rear wipers for up to 10 minutes after ignition switch is turned to OFF position. RAP will shut off if any door is opened during this 10 minute period. Body Control Module (BCM) provides RAP function through RAP feed circuit to PWR WDO circuit breaker (25-amp), RADIO fuse (25-amp), WIPERS fuse (25-amp) and RR WIPER fuse (25-amp) located in instrument panel fuse block. See COMPONENT LOCATIONS .

DTC 23 will set when BCM detects RAP output on RAP feed circuit is either shorted to ground with RAP enabled or shorted to battery voltage with RAP disabled for more than 50 milliseconds. Code will also set if there is an open in Red wire between Black 6-pin BCM connector C2 terminal "F" and IGNB fuse (50-amp) located in underhood fuse block.

When DTC 23 is set, RAP will not operate (shorted to ground) or RAP will not deactivate when door is opened (shorted to battery). DTC 23 will clear when fault is corrected and memory is cleared.

Body Control Module (BCM) provides a 12-volt output to fasten seat belt indicator, located on instrument cluster, while driver's seat belt is unfastened.

DTC 24 will set when BCM detects fasten seat belt output is either shorted to ground or shorted to voltage for more than 50 milliseconds.

When DTC 24 is set, fasten seat belt indicator will not come on when ignition is on and driver's seat belt is not fastened, or fasten seat belt indicator is always illuminated. DTC 24 will clear when fault is corrected and memory is cleared.

Body Control Module (BCM) provides a 12-volt output to illuminate SECURITY indicator light.

DTC 25/35 will set when BCM detects SECURITY indicator light circuit is either open or shorted to ground for more than 50 milliseconds.

Digital Radio Frequency (RF) message received from Remote Keyless Entry (RKE) system transmitter was valid, and all security checks passed.

DTC 41 will set when RKE system transmitter has been in programming mode out of range of Body Control Module (BCM), or when system transmitter is defective.

Digital Radio Frequency (RF) message received was from a Remote Keyless Entry (RKE) system transmitter not programmed to Body Control Module (BCM). Remote function capabilities will not exist.

DTC 42 will set when RKE system transmitter was used without programming it to BCM. If transmitter was already programmed, transmitter may be defective. DTC 42 will also set if another transmitter was operated in the same area.

BCM should be placed in RKE system programming mode. Both transmitters must be programmed to BCM simultaneously.

The digital RF message received was from a Remote Keyless Entry (RKE) system transmitter that is programmed to Body Control Module (BCM), but message did not pass sumcheck security test. No remote function capabilities will exist.

DTC 43 will set when remote keyless entry system transmitter is out of synchronization with BCM. Fault may be caused by transmitter buttons being pressed more than 256 times while out of range of BCM. Fault may also be caused by transmitter battery being changed and buttons being pressed more than 10 times while out of range of BCM. Fault may also be present if digital RF message has an error due to outside noise from surrounding environment, or if transmitter is defective.

Most likely cause is that RKE system transmitter is out of synchronization. To synchronize transmitter, press LOCK and UNLOCK buttons on transmitter simultaneously for 5 seconds.

The digital RF message received was from a Remote Keyless Entry (RKE) system transmitter that is programmed to Body Control Module (BCM), but message did not pass encryption security test. No remote function capabilities will exist.

DTC 44 will set when RKE system transmitter has been in program or re-synchronize mode while out of range of BCM. Fault may also be present if digital RF message has an error due to outside noise from surrounding environment, or if transmitter is defective.

Most likely cause is that RKE system transmitter is out of synchronization. To re-synchronize transmitter, press LOCK and UNLOCK buttons on transmitter simultaneously for 5 seconds.

At least 50 consecutive digital RF messages were received from a Remote Keyless Entry (RKE) system transmitter that is programmed to Body Control Module (BCM), but message did not pass encryption security test. BCM will cease processing any RKE system transmitter digital RF messages (good or bad) for 20 seconds. No remote function capabilities will exist.

DTC 45 will set when RKE system transmitter has been in program or re-synchronize mode while out of range of BCM. Fault may also be present if transmitter is defective.

Most likely cause is that RKE system transmitter is out of synchronization with BCM. To re-synchronize transmitter, press LOCK and UNLOCK buttons on transmitter simultaneously for 5 seconds.

DTC 55 is displayed to indicate the next 2 DTCs displayed are Body Control Module (BCM) hardware and software configurations. First number following DTC 55 is the hardware configuration. The second number is the software configuration. DTC 55 and both configuration numbers will always be displayed. After configuration numbers are displayed, entire DTC list will repeat beginning with DTC 12.

Modules connected to the class 2 serial data line monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged between modules. In addition, node alive (state of health) messages are transmitted by each module on the class 2 serial data line about once every 2 seconds.

When a module detects no messages and a low voltage on the class 2 serial data line for about 3 seconds, DTC U1300 is set, the setting of all other class 2 communication DTCs in inhibited and the module uses default values for all parameters received on the class 2 serial data line. A current DTC U1300 will clear when a valid class 2 message is detected on serial data line or at the end of the current ignition cycle. A history DTC U1300 can only be cleared using scan tool.

Modules connected to the class 2 serial data line monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged between modules. In addition, node alive (state of health) messages are transmitted by each module on the class 2 serial data line about once every 2 seconds.

When a module detects no messages and a high voltage on the class 2 serial data line for about 3 seconds, DTC U1301 is set, the setting of all other class 2 communication DTCs in inhibited and the module uses default values for all parameters received on the class 2 serial data line. A current DTC U1301 will clear when a valid class 2 message is detected on serial data line or at the end of the current ignition cycle. A history DTC U1301 can only be cleared using scan tool.

Modules connected to class 2 serial data line monitor serial data communications during normal vehicle operation. Operating information and commands are exchanged between modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State Of Health (SOH) monitoring (node alive messages).

Once an identification number is learned by a module, it will monitor for that module's node alive message. Each module on the class 2 serial data line which is powered and performing functions that require detection of a communications malfunction is required to send a node alive message every 2 seconds.

When no message is detected by from a learned identification number for 5 seconds, DTC U1XXX (where XXX is equal to the identification number) is set. Use DTC ID NUMBER table to determine which module is not communicating.

When more than one loss of communication DTC is set in one or more modules, perform diagnosis in the following order

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

DTC ID NUMBER