Body Control Modules: Overview GMC Yukon Denali

DESCRIPTION

Note. This article applies to "new style" "C" & "K" series vehicles equipped with 4.3L, 4.8L, 5.3L and 6.0L engines. For "old style" "C" & "K" series vehicles, see BODY CONTROL MODULES -- ESCALADE, TAHOE & YUKON DENALI 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™ 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. Automatic Transfer Case (ATC) module.
4. Powertrain Control Module (PCM)/Vehicle Control Module (VCM).
5. Inflatable restraint Sensing and Diagnostic Module (SDM).
6. Electronic Brake Control Module (EBCM).
7. Suspension Control Module (SCM).
8. Heating, Ventilation and Air Conditioning (HVAC) control module.

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.

The BCM provides a constant 5-volt output reference voltage to the ambient light sensor. The BCM monitors the output voltage and sets a DTC when voltage is not 4.75-5.25 volts. The conditions for setting DTC are

1. System voltage is 9-16 volts.
2. Ignition on.
3. Parking brake released.
4. Headlight switch is in AUTO position.
5. Voltage on reference circuit is less than 4.75 volts for .5 second (DTC B1397).
6. Voltage on reference circuit is more than 5.25 volts for .5 second (DTC B1398).

When DTC is set, Daytime Running Lights (DRL) will not operate.

The battery rundown protection function allows the BCM to disable courtesy lights, cargo lights and underhood light 40 minutes after ignition is turned off. The BCM monitors the output of the inadvertent power relay on Orange wire at BCM connector C2 terminal B12.

When BCM commands the inadvertent power relay on and there is no voltage at BCM connector C2 terminal B12 for .5 second, DTC will be set. DTC will not set if system voltage is not 9-16 volts. When DTC is set, courtesy lights, cargo lights and underhood light will be disabled.

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.

When a critical operating parameter is not received by a module, the module uses a default value for that parameter. When a module does not associate an identification number with at least one critical parameter within about 5 seconds of beginning communication, DTC U1000 or U1255 is set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.

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 UXXX (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

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.