This vehicle is equipped with a body control module (BCM) capable of performing various body control functions. The following is a list of operations and features of the BCM controls:

The battery guard feature provides protection from excessive battery drain caused by high current loads being accidently left on. This protection is accomplished by controlling the battery power to the selected modules after each ignition cycle.

The body control module (BCM) provides power regulation through the use of the battery guard relay to the following components:

The battery guard relay supplies the above components with power whenever the BCM is awake (Controlled Power Mode) and for 15 minutes after the ignition transitions to the LOCK position (High Power Mode).

Wake-ups are signals that turn the BCM ON and cause the BCM to begin active control and/or monitoring. During the controlled power mode, the BCM is awake and controls the battery guard relay output. Battery power is supplied to the above components during this mode.

When the ignition is switched to the LOCK position, the BCM enters the high power mode. During the high power mode, the BCM controls the battery guard relay output for 15 minutes, still supplying battery power to the selected components.

Before the 15 minute time period expires, the BCM issues a controlled power shutdown report on the Class 2 data bus indicating that the BCM will switch off the high power output.

The 15-minute time period may restart when one of the following occurs:

The retained accessory power (RAP) feature allows the use of the entertainment/phone system after the ignition is turned OFF. The BCM controls this feature via the Class 2 serial data circuit.

The RAP feature is enabled when the ignition is in the ON position. RAP will not enable, however, when the door open and the ignition is turned to the LOCK position.

The RAP feature terminates when one of the following occurs:

Replace the BCM only if diagnostics in this section specifically direct you. Most system problems are traceable to faulty wiring, to connectors, or to components. The BCM itself is very reliable and is not likely the cause of a fault. Do NOT replace the BCM before performing a complete diagnosis or recurrence of the fault will likely occur. Note: Every input to the BCM is short to ground and short to battery protected. Every output from the BCM is short to ground and short to battery protected. Therefore, the BCM does not automatically need to be replaced when a short is found in the wiring connected to the BCM . Repair the wiring first, then test the system for any further problem.

When you replace a BCM, program the BCM with the Vehicle Identification Number (VIN) of the vehicle receiving the new BCM. The RadioTheftlock feature uses the VIN in order to verify that the radio has not been removed and improperly installed in another vehicle.

Use a scan tool in order to program the BCM . Enter the vehicle's VIN in the scan tool and then transmit the VIN via a Class  2 physical message to the BCM. The BCM stores the VIN in EEPROM. The first communication between the BCM and radio re-arms the radio theftlock feature. In order to set up a new BCM, select SETUP NEW BCM from the BCM Special Functions menu and follow the instructions on the screen.

The BCM and the radio communicate via Class 2 functional messages. When the ignition is switched to the ON position, the radio requests part of the VIN from the BCM. The BCM reports back the requested VIN to the radio where the information is compared to a VIN stored in the radio EEPROM.

If the VIN from the BCM does not match, the radio disables all entertainment functions and when the ignition is ON displays a message on the VF display that it is in the LOCKED mode. The radio remains in this mode until it receives a VIN that matches the original VIN stored in EEPROM.

When the radio is replaced, the location in the radio EEPROM, reserved for the VIN, must be in the default mode. If not, a scan tool must be used to reset it. In the default mode, the first communication between the BCM and radio re-arms the Radio Theftlock.

In the event there is a power loss to the BCM or radio, the Radio Theftlock re-arms after the first communication between the two devices, after power is restored. If Class 2 communication does not resume, the radio will go into the LOCKED mode.

OnStar is a new on-vehicle communications system which is a dealer installed option. The BCM acts as the interface between the OnStar and the vehicle architecture. It receives the following Class 2 messages from the Vehicle Interface Unit (VIU).

The BCM sends Class 2 messages to the VIU on:

The BCM is hard wired to the Inflatable Restraint Sensing Module. When you switch the ignition to the ON position, the restraint module sends voltage pulses as a test message and verifies the circuit to the BCM. In the event of a restraint system activation (in this case an airbag deployment), the BCM will continue to receive the voltage pulses but at a greater number and repetitive rate. The BCM then sends a Class 2 message to the VIU that a restraint system has activated.