Abbreviations are used in this section to represent the following components:

The body control module (BCM) controls various vehicle electrical systems or features from one central module, instead of using individual modules for each system or feature.

The BCM is located beneath the instrument panel cluster (IPC) to the right of the steering column and is mounted in a flat, horizontal position. The BCM fitted to various levels of vehicles is physically the same, but with different functions in the BCM software.

Important: Disconnect and remove all vehicle modules, after disconnecting the battery, before performing any welding anywhere on the vehicle. Failing to do this can cause permanent damage to the modules.

If the BCM is to be replaced, ensure that the correct BCM is used. The last 3 digits of the BCM part number identify the BCM.

The engine control module (ECM) uses GMLAN communication, while all other control modules, including the body control module (BCM), communicate serial data in universal asynchronous receiver and transmitter (UART). The GMLAN communication is more sophisticated than UART and as such, requires a powertrain interface module (PIM) to convert GMLAN into UART. In simple terms, the ECM communicates in a different language than the other control modules and therefore requires the PIM to translate information into the same language as the other control modules.

The PIM is mounted behind the passenger-side shroud lower trim assembly. Specific software has been developed for use with the scan tool to assist with fault-finding vehicle electrical systems, including the various BCM functions and controls. Scan tool connection for BCM serial data communication is via the data link connector (DLC), attached to the lower right-hand trim of the instrument panel (I/P), to the right of the steering column.

A pull-up resistor has one end connected to a voltage and the other end connected to a switch. The pull-up resistor allows the body control module (BCM) to sense whether a switch is open or closed. A voltage is applied to the switch through the pull-up resistor so that when the switch closes there is not a short-circuit between the power and ground.

The BCM registers the voltage at the bottom of the pull-up to determine if the switch is open or closed. When the switch is open, there is no current flowing in the resistor and therefore no voltage drop occurs across the resistor so the voltage sensed by the BCM is the supply voltage. When the switch closes, a current flows in the pull-up resistor causing a voltage drop across the resistor. Because the bottom of the pull-up resistor is shorted to ground, the voltage at the bottom of the pull-up resistor is 0 volt. This indicates to the BCM that the switch is closed.

A pull-down resistor has one end connected to a switch and the other end connected to ground. The pull-down resistor allows the body control module (BCM) to sense whether a switch is open or closed. When the switch is closed, a voltage is applied to the circuit and current flows through the pull-down resistor so there is no short-circuit to ground.

The BCM registers whether the switch is open or closed by measuring the voltage at the top of the pull-down resistor. When the switch is closed, a current flows through the pull-down resistor, which causes a voltage drop across the resistor. Because the top of the pull-down resistor is connected to 12 volts via the switch, the voltage at the top of the resistor is 12 volts. When the switch is open, no current flows in the pull-down resistor and the voltage at the top of the resistor is then 0 volt.

A pulsed pull-up circuit is similar to a conventional pull-up circuit, however the pull-up resistor has its voltage supply end connected to the battery voltage via a switching transistor. At the output terminal side of the pull-up resistor, circuits such as the door ajar switches, door locks, power windows, etc. are typical circuits using pulsed pull-up inputs to the body control module (BCM).

The purpose of the pulsed pull-up circuit is to briefly sample if a switch connected to it is closed or not. By using only a small of amount of sampling time in which the switching transistor is turned ON, the circuit thereby reduces the current flowing in this type of circuit. When many of these circuits are sampled in the same manner, the overall battery current is reduced considerably. This current reduction has quite a significant effect on battery life when the vehicle is not being used for extended periods. In operation, if the externally connected switch is closed continuously, no current flows through the pull-up resistor because the switching transistor has not been turned ON by the microprocessor. Periodically, approximately 100 sampling pulses per second, the switching transistor is turned ON for a brief time to enable current to flow in the resistor. This action then allows the microprocessor input circuit to sense whether there is voltage across the switch contact or not. Once it has made this measurement, the sampling pulse is removed from the switching transistor and current ceases to flow in the resistor until the next sampling pulse repeats the above process.

The vehicle system control modules, as well as the TECH 2 diagnostic scan tool, communicate with each other via the serial communication lines using serial data. Serial data transfers information, one bit at a time, over a single line called the data bus. Except for the engine control module (ECM), all control modules communicating on the data bus use universal asynchronous receiver and transmitter (UART) communication. The UART is a 5-volt data line that toggles the voltage to ground (0 volt) at a fixed bit pulse width during communication. The UART transmits data at the rate of 8.2 kilobits per second (8192 bits per second). In UART communications, when there is no communication on the data line, the system voltage is 5 volts.

The body control module (BCM) battery saver mode provides vehicle battery protection through reduced current consumption. After a preset delay period of 15 minutes has elapsed, as controlled by the shut down timer, the BCM:

If the BCM is in battery saver mode, all the BCM inputs are re-enabled when:

To provide additional battery protection, the vehicle is delivered with the body control module (BCM) in pre-delivery mode. In this mode the battery saver period is set to 3 minutes. Pre-delivery mode is disabled once the vehicle has travelled for a total of 30 minutes at speeds above 20 km/h (12 mph). This value is estimated to be the equivalent period prior to customer delivery. Pre-delivery mode may be enabled or disabled by using the trip computer buttons and multifunction display (MFD). Pre-delivery mode may also be enabled and disabled using a scan tool.