The body control module (BCM) controls a number of functional systems. The BCM is connected to the class 2 serial data line. Many control signals are implemented by class 2 messages.

The BCM functions include the following:

The power mode is the information used by the various control modules on the vehicle to determine operation. If a control module does not receive a power mode signal, either a serial data message or a hard wire input, the control module does not operate.

No operations using two or more modules is possible without a power mode decision by the power mode master (PMM) and a power mode message from the PMM. The possible power modes are:

The power mode signal may be as simple as a battery positive voltage input wired to a particular ignition switch contact. If this is also the battery positive voltage supply to the module/device, the module/device will only operate with the ignition contact closed to battery positive voltage. An example of this is the starter relay when it is wired directly to the CRANK/START contact of the ignition switch. When the CRANK/START contacts are closed, the starter relay is energized and provides a current source to the starter and starter solenoid. When the ignition switch leaves the CRANK/START position, the switch contacts open and the starter relay is de-energized. This removes the current source from the starter and solenoid and the starter operation stops.

On vehicles that have several control modules connected by serial data circuits, one module is the power mode master (PMM). On this vehicle the PMM is the body control module (BCM). The PMM receives three signals/circuits from the ignition switch. These are the Unlock IGN0, Run/Crank IGN1 and Crank ignition switch signals/circuits.

To determine the correct power mode the PMM uses:

The chart below indicates the modes detected and transmitted by the PMM:

Since the operation of the vehicle systems depends on the power mode, there is a fail-safe plan in place should the PMM fail to send a power mode message. The fail-safe plan covers those modules using exclusively serial data control of power mode as well as those modules with discrete ignition signal inputs.

Serial Data Messages

The modules that depend exclusively on serial data messages for power modes stay in the state dictated by the last valid PMM message until they can check for the engine run flag status on the serial data circuits. If the PMM fails, the modules monitor the serial data circuit for the engine run flag serial data. If the engine run flag serial data is true, indicating that the engine is running, the modules fail-safe to RUN. In this state the modules and their subsystems can support all operator requirements. If the engine run flag serial data is false, indicating that the engine is not running, the modules fail-safe to OFF-AWAKE. In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.

Discrete Ignition Signals

Those modules that have discrete ignition signal inputs also remain in the state dictated by the last valid PMM message received on the serial data circuits. They then check the state of their discrete ignition input to determine the current valid state. If the discrete ignition input is active, battery positive voltage, the modules will fail-safe to the RUN power mode. If the discrete ignition input is not active, open or 0 voltage, the modules will fail-safe to OFF-AWAKE. In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.

The power management function is designed to monitor the vehicle electrical load and determine when the battery is potentially in a high discharge condition. This is accomplished by using a high accuracy battery voltage reading as an indicator of battery discharge rate. The following six levels of load management will execute in the load management control algorithm when there is a high discharge condition:

1. The first action requests a vehicle idle speed increase to the powertrain control module (PCM) in order to raise alternator output.
2. The second action requests a greater vehicle idle speed increase to the PCM in order to raise alternator output.
3. The third action begins to shed vehicle loads in an attempt to remedy the heavy discharge condition.
4. The fourth action requests another vehicle idle speed increase to the PCM in order to raise further the alternator output.
5. The fifth action begins to shed further vehicle loads in an attempt to remedy the heavy discharge condition.
6. If the above five corrective actions fail, the sixth action of power management further sheds loads in a final attempt to remedy the high discharge condition.

Loads subject to reduction include the following:

The power mode master (PMM) calculates the battery temperature, voltage and charging rate at all times while the engine is running. The PMM calculates the battery temperature by factoring in:

If the battery temperature is below set limits, the PMM institutes steps to control the load.

The PMM calculates the voltage of the battery by making constant measurements and using the measurements to calculate the true battery voltage. If the PMM detects a low voltage, the PMM institutes steps to control the load.

The PMM calculates the discharge rate, or draw, on the battery by making constant measurements and using the measurements to calculate the discharge rate in amp/hours. If the PMM detects a high current draw from the battery, the PMM institutes steps to control the load.

The PMM will either request an increase in the engine idle speed to the PCM or the PMM will cycle or turn off loads, called the load-shed function, in order to preserve the vehicle electrical system operation. The criteria used by the PMM to regulate this electrical load management are outlined below:

Each load management function, either idle boost or load-shed, is discrete. No two functions are implemented at the same time.

During each load management function, the PMM checks the battery temperature, battery voltage and amp-hour calculations and determines if the PMM should implement a different power management function.

Idle Boost Functions

The PMM sends a serial data request to the PCM to increase the idle speed. The PCM then adjusts the idle speed by using a special program and idle speed ramp calculations in order to prevent driveability and safety concerns. The idle speed boost and cancel function will vary from vehicle to vehicle and from one moment to another on the same vehicle. This happens because the PCM responds to changes in the inputs from the sensors used to control the powertrain.

Load Shed Function

The PMM executes the load shed function, by controlling the relay coil or the inhibit circuit of the following devices.