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 via class 2 messages. The BCM, which is the power mode master (PMM), is responsible for sending the power mode messages on the serial data line to other modules. The BCM functions include the following:

The power mode is the information used by the various control modules on the vehicle to determine operation. The power mode reflects the ignition switch position as well as any power modes that are customer convenience items, i. e. retained accessory power (RAP). 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.

Simple Power Mode Example

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 Accessory (ACC) 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 PMM calculates the battery temperature, voltage and charging rate at all times while the engine is running. The BCM 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.

One of the highest loads on the electrical system is the resistance load of heating elements. The PMM controls the heating elements in the outside rear view mirrors and the rear window, as well as the heated seats, by controlling the relay coil controlling power to these devices.

The PMM will either request an increase in the engine idle speed from the PCM or the PMM will 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.

The battery guard function provides protection from battery drain. The system includes the following functions:

The wake-ups are signals that turn ON the BCM. The signals cause active control or monitoring by the BCM. The BCM wake-ups can include the following signals:

Controlled Power consists of the following modes:

The BCM executes the high power mode by grounding the controlled power relay coil control circuit of the CONTROLLED POWER relay whenever the BCM is awake. This circuit also is grounded for 15 minutes after you turn the ignition switch to the LOCK position in order to obtain the RAP function. After 15 minutes elapse, the BCM enter a sleepstate and transitions to the low power mode. The components under controlled power then receive any necessary voltage from the battery positive voltage circuit of the BCM. If the BCM is in the low power mode, and the BCM detects that the current exceeds approximately 2 amperes, the BCM wakes up and re-enters the high power mode. If the vehicle remains inactive with the ignition in the LOCK position and without any BCM wake-ups for a period of 3 days, the BCM deactivates the low power mode. The following components are under controlled power:

After 10 minutes, the BCM turns off inadvertent power in order to preserve battery power. The 10 minute timer starts after one of the following conditions is present:

If any of these conditions occur during the timer period, the timer resets to 10 minutes.

Storage mode allows the vehicle to be stored for long periods of time. The storage mode is enabled through the DIC. 20 minutes after the storage mode has been enabled, the BCM disables both the low power and the high power relays. RCDLR functions are not available during storage mode and the BCM will set DTC B2250 as history.