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Sending you system discription and operation from the book. It will explain it better than I ever could. This will give you a pretty good uderstanding of how and why the system works. And when it does what. Note, the blue high lights won't open for you. GM access only. Thanks
Electrical Power Management (EPM) Overview
The electrical power management (EPM) system is designed to monitor and control the charging system and send diagnostic messages to alert the driver of possible problems with the battery and generator. This EPM system primarily utilizes existing on-board computer capability to maximize the effectiveness of the generator, manage the load, improve battery state-of-charge (SOC) and life, and minimize the system's impact on fuel economy. The EPM system performs 3 functions:
The battery's condition is estimated during key-off and during key-on. During key-off the SOC of the battery is determined by measuring the open-circuit voltage. The SOC is a function of the acid concentration and the internal resistance of the battery, and is estimated by reading the battery open circuit voltage, when the battery has been at rest for several hours.
The SOC can be used as a diagnostic tool to tell the customer or the dealer the condition of the battery. Throughout key-on, the algorithm continuously estimates SOC based on adjusted net amp hours, battery capacity, initial SOC, and temperature.
While running, the battery's degree of discharge is primarily determined by a battery current sensor, which is integrated to obtain net amp hours.
In addition, the EPM function is designed to perform regulated voltage control (RVC) to improve battery SOC, battery life, and fuel economy. This is accomplished by using knowledge of the battery's SOC and temperature to set the charging voltage to an optimum battery voltage level for recharging, without detriment to battery life.
The Charging System Description and Operation is divided into 3 sections. The first section describes the charging system components and their integration into the EPM. The second section describes charging system operation. The third section describes the instrument panel cluster (IPC) operation of the charge indicator, driver information center (DIC) messages and voltmeter operation.
Charging System Components
The generator is a serviceable component. If there is a diagnosed failure of the generator, it must be replaced as an assembly. The engine drive belt drives the generator. When the rotor is spun, it induces an alternating current (AC) into the stator windings. The AC voltage is then sent through a series of diodes for rectification. The rectified voltage has been converted into a direct current (DC) for use by the vehicle's electrical system to maintain electrical loads and battery charge. The voltage regulator, integral to the generator, controls the output of the generator. It is not serviceable. The voltage regulator controls the amount of current provided to the rotor. If the generator has field control circuit failure, the generator defaults to an output voltage of 13.8 volts.
Body Control Module (BCM)
The body control module (BCM) is a class 2 device. It communicates with the powertrain control module (PCM) and the instrument panel cluster (IPC) for electrical power management (EPM) operation. The BCM determines the output of the generator and sends the information to the PCM for control of the generator field control circuit. It monitors the generator field duty cycle signal circuit information sent from the PCM for control of the generator. It monitors a battery current sensor, the battery positive voltage circuit, and estimated battery temperature to determine battery state-of-charge (SOC). The BCM performs idle boost and load management operations.
Battery Current Sensor
The battery current sensor is a serviceable component that is connected to the negative battery cable at the battery. The battery current sensor is a 3-wire hall effect current sensor. The battery current sensor monitors the battery current. It directly inputs to the BCM. It creates a 5-volt pulse width modulated (PWM) signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-95 percent. Between 0-5 percent and 95-100 percent are for diagnostic purposes.
Powertrain Control Module (PCM)
The PCM directly controls the generator field control circuit input to the generator. It monitors the generator's generator field duty cycle signal circuit and sends the information to the BCM. The PCM will override the BCM control of the generator when one of the following conditions is met:
Instrument Panel Cluster (IPC)
The IPC provides a means of customer notification in case of a failure and a voltmeter. There are 2 means of notification, a charge indicator and a driver information center (DIC) message of SERVICE CHARGING SYSTEM and CHARGING SYSTEM FAULT.
Charging System Operation
The purpose of the charging system is to maintain the battery charge and vehicle loads. There are 6 modes of operation and they include:
The powertrain control module (PCM) controls the generator through the generator field control circuit. It monitors the generator performance through the generator field duty cycle signal circuit. The signal is a 5-volt pulse width modulated (PWM) signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-95 percent. Between 0-5 percent and 95-100 percent are for diagnostic purposes. The following table shows the commanded duty cycle and output voltage of the generator:
Commanded Duty Cycle
Generator Output Voltage
The generator provides a feedback signal of the generator voltage output through the generator field duty cycle signal circuit to the PCM, this information is sent to the body control module (BCM). The signal is a 5-volt PWM signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-99 percent. Between 0-5 percent and 100 percent are for diagnostic purposes.
The BCM will enter Charge Mode whenever one of the following conditions is met:
Once one of these conditions is met, the generator battery control module will set the targeted generator output voltage to 13.4 volts and then ramp that voltage up to 14.5 volts at a rate of 50 mV per second.
Fuel Economy Mode
The BCM will enter Fuel Economy Mode when the calculated ambient air temperature is above 0°C (32°F), the calculated battery current is less than 15 amperes and greater than -8 amperes, and the battery state-of-charge (SOC) is greater than 80 percent. Its targeted generator output voltage is 13 volts. The BCM will exit this mode once the criteria is met for Charge Mode.
Voltage Reduction Mode
The BCM will enter Voltage Reduction Mode when the calculated ambient air temperature is above 0°C (32°F); the calculated battery current is less than 2 amperes and greater than -7 amperes, and the generator field duty cycle is less than 99 percent. Its targeted generator output voltage is 12.9 volts. The BCM will exit this mode once the criteria are met for Charge Mode.
Start Up Mode
After the engine has started, the BCM sets a targeted generator output voltage of 14.5 volts for 20 seconds.
Windshield Deice Mode
After the engine has run for more than 10 seconds, the BCM sets a targeted generator output voltage of 13.8 volts if the calculated ambient air temperature is less that 0°C (32°F). The BCM will stay in this mode until the engine coolant temperature reaches 75°C (167°F) for 10 minutes.
Battery Sulfation Mode
The BCM will enter this mode when the interpreted generator output voltage is less than 13.2 volts, for 45 minutes. Once in this mode, the BCM will set a targeted output voltage of 13.8 volts, for 5 minutes. The BCM will then determine which mode to enter depending on voltage requirements.
Instrument Panel Cluster (IPC) Operation
Charge Indicator Operation
The instrument panel cluster (IPC) illuminates the charge indicator in the message center when the one or more of the following occurs:
Charging System Failure
The BCM and the PCM will send a class 2 message to the IPC for the CHARGING SYSTEM FAILURE message to be displayed. It is commanded ON when a charging system DTC is a current DTC. The message is turned OFF when the conditions for clearing the DTC have been met.
The IPC displays the system voltage as received from the BCM over the class 2 serial data circuit. If there is no communication with the BCM, then the display will read all dashes until communication is restored.
Battery Saver Active
The BATTERY SAVER ACTIVE message will display on the DIC when the vehicle enters a load shed 2 event. Refer to Load Shed System Description and Operation for load shed 2 setting criteria.