I apologize for the delay, the site I was getting the info from has been down, but here ya go! You can see in certain situations it's completely normal to have quite a low voltage compared to older charging systems with static voltage regulation.

2008 Pontiac Grand Prix 5.3L Eng GXP





BATTERY, CHARGING SYSTEM AND STARTING SYSTEM


DIAGNOSTIC INFORMATION AND PROCEDURES > BATTERY INSPECTION/TEST > CIRCUIT/SYSTEM TESTING


CAUTION: Unless directed otherwise, the ignition and start switch must be in the OFF or LOCK position, and all electrical loads must be OFF before servicing any electrical component. Disconnect the negative battery cable to prevent an electrical spark should a tool or equipment come in contact with an exposed electrical terminal. Failure to follow these precautions may result in personal injury and/or damage to the vehicle or its components.

  • Inspect the battery for a cracked, broken, or damaged case, which may be indicated by battery acid leakage.
    • If there is any apparent damage, replace the battery.

  • Verify the cold cranking amperage (CCA), and reserve capacity (RC) and/or amp hour (AH) rating of the battery to the original battery or original equipment (OE) specification. Refer to Battery Usage .
    • If the battery does not meet or exceed specifications, replace the battery.

  • Verify that the battery cables are clean and tight. The battery terminal bolts should be torqued as specified in Fastener Tightening Specifications .
    • If the battery cable(s) need to be cleaned, clean as required and tighten as specified.


    • If the battery cable(s) are damaged, replace then tighten as specified.

  • Install the J 42000 and follow directions supplied by the tester. See Special Tools .
    • If the tester calls for charging the battery, refer to Battery Charging .

DESCRIPTION AND OPERATION > CHARGING SYSTEM DESCRIPTION AND OPERATION > 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:

  • Charge Mode
  • Fuel Economy Mode
  • Voltage Reduction Mode
  • Start Up Mode
  • Windshield Deice Mode
  • Battery Sulfation Mode

The ECM/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 PWM signal of 128 Hz with a duty cycle of 0-100%. Normal duty cycle is between 5-95%. Between 0-5% and 95-100% are for diagnostic purposes. The following table shows the commanded duty cycle and output voltage of the generator:
Commanded Duty Cycle Generator Output Voltage
10 % 11.0 V
20 % 11.56 V
30 % 12.12 V
40 % 12.68 V
50 % 13.25 V
60 % 13.81 V
70 % 14.37 V
80 % 14.94 V
90 % 15.5 V
The generator provides a feedback signal of the generator voltage output through the generator field duty cycle signal circuit to the ECM/PCM, this information is sent to the BCM. The signal is a 5 volt PWM signal of 128 Hz with a duty cycle of 0-100%. Normal duty cycle is between 5-99%. Between 0-5% and 100% are for diagnostic purposes.
DESCRIPTION AND OPERATION > CHARGING SYSTEM DESCRIPTION AND OPERATION > CHARGING SYSTEM OPERATION > CHARGE MODE


The BCM will enter Charge Mode when ever one of the following conditions are met:

  • The interpreted fuel rate is greater than 21 g/s and the throttle position is greater than 90%.
  • The headlamps are ON, low or high beam.
  • The wipers are ON for more than 8 seconds.
  • The electric cooling fans are on high speed.
  • The rear defogger is ON.

Once one of these conditions are 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.
DESCRIPTION AND OPERATION > CHARGING SYSTEM DESCRIPTION AND OPERATION > CHARGING SYSTEM OPERATION > 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 SOC is greater than 80%. Its targeted generator output voltage is 13.0 volts. The BCM will exit this mode once the criteria are met for Charge Mode.
DESCRIPTION AND OPERATION > CHARGING SYSTEM DESCRIPTION AND OPERATION > CHARGING SYSTEM OPERATION > 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%. Its targeted generator output voltage is 12.9 volts. The BCM will exit this mode once the criteria are met for Charge Mode.
DESCRIPTION AND OPERATION > CHARGING SYSTEM DESCRIPTION AND OPERATION > CHARGING SYSTEM OPERATION > START UP MODE


After the engine has started the BCM sets a targeted generator output voltage of 14.5 volts for 20 seconds.
DESCRIPTION AND OPERATION > CHARGING SYSTEM DESCRIPTION AND OPERATION > CHARGING SYSTEM OPERATION > 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.
DESCRIPTION AND OPERATION > CHARGING SYSTEM DESCRIPTION AND OPERATION > CHARGING SYSTEM OPERATION > 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.
DESCRIPTION AND OPERATION > ELECTRICAL POWER MANAGEMENT DESCRIPTION AND OPERATION > ELECTRICAL POWER MANAGEMENT


The electrical power management (EPM) is used to monitor and control the charging system and alert the driver of possible problems within the charging system. The EPM system makes the most efficient use of the generator output, improves the battery state-of-charge (SOC), extends battery life, and manages system electrical loads.
The load shed operation is a means of reducing electrical loads during a low voltage or low battery SOC condition.
The idle boost operation is a means of improving generator performance during a low voltage or low battery SOC condition.
Each EPM function, either idle boost or load-shed, is discrete. No 2 functions are active at the same time. Idle boost is activated in incremental steps, idle boost 1 must be active before idle boost 2 can be active. The criteria used by the instrument panel module (IPM) to regulate EPM are outlined below:
Function Battery Temperature Calculation Battery Voltage Calculation Amp-hour Calculation Action Taken
Idle Boost 1 Start Less Than -15°C (5°F) Less Than 13 V - First level Idle boost requested
Idle Boost 1 Start - - Battery has a net loss greater than 0.6 AH First level Idle boost requested
Idle Boost 1 Start - Less Than 10.9 V - First level Idle boost requested
Idle Boost 1 End Greater Than -15°C (5°F) Greater Than 12.0 V Battery has a net loss less than 0.2 AH First level Idle boost request cancelled
Load Shed 1 Start - - Battery has a net loss of 4.0 AH Rear Defrost, Heated Mirrors, Heated Seats cycled OFF for 20% of their cycle
Load Shed 1 Start - Less Than 10.9 V - Rear Defrost, Heated Mirrors, Heated Seats cycled OFF for 20% of their cycle
Load Shed 1 End - Greater Than 12.0 V Battery has a net loss of less than 2.0 AH Clear Load Shed 1
Idle Boost 2 Start - - Battery has a net loss greater than 1.6 AH Second level Idle boost requested
Idle Boost 2 Start - Less Than 10.9 V - Second level Idle boost requested
Idle Boost 2 End - Greater Than 12.0 V Battery has a net loss less than 0.8 AH Second level Idle boost request cancelled
Idle Boost 3 Start - - Battery has a net loss of 10.0 AH Third level Idle boost requested
Idle Boost 3 Start - Less Than 10.9 V - Third level Idle boost requested
Idle Boost 3 End - Greater Than 12.0 V Battery has a net loss of less than 6.0 AH Third level Idle boost request cancelled
Load Shed 2 Start - Less Than 10.9 V Battery has a net loss greater than 12 AH Rear Defrost, Heated Mirrors, Heated Seats cycled OFF for 50% of their cycle. The BATTERY SAVER ACTIVE message will be displayed on the DIC
Load Shed 2 Start - Less Than 10.9 V - Rear Defrost, Heated Mirrors, Heated Seats cycled OFF for 50% of their cycle. The BATTERY SAVER ACTIVE message will be displayed on the DIC
Load Shed 2 End - Greater Than 12.6 V Battery has a net loss of less than 10.5 AH Clear Load Shed 2
Load Shed 3 Start - Less Than 11.9 V Battery has a net loss greater than 20 AH Rear Defrost, Heated Mirrors, Heated Seats cycled OFF for 100% of their cycle. The BATTERY SAVER ACTIVE message will be displayed on the DIC
Load Shed 3 End - Greater Than 12.6 V Battery has a net loss of less than 15 AH Clear Load Shed 3