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Old 04-25-2013, 09:45 AM   #1
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2008 G6 3.5 charging issue

Im working on my sisters g6. Its dying as shes driving it. Threw a multimeter on the battery while it was running. it started at 12.5 volts and continued dropping. The alternator should have it up over 14 volts. I had the alternator tested and its fine.
So now im at a loss. Help me please!
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Old 04-25-2013, 10:23 PM   #2
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I found this when 'I thought" I too had a bad alt. The new charging system sure gave me a scare 230 miles from home. Hope the following helps,

Charging System Description and Operation (Generator/Battery Control Module)
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, to manage the load, improve battery state-of-charge (SOC) and life, and minimize the systems impact on fuel economy. The EPM System performs 3 functions:
• It monitors the battery voltage and estimates the battery condition.
• It takes corrective actions by adjusting the regulated voltage.
• It performs diagnostics and driver notification.

The battery 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 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 batteries 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 operation of the charge indicator, driver information center 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 vehicles electrical system to maintain electrical loads and the 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.
Generator Battery Control Module

The generator battery control module is a class 2 device. It communicates with the powertrain control module (PCM), instrument panel cluster and the body control module for electrical power management (EPM) operation. It is a serviceable component that is connected to the negative battery cable at the battery. It directly controls the generator field control circuit, charge indicator control, input to the generator. It continuously monitors the generator field duty cycle signal circuit and the battery voltage. If the generator battery control module loses communication with the PCM, the default voltage will be set to 13.8 volts and the module will set U1016. If the generator battery control module loses communication with the body control module (BCM), the module will set U1064.
Powertrain Control Module (PCM)

The PCM provides information over the class 2 serial data circuit to the generator battery control module. The generator battery control module monitors the following data parameters provided by the PCM:
• Intake air temperature
• Fuel grams per second
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Old 04-25-2013, 11:24 PM   #3
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remove all connections to the alternator & the battery & clean them. clean both ends of all cables. don't make the mistake of assuming they are fine because they don't look corroded. have the battery load tested.

Eric has covered charging system troubleshooting in good detail:

Last edited by greenman; 04-25-2013 at 11:38 PM.
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