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Bob
Bob, Auto Service Technician
Category: Car
Satisfied Customers: 4087
Experience:  40 year GM Tech. +30 Certified. ASE Master Tech.(expired, retired) Medium Duty, Heavy Line, Retired
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Pontiac montana: van..3.4 liter, with battery light..volts

Customer Question

05 pontiac montana van 3.4 liter, with battery light on, had alternator checked both in and out of van, had 14.5 volts and 115 amps, testor showed a peak voltage of 18 volts, can unhook alternator plug and battery light will go out and everything is fine, rehook alternator and battery voltage goes from 12 to 15 volts, is this too high? battery light will stay on and windows and locks quit working, transmission goes into 2nd gear only in drive like "limp home" mode?
last week at idle would register 19 volts and boiled some acid out of the battery.
Submitted: 7 years ago.
Category: Car
Expert:  Bob replied 7 years ago.
HiCustomer Sorry but I think it is time you looked into a different rebuilder. The only things that regulate the voltage and amperage out put of your alternator are all built right into it. 15 volts is too much charging voltage and will sooner or later warp the plates in your battery and you will be replacing that too before long if this continues. The fact that the light goes out when you unplug the alternator tells me that you have a minimum of a bad diode in the alternator. The voltage regulator for the system is also build into the case of the alternator. Maybe the test equipment is only as good as the person who is using it, but I am sure that you have an alternator problem, regardless of what the on and off car tests are showing. A spark when hooking up the battery post on the alternator is also not a good sign. I think maybe you should take the whole rig to another place to be tested and tell them that you want the system load tested. Under no circumstances should the output be at 19 volts. The only way the unit can exceed it'd rated capacity, which it is doing, is if it has an internal problem.
Customer: replied 7 years ago.
The small spark is when I rehook the battery cable to the battery, I was looking to see any large drains.
I rechecked the system with my volt meter, battery is 12.5 volts with the motor off, and 14.5 with the motor running and the alternator plugged in, the alternator has two wires one orange with a s on the receptacle for the switch(?) showing 12 volts and one red with an "L" showing 5 volts, I checked these with the motor running and the plug unplugged, they have 0 voltage with the motor not running.
The alternator was checked by AutoZone, Advance Auto and a rebuild shop. I took it to all three places trying to find someone who had seen the problem. The alternator is $250 for a lifetime warrenty rebuild, not happy about the price but would pay it if it would fix it for sure, hate to spend this and then be something else, Would a GM dealer have any other (better) equipment to check the whole system?
Expert:  Bob replied 7 years ago.
I don't know what kind of equipment the parts stores are using so I wouldn't venture a guess. But I do know that no body is going to know the system better than a GM dealer. (as much as I hate to admit it). My other concern is that even if you did happen to have a voltage draw, that would not account for the over charging condition that you have experienced. Question, when you test at the battery with your volt meter, are the readings the same as what the dash gage is showing? But, again, even if the dash gage was off, you should not have a problem where the system charges at such a high rate that it boils the acid in the battery like it did. The voltage regulator inside the alternator is designed to keep it from doing just that, by regulating the output. There is another test that can be done on the car where they can ground the voltage regulator to the alternator case, while hooked to the tester to see what the maximum output the regulator is allowing is. Has anyone bothered to check that?
Customer: replied 7 years ago.
What should the alternator charging voltage be?
Expert:  Bob replied 7 years ago.
Normal charging voltage on your system should be between 13.2 and 14.5 volts, give or take a couple of tenths. Amperage should be close to 90% of rated capacity, too many different ratings to know what you may have. As low as 12.8 volts and as high as 14.8 would not make me worry as it will fluctuate depending on circumstances and load applied at the time, but it should never, never be anyplace close to 19.0 volts.
Customer: replied 7 years ago.
Is there anything (signal) from the plug/computer that would make it overcharge?
plug wires has one orange wire "s" with 12 v and one red "L" with 5 volts)

I assume the "S" is power from the switch and the L is? How does the computer know to turn on the battery light.
Expert:  Bob replied 7 years ago.

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

Document ID#(NNN) NNN-NNNN
2005 Pontiac Montana SV6 Ext.

 

Charging System Description and Operation

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:

  • 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'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

Generator

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:

  • The engine cooling fans are on high speed.
  • There is a high fuel demand.
  • The calculated ambient air temperature is less that 0°C (32°F).

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:

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

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

10%

11 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 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.

Charge Mode

The BCM will enter Charge Mode whenever one of the following conditions is met:

  • The interpreted fuel rate is greater than 21 g/s and the throttle position is greater than 90 percent.
  • 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 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:

  • The powertrain control module (PCM) detects that the generator output is less than 11 volts or greater than 16 volts. The IPC receives a class 2 message from the PCM requesting illumination.
  • The IPC determines that the system voltage is less than 11 volts or greater than 16 volts for more than 30 seconds. The IPC receives a class 2 message from the body control module (BCM) indicating there is a system voltage range concern.
  • The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.
  • The ignition is ON, with the engine OFF.

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.

Battery Voltage

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.


Document ID#(NNN) NNN-NNNN
2005 Pontiac Montana SV6 Ext.

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