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    Alternator Charge Question

    OK, I did the on both Google and here on the ca forum and cannot seem to locate a good answer...sooo....

    Why is it that my alternator puts out about 14.5-14.9 volts when the truck is just started and then drops to about 13 volts after everything is running warm? Is this a problem or standard behavior for an alternator?




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    Re: Alternator Charge Question

    Quote Originally Posted by jkbeaumont1 View Post
    OK, I did the on both Google and here on the ca forum and cannot seem to locate a good answer...sooo....

    Why is it that my alternator puts out about 14.5-14.9 volts when the truck is just started and then drops to about 13 volts after everything is running warm? Is this a problem or standard behavior for an alternator?
    standard behavior but mine puts out 15.4 cold and 15.1 hot



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    Re: Alternator Charge Question

    I've always thought it was because batterys lose a bit of charge from sitting and it is topping them back off real quick.

    But it's just a guess on my part/



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    Re: Alternator Charge Question

    Hmmm good question.



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    Re: Alternator Charge Question

    Well, it's not like I have an overwhelming car audio system in my '08 Silverado, so I am sure that my stock OEM 165AMP alternator cannot be that stressed. I was just curious if this is standard behavior or not. I thought I read somewhere that alternators supply voltage a bit differently cold vs. hot, but I didn't think they would swing by almost 2 volts. There are certainly no issues (no warning lights, diagnostic codes, etc.) in my truck, I just noticed it today and decided to check it with a DMM when I got home.

    I arrived, placed a DMM on the battery and it was around 13 volts, just like the analog voltage meter on the dash was reporting. I shut the truck off for about a minute, re-started and it went back up to 14.5+ volts, then over the course of a couple of minutes it settled back down to about 13 volts. Vewy, vewy curious.




    Eddie Izzard: Squirrels always eat nuts with two hands, always two hands, "arararar", and occasionally, they stop and go...oh, uh, ah, as if they're going, "Did I leave the gas on? No! No, I'm a fucking squirrel!"

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    Re: Alternator Charge Question

    I sit at 14.9 cold. On a hot day (70 degrees) I sit around 13.9. That's with no audio at all.




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    Re: Alternator Charge Question

    Quote Originally Posted by terra View Post
    I sit at 14.9 cold. On a hot day (70 degrees) I sit around 13.9. That's with no audio at all.
    Well, I did have the tunes cranking pretty good and the A/C was on...I guess that could explain it?

    I guess I'll need to perform some more tests with little to no load and see how it behaves.




    Eddie Izzard: Squirrels always eat nuts with two hands, always two hands, "arararar", and occasionally, they stop and go...oh, uh, ah, as if they're going, "Did I leave the gas on? No! No, I'm a fucking squirrel!"

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    Re: Alternator Charge Question

    Got some very good information for any other late model GM owners out there!

    ********************************************
    From the 2CarPros Forum:

    I would say this is very normal. Remember while the truck is sitting other things are still “slightly” drawing a little current. Plus the battery is sitting slowly going down….

    So when you first start….that is normal for it to take a little more charge. Circuits are cold, many things are charging up. You use the battery to start the vehicle….that is a big draw.

    Then as you use thing the alternator will cycle giving you more/less power as needed. Here is a chart to help.

    The engine control module (ECM) controls the generator through the generator turn on signal. It monitors the generator performance though the generator field duty cycle signal circuit. The signal is a 5 volt pulse width modulation (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%, 11V
    20%, 11.56V
    30%, 12.12V
    40%, 12.68V
    50%, 13.25V
    60%, 13.81V
    70%, 14.37V
    80%, 14.94V
    90%, 15.5V

    Unless you have a DTC or are experiencing some problems…….I would not be concerned.

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

    • The wipers are ON for more than 3 seconds.

    • The GMLAN Climate Control Voltage Boost Mode Request is true, as sensed by the HVAC control head. High speed cooling fan, rear defogger and HVAC high speed blower operation can cause the BCM to enter the Charge Mode.

    • The estimated battery temperature is less than 0°C (32°F).

    • Battery state of charge is less than 80 percent.

    • Vehicle speed is greater than 145 km/h (90 mph)

    • Current sensor fault exists

    • System voltage was determined to be below 12.56 volts

    • Tow/Haul mode is enabled

    When any one of these conditions is met, the system will set targeted generator output voltage to a charging voltage between 13.9-15.5 volts, depending on the battery state of charge and estimated battery temperature.

    Battery Voltage Gauge Operation
    The IPC displays the system voltage as received from the BCM over the GMLAN serial data circuit. If there is no communication with the BCM then the gauge will indicate minimum.

    This vehicle is equipped with a regulated voltage control (RVC) system. This system turns off the generator when it is not required in order to improve fuel economy. The generator will turn back on when additional voltage is required. This will cause the voltmeter to fluctuate between 12 and 14 volts as opposed to non-regulated systems which usually maintain a more consistent reading of 14 volts. This fluctuation with the RVC system is normal system operation and NO repairs should be attempted. For max output of the alternator is 145 amp with KG3 or 160 amp with KW1, these are the RPO codes located in the glove box just check and see if it has KW1 or KG3 and that will tell you.

    Mine is a KG3 so my stock alternator is 145 AMPs.




    Eddie Izzard: Squirrels always eat nuts with two hands, always two hands, "arararar", and occasionally, they stop and go...oh, uh, ah, as if they're going, "Did I leave the gas on? No! No, I'm a fucking squirrel!"

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