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Volt Meter Fluctuating????

Discussion in '1973-1991 K5 | Suburban | Truck' started by TysonDawg, Aug 6, 2002.

  1. TysonDawg

    TysonDawg Newbie

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    I'm new to this forum. I have found an answer to every question I have ever needed until now. Can someone please tell me what would cause my volt meter to fluctuate while driving? It doesn't always happen but it does the motor seems to idle rough as the volts go down and becomes smooth again once the volts come back up. Please help.
     
  2. wedgeindy

    wedgeindy Registered Member

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    I had this problem on my '89. First keep in mind that the volt meter will fluctuate with current usage, so things like the fan, A/C compressing kicking on and off will make the Voltmeter go up and down.

    Next thing I did was to pull the battery and clean the terminals with a water and baking soda mix (and the battery tray while I was at it). This will neutralize the battery acid and allow you to clean stuff up. But the battery back in and make sure everything is snug.

    Next Clean the motor and chassis ground to make sure you're getting a good feed.

    Hope this helps!

    Tony
     
  3. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    I think you problem has nothing to do with the Voltage. It seems to work as it should. You say the idle goes down and it starts to run rough. Well once the engine drops below 600 or so RPMs the Alternator is not spinning fast enough to keep up with demand. Alternators need to have about 1500 RPMS of engine speed to reach full output. At Idle they are probably only producing about 50% capaicity. well if you have enough on like lites and A/C then your probably pulling as much power as it can produce at that speed.
    I think you have some sort of engine problem. I would start by reading the codes out of the computer. Eventhough the light on the dash may be off there could still be some intermitant issue that the ECU has not deamed serious enough to flip on the light but those codes may still be available.
     
  4. Rob 85K5

    Rob 85K5 1/2 ton status GMOTM Winner

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    First, check the belt to see if it is loose. At low rpms if the belt is too loose the alt will either not turn or turn very little. At the higher rpms, the belt will spin faster and catch the pulley more and spin faster, still not full capacity though.

    I learned this the hard way on my boat.
     
  5. TopOff

    TopOff 1/2 ton status

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    Could be a bad regulator in the alternator.

    After checking all the above mentioned... take it to your favorite local parts store and have them check the alternator under load.

    Good luck.
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    You can do the same thing with a voltmeter (yes, they are one of my "necessity" tools) by running it with accessories on and off, and watching/measuring voltage off the single red wire on the back of the alternator.
     
  7. TopOff

    TopOff 1/2 ton status

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    My battery and single wire on the alternator are plugged into each other. If the alternator is not putting out enough amps, it can still be putting out enough voltage.

    I don't have a good easy way to check for amps... so I would have to take my alternator (or truck) to my favorite local auto parts store.
     
  8. TysonDawg

    TysonDawg Newbie

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    I want to thank everyone for their help I will check all of the above suggestions. To make thing a little clearer the volts gauge will dance around while driving and while at an idle. Also this will not happen all the time. I will be driving a for 30 minutes with no problems them I look over and the gauge is jumping around for a minute or so them go back to charging at approx 14 volts. Again thanks for all your help.
     
  9. TopOff

    TopOff 1/2 ton status

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    Could also be an instrument cluster problem...

    My temp gauge did this...
     
  10. or2play

    or2play 1/2 ton status

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    One thing you can check is the voltage across the battery. At idle you should be getting right around 13 volts. As you start turning on electrical accessories the voltage across the battery will drop.
     

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