Low idle voltage

Discussion in 'Electrical & Audio' started by bigcountry78, Oct 9, 2018.

  1. bigcountry78

    bigcountry78 Full Access Member

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    Kast night after work I was waiting on my truck to warm up before I left, with the headlights and radio on, my volt meter was just barely above the red. Is this normal, or is there something that would cause this? Battery and alternator are good, it reads well over 13 volts driving down the road. It just drops when idling.

    FAA03D81-3406-4514-BD32-F799A32CF9C4.jpeg
     
  2. yevgenievich

    yevgenievich Full Access Member

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    Pulley size on alternator wrong size for motor/belt or alternator failing
     
  3. Charlie

    Charlie Full Access Member

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    If you have a multimeter or access to one, test battery while idling and reving to see if there is much difference. Idling should read approximately 13.5 - 14.5 V. If reading increases at higher RPM's and decreases at idle, I would say alternator bad.
     
    Last edited: Oct 9, 2018
  4. bucket

    bucket Super Moderator Staff Member

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    That and it's a quick way to tell if the dash gauge is accurate at all.
     
  5. yevgenievich

    yevgenievich Full Access Member

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    Actually, talking about gauge accuracy. A poor body ground will make the gauge read decently lower than it should
     
  6. bigcountry78

    bigcountry78 Full Access Member

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    Which ground should I check?
     
  7. yevgenievich

    yevgenievich Full Access Member

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    There is a ground to the core support, to the engine block and from engine block to firewall by rear passenger side head. But test with a voltmeter should be done first
     
    Last edited: Oct 10, 2018
  8. Snoots

    Snoots Supporting Member Supporting Member

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    All of them.

    Not trying to be a wise-arse. But it is best to check them all.

    Just start at the front and work you way to the back one-by-one. Take 'em off, clean the surfaces, add a touch of copper infused anti-seize and reinstall.

    If ANY of the screws or bolts that hold down a ground wire are rusted, replace it. It ain't worth the headaches to try and clean them up. If they're rusty, they're toast.
     
    Last edited: Oct 10, 2018
    bigcountry78 and Paladin like this.
  9. 77 K20

    77 K20 Full Access Member

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    I had that same problem on my truck. Idling at a stop light with heater fan on, headlights on, and heaven forbid its raining and I have the wipers on. Volt gauge would be down to where yours was or even lower.
    I bought a smaller pulley for the alternator. This helped a bit- then when I added fuel injection I went from the stock 10si alternator to the 12si alternator. It makes a LOT more current at low (idle) RPM.

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    Typical OEM ratios are around 3 to 1, which simply means the crank pulley is three times the diameter of the alternator’s pulley. So in my case the engine would idle at 550 RPM and the alternator is spinning about 1650 RPM. So maybe I was making 16 amps? Slightly smaller puller and the 12si then will put out maybe 30 amps.
     
    Last edited: Oct 20, 2018 at 12:49 AM

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