2004 peterbilt voltage drop

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Peterbilt04379, Nov 25, 2021.

  1. Peterbilt04379

    Peterbilt04379 Bobtail Member

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    Nov 25, 2021
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    I have a 2004 Pete with a C-15 in it. Whenever I turn on my headlights, clearance lights, and my blower my voltage drops down to around 13 which I feel is plenty. But it will start beeping at me sometimes. It’s not all the time and it will beep and then kill my tach, Speedo, water temp gauges. It has a red square low batt on the dash it doesn’t flash or anything. It only does it once it gets around 12.8 or 13 volts with everything on. That’s what makes me think it some kind of safety. Any help would be great. Thank you
     
    blairandgretchen Thanks this.
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  3. joey8686

    joey8686 Light Load Member

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    Jul 1, 2014
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    Needs a new Alternator
     
  4. blairandgretchen

    blairandgretchen Road Train Member

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    South west Missouri
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    Aaaah - something comes to mind - Accessory Power Relay - black box behind’A’ panel - check/replace.
     
    Goodysnap Thanks this.
  5. Peterbilt04379

    Peterbilt04379 Bobtail Member

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    Nov 25, 2021
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    I will test voltage out of alternator again but last time I checked it was fine but will double check it. thank you

    as for the accessory relay I assume your talking about the panel down by the clutch? Thank you also appreciate your guys help for some where to look
     
    blairandgretchen Thanks this.
  6. blairandgretchen

    blairandgretchen Road Train Member

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    South west Missouri
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    No - dash panels - ABC left to right. Pull far left panel and is behind there. Mine is 1995, if 2004 is the same, that little relay box upset guages, wipers, lights etc if on the blink.
     
  7. Peterbilt04379

    Peterbilt04379 Bobtail Member

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    Nov 25, 2021
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    Okay I will check that. It doesn’t effect my lights. Just my gauges, blower motor, wipers. Lights might be wired different.
     
    blairandgretchen Thanks this.
  8. BoxCarKidd

    BoxCarKidd Road Train Member

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    Check the alternator voltage when you are having the problem. That should be OK but tested along with amperage under load on the batteries is better.
    Check grounds circuits to cab and other possible components.
     
  9. blairandgretchen

    blairandgretchen Road Train Member

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    Dec 9, 2011
    South west Missouri
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    Yeah, that was it - voltage was showing 10-11, and wipers, blower and guages were cutting out.

    Was a faulty Auxiliary Power relay, $25 - and some loose connectors. It was messing up mileage reported to the EOBR too.
     
    joey8686 and BoxCarKidd Thank this.
  10. Peterbilt04379

    Peterbilt04379 Bobtail Member

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    Nov 25, 2021
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    Thank you everyone for the help. I ended up testing voltage out of alternator and I couldn’t get it to ground from the ground wire. The bolt for the ground was just barely holding on went to take it off it literally just broke. Drilled it out and retapped it and everything has been good the last 2 days! Volts are staying up now
     
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