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Voltage differences?

Discussion in 'FAST Support Forum' started by J-440, Dec 4, 2017.

  1. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Started a previous thread on here and am getting some great help but I have another concern regarding voltage drop. I stated earlier that I'm getting 14.2 volts from a 140amp alternator at the terminal, getting 13.9 volts at the battery in the trunk and getting a 10.5 volt reading from my ECM on my laptop. This is with all the accessories on and the car idling at 800rpm.
     
  2. A A

    A A Well-Known Member

    Joined:
    Jun 22, 2014
    Without your having the cables correctly sized, and more specific details, it's difficult to know what all is going on. Follow the advice Richard already gave you. You could need a larger amperage alternator or a pulley resized on the alternator, but you'll never know until the cables are correctly sized.
     
  3. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Ok, but if everything is reading right with the multimeter, then why would the ECU be reading wrong? Not trying to be a pain, just curious is all.
    Oh and I called Powermaster with my alternator specs and they said I was good to go.
     
  4. Fastmanefi

    Fastmanefi Active Member

    Joined:
    Jun 4, 2011
    Location:
    Sonora, CA
    The ECU reads the voltage at the Switched 12V wire, not the power leads. Measure the voltage there with your DVM.
     
  5. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Good deal. Thanks again.
     

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