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Alternator voltage drop...

Discussion in 'FAST Support Forum' started by J-440, Dec 3, 2017.

  1. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Re-Edit.

    Have a FAST XFI setup and am experiencing severe voltage drop. I'm reading 14.2 volts at the alternator, 13.8 at the battery in trunk. When everything is on, I get 10.5 volts from my laptop. I've already been through 1 battery and 1 alternator and the car only has 350 miles on it. It's a 140amp alternator and Powermaster says it'll work for my car. I've got 4 grounds and am still getting voltage drop. Is there a key element from the FAST harness I'm missing?
     
    Last edited: Dec 3, 2017
  2. crash520

    crash520 Member

    Joined:
    Dec 16, 2016
    There should only be one red and one black going to the battery, they will be the two largest size wires.
    What size alternator are you using?
    What size charge cable is connecting the alternator to the battery?
    Is the battery front or rear mounted?
     
  3. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Sorry, the extra wires to the terminals are for other things. The alternator is a 140amp, there's a 4 gauge from the alternator to the starter, and the battery is in the trunk.
    I've also heard that a 1 gauge wire from the motor to the frame, then from the motor to the body will help for more grounding. Still doing some testing.
     
  4. Fastmanefi

    Fastmanefi Active Member

    Joined:
    Jun 4, 2011
    Location:
    Sonora, CA
    The basic problem comes from using a one-wire alternator and high resistance in the system. 1 and 4 gauge is NOT acceptable. At that current level the minimum cable size should be 2/0. BOTH the cables from the alternator to the battery need to be very large to handle the voltage drop over that length. I advise to NEVER rely on the chassis for the ground return. Run a separate black cable to the battery.

    One wire alternators sense at their terminal. If you had an alternator with a separate sense wire you could connect that to the ECU power point and it would do what it had to get that voltage correct, but you can stay with a one-wire if you get the rest of the system correct.
     
    Last edited: Dec 3, 2017
  5. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    Ok, this separate sense wire. It connects at the alternator terminal to where? And getting the rest of the system correct...do I still need to run 1 gauge wires from the alternator to the battery? It sounds like you need me to do both?
     
  6. Fastmanefi

    Fastmanefi Active Member

    Joined:
    Jun 4, 2011
    Location:
    Sonora, CA
    You may not have a separate sense wire. Find that out first.
    A I gauge is border line for that long a run.
     
  7. J-440

    J-440 Member

    Joined:
    Jun 16, 2012
    I've got a GM design alt for my Mopar. A plug with 3 wires plugs into the back. 1 wire is not used (that's the low voltage light), 1 wire goes to the fuse box, and the last wire plugs to the alternator main terminal. Then I have a 4 gauge going from the main alt terminal to the starter.
    Another thing, when everything's on, the alternator reads 14.2, the battery reads 13.9 and when my laptop is plugged in it reads 10.5. What's that all about?
     
    Last edited: Dec 3, 2017

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