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Fuel gauge calibrate

Discussion in 'Intermediate CJ-5/6/7/8' started by dmfrazee, Mar 2, 2009.

  1. Mar 2, 2009
    dmfrazee

    dmfrazee Jeep Builder

    Smithfield Pa.
    Joined:
    Dec 17, 2008
    Messages:
    16
    :rofl:Hello,
    I purchased a new fuel gauge for my 1979 Jeep CJ. It said in the instructions it must be calibrated to work right but not how to do it. Does anyone know how to do this? It's a gauge that goes in speedometer cluster.
    Thanks,
    Don
     
  2. Mar 2, 2009
    trickpatrick

    trickpatrick Done? LOL

    North Idaho USA
    Joined:
    Nov 29, 2006
    Messages:
    838
    No you dont have to calibrate it.
    Hook it up exactly like the old one comes out.
    It needs to have less then12 volts.
    And it is acomplished with the third little item on the back of the speedo which takes it down to around 7 volts IIRC.
    Make sure that is working before hooking up it will melt the new gauge.

    But the sender and the gauge must match.

    Here is my simple discription of how they work together.

    The guage is a litebulb, and the sender is a dimmer switch.

    While the gas tank is full the dimmer switch is all the way up and the lite is brite.
    As the dimmer ( sender ) goes down the the lite ( guage ) goes down too.


    This is resistance or cutting back on the flow of electricity.

    On a early jeep system when the tank is full and there is little resistance it would measure around 10 ohm's, and when empty most resistance it would be around 75 ohm's.

    The sender and the gauge are really pretty cheap peices and the exact measurement between 2 units will vary some.

    For the most part it wont matter much.
    but you can bend the sender floating arm some to get the guage to read a little more acurate if needed.
    I like to know when I am really empty so I would bend mine to match empty on the guage the best I could.


    But here is where things can get messed up.

    There are at least 3 common sending setups.

    they all work the same but they use a different range of resistance #'s

    So if they dont match.
    for instance you have an early sender, Jeep, and a AMC guage.
    They wont be in the same range of resistance and the guage will read say half full when empty.
    And you cant adjust the float arm enough to make it work.

    IIRC (Note:Looked this up and edited with #'s)

    General Motors : 0 to 90 Ohm's
    Ford and Chrysler: 73 to 10 Ohm's
    AMC and Stewart Warner: 240 to 33 Ohm's

    Each different.
    Early Jeep is same as Ford.
    I would think Intermediate Jeeps would be AMC?
    It must have changed there somewhere.

    The rub is jeeps tend to be a miss match of parts over time.

    SO. I would want to know what my gas tank sender was giving for #'s before Just assuming my new guage will match it.

    This is easy to do.
    use a test meter that can measure ohm's
    One lead goes to the sender wire coming from the tank.
    And one to ground.
    Measure it when full and when empty and see what your range of ohm's are from the sender.
    Then match your guage to it.

    Remember the numbers arent exact but in the right range.
     
    Last edited: Mar 5, 2009
  3. Mar 2, 2009
    CT

    CT Member

    Joined:
    Apr 28, 2004
    Messages:
    238
    Those things always made me a bit nervous when working on them. I was afraid of hooking up the wires wrong to ten gallons of gas sitting right under me. I could see myself being launched as another satellite when turning on the key for the first time after replacing a sender.

    Thanks for the great explanation.
     
  4. Mar 5, 2009
    trickpatrick

    trickpatrick Done? LOL

    North Idaho USA
    Joined:
    Nov 29, 2006
    Messages:
    838
    Filled in the resistance #'s from the book.
    To earlier reply.:)
     
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