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Thread: How do you read the fuel sender ohms?

  1. #1
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    Default How do you read the fuel sender ohms?

    May seem like a silly question, but I don't know where the meter leads should be to take the ohm reading on my fuel sending unit. When I have them both touching the metal plate I get the same reading of 100 w/ the tank full and with it empy. When I have one lead on metal and one on the post I get no reading (full or empty).
    Attachment 9531
    P.S. I've done some searching on this, but haven't found the answer yet. Thanks.
    Mike in Vista, CA
    1960 Willys CJ6B

  2. #2
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    Default Re: How do you read the fuel sender ohms?

    try inbetween the wire and the post, key on, power to wire
    you might have to ground the sender with an additional wire

  3. #3
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    Default Re: How do you read the fuel sender ohms?

    Power to the white wire? I thought power to the tank was a no no?
    Mike in Vista, CA
    1960 Willys CJ6B

  4. #4
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    Default Re: How do you read the fuel sender ohms?

    white wire looks like your ground; ground it somewhere, use jumper wire, whatever
    touch one probe lead to the hot sender wire from vehicle
    the other lead to the post where the hot wire attaches
    your ohm meter now becomes your fuel gauge
    as you move the float arm, full, half, empty, you should register different ohm readings/values
    it won't matter which probe lead you use to make these connections, red or black

  5. #5
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    Default Re: How do you read the fuel sender ohms?

    Okay. Just to make sure I don'rt blow up the garage...
    "the hot sender wire from vehicle" can just be a 12V source, with the ohm meter between the 12 volts and the post? I don't know if the gas gauge is even working, so I was trying to leave that out of the equation and just check my ohms full and empty, and see if the sending unit is working.
    Mike in Vista, CA
    1960 Willys CJ6B

  6. #6
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    Default Re: How do you read the fuel sender ohms?

    Quote Originally Posted by mpc
    Okay. Just to make sure I don'rt blow up the garage...
    "the hot sender wire from vehicle" can just be a 12V source, with the ohm meter between the 12 volts and the post? I don't know if the gas gauge is even working, so I was trying to leave that out of the equation and just check my ohms full and empty, and see if the sending unit is working.

    *DO NOT* Connect that wire to 12 volts!!!

    Bad Idea, Very Bad Idea .

    Between the sender & the 12 volt system theres a regulator & the internal resistance of the guage itself that limits the amount of current to the sender.



    It *might* actually be safe to apply battery voltage directly to the sender, Or It Might Not.

    The worst thing that *might* happen is you burn out the sender, Or It Might Not.

    Either way, do you *Really* want to find out for sure???

    You should be able to read the sender resistance from the insulated screw to bare metal on the tank, not sure on flatties but on later CJs it will vary between 0 to 80 +/- ohms.

    That meter you are using will need to be "zeroed", touch the lead tips together & adjust the "Zero" control untill the needle is pointing at the 0 on the right hand side. I notice theres two range settings for Ohms on your meter, can't read what they are but if one of them is "100" or "1000" or "1K" you'll be in the ballpark.

    Don't try to read resistance with any power applied to the sender, in fact if you have a wire coming from the guage dissconnect it or it will mess up the meter reading, & maybe mess up the meter itself .

    I'd pull the sender out & give the old eyeball treatment, from what you said about the reading from the center post sounds like the sender is having issues.

    H.
    Tonka
    1955 CJ5
    F134
    S/N 378
    tantel.ca/Tonka.html

  7. #7
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    Default Re: How do you read the fuel sender ohms?

    Howard
    have been talking to Mike over landline
    thanks for the warning, he is aware of the voltage reduction

  8. #8
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    Default Re: How do you read the fuel sender ohms?

    Yep, I understand. Thank you both for the instructions.
    Okay here's what I've got:
    First, upon testing the fuel gauge by turning the ignition on and puting a test light to the gauge, instead of the tester lighting up, it apparently grounded out and the gauge would peg "full". Not what I was expecting, so I believe the gauge is bad. When I purposely grounded the fuel gauge, it also pegged full, which is what I expected.

    Now for the sending unit. I took it out of the tank. It is covered w/ rust and the brass float has some gas in it, so it is toast. I went ahead and hooked up the ohm meter between the gauge and the post on the sending unit anyway. With the ignition on and the sending unit grounded, I got no readings on the ohm meter. Also tried moving the float manually - nada. I had the meter on the x1k setting and I zeroed it out before hand.

    Sending unit dead? Yes.
    Gas gauge dead? Not sure, but something's wrong with it.
    Mike in Vista, CA
    1960 Willys CJ6B

  9. #9
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    Default Re: How do you read the fuel sender ohms?

    to Safely test your guage, make sure the power lead has a good 12 volts going to it when the ignition is on. with the meter set on volts and the ignition on, read from the terminal (with the strap that goes to the temp guage) and a good ground, you should get around 5 volts. if you do the regulator in the fuel guage is working, if not the guage will have to be replaced.

    if the regulator is good; with the ingnition on, temporarily ground the S (sender) terminal on the guage, this should peg the guage, if not the guage is bad and must be replaced.

  10. #10
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    Default Re: How do you read the fuel sender ohms?

    I was wrong
    Howard straightened me out about the ohm meter being powered with it's own battery, not the vehicle power.
    I was thinking volts, not ohms, sorry Mike.
    is the corner available, going to stand in the corner now

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