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Question About Fuel Gauges And Ohm Meters....

Discussion in 'Early CJ5 and CJ6 Tech' started by SoCalNickG, Mar 17, 2020.

  1. Mar 17, 2020
    SoCalNickG

    SoCalNickG Member 2023 Sponsor 2022 Sponsor

    Whittier, CA.
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    Please educate me. I have worked on cars (hobby not professional) for a long time. I am not that well versed on auto electrical and testing.
    My 71 CJ5 has both the stock rear gas tank and the under seat tank added. The PO put two fuel gauges left of steering column. Neither fuel gauge was working. I cleaned the under seat tank and replaced the sending unit (brass float was eaten through) and that gauge now works. I got lucky once.
    Moving to the rear tank, I cleaned the tank and purchased a new sending unit (old unit had a plastic float full of gas). I checked the new sending unit with a digital ohm meter, and found about 78.4 at 'empty' and 9.8 at 'full' before installing. After the tank was installed I checked again and got right at 70 when checking between the sending unit wire and grounding to a bolt on the frame with an empty tank. The gauge still doesn't work. I have pulled the gauge from the dash, now my question.
    If I disconnect the wires from the offending fuel gauge and connect my ohm meter, should I read the same 70 resistance that I saw at the wire connections at the tank? Then if I go to the gas station and fill the tank, should the ohm meter move to 10 as the tank fills?
    I think this is correct but I want to hear from someone who KNOWS, not me who THINKS it is right.
     
  2. Mar 17, 2020
    John Gedde

    John Gedde Been around the block once or twice...

    Middle Island, NY
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    No, you won't see the 70 ohms quite that way. One wire should have +12 on it and the other wire should have your 70 ohms to ground... Find the wire with the +12 on it first before you try to measure the resistance to ground on the other wire or you risk possibly damaging your meter.

    If you don't see about 70 ohms on one wire or 12V on the other, then you've found your issue - probably wiring.

    Are the two gauges identical? If everything checks out OK as above you could move the wires to the other gauge to see if it works there to find if you have a bad gauge.

    Also, measure the resistance of the non-working gauge when you have the wires off. Let us know what you get and we can help you tell if that gauge is messed up.

    John
     
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  3. Mar 17, 2020
    John Gedde

    John Gedde Been around the block once or twice...

    Middle Island, NY
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    One more thing I thought off. Most gauges have an additional connection: either a third terminal or a ground connection through the case of the gauge... So, the gauge probably needs a ground to work right.

    So, also measure the resistance between the gauge case and each of the two gauge terminals with the wires off. I would not expect any reading to be higher than 150 ohms worst case including between the two terminals.

    John
     
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  4. Mar 17, 2020
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

    Tantallon, Nova...
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    Nov 22, 2003
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    You should see the 70 ohms on the wire from the tank.

    What sort of fuel guages are you using?
     
  5. Mar 17, 2020
    SoCalNickG

    SoCalNickG Member 2023 Sponsor 2022 Sponsor

    Whittier, CA.
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    The gauge that is working is an Equus, it has three wires plus a light bulb stuck in hole with a forth wire. Pretty standard looking gauge. White letters on black face, chrome bezel. E on the left with marks at E, 1/4, 1/2, 3/4 and F.
    The gauge that I am having trouble with has no name on the black face. It has a lighting bolt under the pointer and the E to F arch is colored. red toward the E, yellow in the middle and green toward F. This one only has two wires, I will post pictures and trace more wires when I get home. I think one wire goes to a connector and the other goes to a ground. I will need to lay under the dash and see where these wires go.
    To add a wrinkle to this There is a 3 position toggle switch that can only be in one position for the working gauge to work.
     
  6. Mar 18, 2020
    SoCalNickG

    SoCalNickG Member 2023 Sponsor 2022 Sponsor

    Whittier, CA.
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    Okay now i have a new question.
    The gas gauge that was not working was never hooked up as a gas gauge. It is a Datcom Instrument gauge. it had three terminals; G in the center, I to one side and S to the other. The S terminal has been cut so it cannot be used. It has 2 wires connected, one is 12v from the ignition switch to the I terminal and the second wire goes from the G terminal to ground.
    IMG 1138
    Is this gauge acting like a volt meter? That would explain why when I had the engine running the gauge went to about 3/4 of a tank and the needle went toward full as the RPM went up.
     
  7. Mar 18, 2020
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

    Tantallon, Nova...
    Joined:
    Nov 22, 2003
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    First off I'd pick up another Equus, I have, issues, with mis-matched gauges:shock:

    If they went to the bother of the switch you may have a solenoid valve in there to switch tank feeds.

    I suggest you trace out the wiring & draw a diagram of your setup, with that we could be more help with this.
     
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  8. Mar 18, 2020
    Walt Couch

    Walt Couch sidehill Cordele, Ga. 2024 Sponsor 2023 Sponsor 2022 Sponsor

    cordele, Ga.
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  9. Mar 18, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    I'd abandon trying to identify/characterize the mystery gauge and go with something that's already documented. First I would make the factory tank work with the factory gauge and sender using the factory wiring diagram. Then I'd worry about the aux tank.

    Note that your fuel gauge has to match the in-tank sender, and vice-versa. The sender is a variable resistor that changes resistance with fuel level. The resistance can go up or down with fuel level, depending on the design. The range of resistance can be different, depending on the design. The range and direction of resistance for your factory gauge is given in your FSM.

    Two connections to the sender, one through a wire and one through the body and frame (ground connection). The gauge needs three connections: power and ground (to run the gauge) and a connection to the sender wire.

    It is possible to make the aux tank work with the factory gauge in the big round speedometer, and switch between the two tanks, if you buy and install correctly the right parts. Or, you can use the factory gauge for the factory tank and sender, and add a second gauge.
     
    Last edited: Mar 18, 2020
    65CJ5 SRD likes this.
  10. Mar 19, 2020
    BadGoat

    BadGoat How High Can You Climb?

    Northern Virginia
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    My fuel gauge would not work running the ground through the frame. When I ran the ground wire directly from the gauge to the sending unit it worked fine. Might be worth a try.

    Note that my taillights, turn signals, and reverse lights work fine through the frame.

    Mike
     
  11. Mar 26, 2020
    SoCalNickG

    SoCalNickG Member 2023 Sponsor 2022 Sponsor

    Whittier, CA.
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    Problem solved. I traced wires from the working fuel gauge to a toggle switch under the Speedo cluster. The toggle switch switches between the tanks and the single aftermarket gauge. I don't know if the original fuel gauge in the cluster works, may never know.
    The gauge I thought was a non working fuel gauge was a non working volt meter. No numbers but it has red, yellow and green bands. 12v is at the bottom of green band and needle goes up with RPM.
    2 new sending units plus cleaning connections and I have working fuel and volt gauges.
    Thanks for the help.:bananatool:
     
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