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72 fuel gauge issue
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mach1460



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    Post: #1
    72 fuel gauge issue

    I've had my 72 for 10 years now an the fuel gauge has never worked. Just moved into a new house with a 3 stall and I finally have room to work on my car.
    So I have 12v down to the harness in the driver kick panel, ( red connector, yellow/white wire. Problem is under the driver door sill plate there are two yellow/white wires going to the trunk area where the wires go to the sending unit. By the way I don't have 12 volts to my sending unit. I'm losing my power somewhere between the kick panel and the sending unit.
    Well what's confusing me is the two uyellow/white wires. Way up over the driver side rear wheel well, on the wiring harness I have one white/yellow wire coming out with a bullet style connector that has nothing plugged into it but it has 12v. Thanks for feedback everyone, good to see us 71-73 guys get our own site!!!
    Offline 10-06-2010 07:38 PM
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    Joe



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      Post: #2
      RE: 72 fuel gauge issue

      Someone a long time ago may have mixed up some wires. That 12v bullet connector going nowhere - I wonder if your fuel gauge would peg if you grounded it?
      Wait, never mind - I forgot about the IVR, so there shouldn't be 12v there if it is working correctly. The '72 Shop Manuals never come out and tell you what it's supposed to be, but on earlier Mustangs it was 5v.
      (This post was last modified: 10-06-2010 10:12 PM by Joe.)
      Offline 10-06-2010 10:00 PM
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      72HCODE


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        Post: #3
        RE: 72 fuel gauge issue

        The rear wire harness is pretty simple, but you can't just pull it cause if the dome light. you can back trace, if you feel along the drivers side trunk drop down you will find the sender wires.
        Run you hand and follow them until you get a good look at the wire insulator colors.
        Then go to the kick panel and match the same colors at the connector.

        When you tested the voltage did you pull the connector at the fuel sender? How corroded was the connector inside? Test the ground to the fuel tank also.

        It's also possible that the gauge is dead, test resistance of the sender itself make sure it's not open.

        You may need to pull the rear Seat and quarter panel trim, and the carpet holder/door sill plate, and back trace the wire into the trunk looking for a break.
        Offline 10-07-2010 07:27 AM
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        mach1460



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          Post: #4
          RE: 72 fuel gauge issue

          Sending unit is new, and has 18 ohms of resistance. At the small red connector in the kick panel my y/w wire has 12v. I pulled the sill and checked at the latch side of the door and had 12 volts but there are two y/w wires. Ones is dead. There all in the factory loom and are unmodified. I also checked in the trunk and above the driver rear wheel well, I have 12v at the one y/w wire ( this one stops above the wheel well and has the bullet connector) the other y/w wire leads down to the connector to the sending unit and is dead.
          Offline 10-07-2010 08:02 AM
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          OMS



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            Post: #5
            RE: 72 fuel gauge issue

            Is the one with the bullit connector more green than yellow?
            Green wire with green bullit connector is for the optional trunk light.
            Offline 10-07-2010 08:15 AM
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            72HCODE


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              Post: #6
              RE: 72 fuel gauge issue

              is the connector very rusty look at the rubber plug, inside there are the 2 connectors that fit over the poles on the sender. i had a lot of corrosion inside i took a toothpick and i cut a sliver of sand paper and glued it to the toothpick then cleaned out the connection, and took a small flat head jewelers screw driver and slightly bent in the connector so it really grabbed the sender connector poles. Lastly i coated the sender connector with dielectric grease to prevent more corrosion.

              this may not solve your voltage loss but can help things later if you discover a broken wire up the chain.
              Offline 10-07-2010 09:24 AM
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              Joe



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                Post: #7
                RE: 72 fuel gauge issue

                If there is no power at the sender, it's possible your IVR may be dead. Do you have any other instruments that run off of it? The original owner of mine wasn't big on instruments, so my fuel gauge is the only thing powered by the IVR. (Everything else is idiot lights.) There could also be a problem with the printed circuit card.
                Offline 10-07-2010 10:36 AM
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                72HCODE


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                  Post: #8
                  RE: 72 fuel gauge issue

                  that is a good idea, if you trace all the wires and can't find the problem then you'd have to pull the instrument cluster.

                  the IVR is attached to the back of the panel with little 9Volt battery buttons.

                  you could take the opportunity to replace both the circuit board(copper on mylar film) with a new one, and also replace the IVR with a new one. both are available as reproduction, if you still get nothing that gauge could be messed up.
                  Offline 10-07-2010 11:51 AM
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                  mach1460



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                    Post: #9
                    RE: 72 fuel gauge issue

                    I have my 12v at the red connector in the kick panel but i loose it somewhere between, if you look at my pic you can see the wire that has the bullet connector on it it has 12v but nothing goes to it.
                    here is the sender wiring, no voltage...


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                    (This post was last modified: 10-17-2010 02:51 PM by mach1460.)
                    Offline 10-17-2010 02:42 PM
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                    72HCODE


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                      Post: #10
                      RE: 72 fuel gauge issue

                      it may be a good idea to pull the rear harness and diagnose it with a ohm meter, a wire may have broken or shorted inside the harness covering.
                      Offline 10-17-2010 06:38 PM
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