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  • Electrical: Gas Gauge

    My 1947 Champion has a gas gauge problem. The gas gauge goes way past "full", as extremely pegged, when the gas tank is full. In checking the sending unit (removed from the tank) the resistance measures from about zero to 80 ohms when at the top. The sending unit is free to move and the cork float does float. I have purchased a new gas gauge (from SI) and still have the same problem. I installed a 6 ohm resistor in the wire to the gas gauge from the sending unit in an attempt to balance the load which worked for a while but not for long. Removing the wire from the sending unit does cause the gauge to go to "empty" as does turning off the ignition so I conclude that there is nothing wrong mechanically with the gauge. As the gas tank is near empty the gas gauge shows "empty". BTW, a new sending unit purchased from SI does not have quite the bolt pattern necessary for a tight seal on the tank. Any ideas?

  • #2
    That sounds frustrating. When you say the resistor worked for a while, did it slowly work its way back over the Full mark or just come on to the excess marking one time when you started the vehicle?
    The poor fit would be irritating. On my Lark I noticed a lot of sealant around the sender, maybe that happened with this PO and their sending unit not fitting. Not much help to you, other than the knowledge that that epoxy sealer works very well on a gas tank, I can attest.

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    • #3
      The wire from the gauge to the sending unit is shorted to ground. The sending unit is just a variable resistor to ground. Less resistance as the float rises. Thus with the wire shorted out, the gauge thinks the tank is full. (no resistance)
      Bez Auto Alchemy
      573-318-8948
      http://bezautoalchemy.com


      "Don't believe every internet quote" Abe Lincoln

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      • #4
        On my 1950 Champion the gauge will show over full if there's an open, such as now when the tank is out of the car.
        If I short the wire to ground, then the gauge shows empty.

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        • #5
          Yes, I understand the electrical circuit. If the wire from the gauge to the sending unit were shorted to ground, the gauge would still say "full" when the wire has been disconnected from the sending unit. It says "empty" when the wire is removed. Thanks for comment.

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          • #6
            Fast Eddie;
            if as you say when the wire that goes to the gas tank float is grounded and held to ground the gas gauge correctly indicates a full tank and does not go past full the trouble has to be with the gauge and not the float assembly. In order for the gauge to show more then full something has to be wrong with the gauge.
            Ron

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            • #7
              Thanks Ron, this is about the only conclusion to come to.

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              • #8
                By the way, do not test a gauge by hooking it up directly to a power source, without some resistance in the circuit. Most old gauges are a couple of wire wound heating coils, that heat a bi-metallic strip, that the needle is attached to. It can get too hot and melt the nickel resistance wire, then your gauge will no longer function.
                Bez Auto Alchemy
                573-318-8948
                http://bezautoalchemy.com


                "Don't believe every internet quote" Abe Lincoln

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                • #9
                  Thanks for this tip. I just looked at the wiring diagram and I see no sign of a resister in the circuit but I will heed your suggestion.

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                  • #10
                    The '47 Champion probably has a two-terminal sending unit. I know my '40 does, and they were used on M-trucks and on early 2R trucks, too. Check your wiring diagram closely. On those sending units, the ends of the resistance element are brought out to the terminals, and the wiper (moving contact) is grounded. In electrical nomenclature, the unit is a potentiometer, instead of the more typical rheostat. The gauge has two thermo elements in it, one tends to move the needle towards "Full", the other towards "Empty", connected to the respective ends of the sender. The idea is to make the gauge immune to variations in supply voltage. If the end of the resistance element, or its connecting wire, going to the "Empty-wards" element in the gauge is open, you will get the failure you are seeing.

                    If your sender is indeed two terminals, you should see about 150 ohms between them, and ground should split that. E.G., if one terminal is 70 ohms to ground, the other should be 80 ohms.

                    Obviously, the gauge used with a two-terminal sender has to have three terminals on it: hot, sender 1, and sender 2. Ground is through the case/mounting screws. It's possible that some prior owner replaced a two-terminal sending unit with a single terminal one, and cut or taped off the extra wire, which would also give you much the same effect.
                    Gord Richmond, within Weasel range of the Alberta Badlands

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                    • #11
                      Thank you Gord.
                      The sending unit is a single post and it is for certain the original sending unit. It is defiantly a rheostat and not a potentiometer. The resistance measures (about) 80 ohms. I do understand what you are saying about a potentiometer adding resistance on both ends of the scale. This is not the case with my 47 Champ. There is only one wire connecting the gauge with the sending unit.
                      I do appreciate your insight.

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