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fuel tank sending unit shorting?

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  • pkingco
    Experienced
    • Nov 9 2021
    • 217

    fuel tank sending unit shorting?

    Hi folks. I've been troubleshooting my fuel gauge (not registering any fuel) I've read the threads here and followed testing procedures for the CVR—all good. I have 6 volts at the wire at the sending unit (a new unit). I removed the sending unit and was going to test it with a remote ground, but before I did that I thought I would check to be sure there was no short in it. I'm getting continuity from the post to the body. This isn't correct, is it? The post appears to be isolated with a rubber grommet, so it seems there shouldn't be continuity to the top plate or case.
  • pkingco
    Experienced
    • Nov 9 2021
    • 217

    #2
    Hmm, on closer inspection, the rubber grommet is to keep fuel from seeping out around the post. I dismantled the sender and now see how the rheostat works. But I'm still not clear on how the sending unit grounds.

    Comment

    • jopizz
      Super-Experienced


      • Nov 23 2009
      • 8297

      #3
      The sending unit grounds to the fuel tank. Refer to the diagram to see how the internals work. To test the sender outside of the tank just use a ground wire from the sender case to any ground point. Turn the key on and move the float lever. You should see the gauge move from empty to full if the sender is good.

      John

      FuelGaugeWiring2.jpg
      John Pizzi - Squarebirds Administrator

      Thunderbird Registry #36223
      jopizz@squarebirds.org 856-779-9695

      https://www.squarebirds.org/picture_gallery/TechnicalResourceLibrary/trl.htm

      Comment

      • pkingco
        Experienced
        • Nov 9 2021
        • 217

        #4
        Thanks, John. Bear with me...if there is continuity between the post and the case, and I ground the case...mmm? What am I missing here?

        Comment

        • jopizz
          Super-Experienced


          • Nov 23 2009
          • 8297

          #5
          It's a resistor so you will have some continuity from the post to the case. If it was shorted your gauge would go to the FULL mark as soon as you turned the key on.

          John
          John Pizzi - Squarebirds Administrator

          Thunderbird Registry #36223
          jopizz@squarebirds.org 856-779-9695

          https://www.squarebirds.org/picture_gallery/TechnicalResourceLibrary/trl.htm

          Comment

          • pkingco
            Experienced
            • Nov 9 2021
            • 217

            #6
            Ok, that makes sense. I tested it with the ground wire and the gauge works fine. Pretty sure the issue is that I undercoated the fuel tank and used rubber between it and straps, so I am not getting any ground. I'll run a wire alongside the top wire and ground it to the top of the tank.

            Thanks again!

            Comment

            • simplyconnected
              Administrator
              • May 26 2009
              • 8772

              #7
              Undercoating your fuel tank is a common part of restoring as long as it's done properly. We depend on six volts to push through the gauge, spot welds, rust, connections and undercoat from the CVR to the sending unit and back to the CVR. The gauge simply measures heat produced by the current.

              Both Temp and Fuel sending units should measure 100-10 ohms from cold/empty to hot/full. Both gauges are identical and so are their sending unit resistances. Instead of using continuity, use an ohm meter, inexpensive meters work well too.

              For grounding, use this method:

              IMG_2953.jpg
              IMG_2954.jpg

              IMG_2959.jpg

              These pictures were the work of NYsquarebird58 (Marcelo Laviano, in NY). He used simple #14 THHN wire with stainless screws and nuts (brass works too). All are available at your local box store. If you have questions, please ask. - Dave​​
              Member, Sons of the American Revolution

              CLICK HERE to see my custom hydraulic roller 390 FE build.

              "We've got to pause and ask ourselves: How much clean air do we need?"
              --Lee Iacocca

              From: Royal Oak, Michigan

              Comment

              • pkingco
                Experienced
                • Nov 9 2021
                • 217

                #8
                Thanks for sharing those photos, Dave. Looks like a good approach.

                Comment

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