Yet another fuel gauge thread...but different

Electrical & Ignition

  1. wsutard

    wsutard Well-Known Member

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    but I want to make this one different. I have been reading all the fuel gauge posts on here and two things are clear to me. 1. there are a ton of smart people on here, 2. that throw out ideas all at once. All the great info is so hard to keep track of.

    My request, can an expert please take a shot at making a check off list on how to diagnose a non functional fuel gauge?

    Seems like there are three culprits in order of probability, 1. ground wire. 2. sending unit, 3. fuel gauge.

    If all the experts can agree on a check list then we might be able to make it a sticky thread and help everyone who seems to never be able to figure out their fuel gauge.

    Until then, I will just fill up every other time I drive.
     
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    • Beekeeper

      Beekeeper FBBO Gold Member FBBO Gold Member

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      TESTING FUEL,OIL AND TEMP GAUGES AND CIRCUITS

      1. Check for low resistance ground between chassis and sending unit.
      a. If no ground or high resistance check ground strap at sending unit.
      2. If a good ground is present check the ohm value at sender between sender wire and metal of sending unit.
      a. Resistance should be between 10-73 ohms. a high reading points to bad sending unit.
      b. Open circuit is a bad sending unit
      3. To test the gauge
      a. Disconnect wire from sending unit and turn the ignition switch to run. The next step is much easier with an extra hand. with someone watching the gauge ground the sending unit wire to the chassis and note deflection of the gauge it should move to full rather quickly note (do not ground the sending unit wire any longer than necessary to reach full position damage to gauge or regulator may occur). If the gauge does nothing there are three main causes of the malfunction
      a. Faulty wiring (high resistance and/or bad connection
      b. Faulty voltage regulator
      c. Faulty gauge (open circuit)
      d. Faulty ignition switch (the gauge feed comes through the ignition switch)
      usually dark blue with a tracer.
      4. Voltage regulator
      a. If any of the other gauges are working i.e. Oil pressure,water temp the regulator is most likely working and if none of them are working it's either bad or is not getting a ground or 12v supply.
      4. To test wiring
      a. Turn ignition switch to off and ohm the sending unit wire to chassis it should be infinity
      b. Remove the drivers side kick panel to access the wiring harness connector for the rear harness. Refer to service manual for wire color for sending unit (usually dark blue)
      c. disconnect the rear harness and ohm out the sending unit wire from connector back to tank
      should be very low resistance. From kick panel connector up to dash you will need to reference
      the factory service manual to aid in troubleshooting the circuit.

      This troubleshooting method works for the oil and temp gauges also.


      that's about as far as I can go for the typing I'm sure I missed something
       
      Last edited: Feb 1, 2017
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      • wsutard

        wsutard Well-Known Member

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        Great stuff. We should get this to be a sticky thread. Do they do those on this forum?
         
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        • wsutard

          wsutard Well-Known Member

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          Well, ran all the tests. Passed them all...except the sweep test. It did sweep but it wasn't smooth. It looks like the needle is sitting on the gauge face which is causing it to catch and jump when it tries to sweep. I had to tap the cluster to get the needle to fall back down when I turned the key off.

          I guess I am in the market for a new cluster. My speedo sucks also, to the point I had to disconnect it from the cable to keep it from whining at me all the time.
           
        • bm02tj

          bm02tj Well-Known Member

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          Go to your local electronics store and buy a few resistors then you can check calibration and function with no worry of hurting it
           
        • Beekeeper

          Beekeeper FBBO Gold Member FBBO Gold Member

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          Yep 10 ohm for full 23.5 for half and 72 for empty
           
        • wsutard

          wsutard Well-Known Member

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          Here is an update. I need more advice as o still can't track down the issue. I think my next step is a new sending unit as it seems to be the only possible issue left.

          The unit grounds tested good. I tested all the way to where the fuel line bolts to the body. I cleaned the ground strap and the fuel line bolt and location.

          The ohms at the sending unit tested within range. I was at about 60ohms.

          Wire continuity from the kick panel back to the tank tested good.

          Wire from the kick panel to the gauge tested good when I grounded the gauge.

          My gauge was kind of messed up so I had the cluster rebuilt (other stuff was bad in it) according to the shop all the gauges are fixed and within spec.

          All this and no change, still no reading.
           
        • bigmanjbmopar

          bigmanjbmopar Newb with a view

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          standard or rally cluster?
           
        • wsutard

          wsutard Well-Known Member

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          Standard.
           
        • kiwigtx

          kiwigtx International Mod Staff Member FBBO Gold Member

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          We can sort that out.:)

          The sweep test should include checking the needle is not bent or twisted - I had to straighten the needle in a couple of gauges of my A100 - different dash layout, but the same principal.:thumbsup:
           
        • wsutard

          wsutard Well-Known Member

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          I just had the whole cluster rebuilt by a reputable shop in my area. They specialize in Mopat clusters. It is all good so the problems is somewhere rear of that.
           
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          • bigmanjbmopar

            bigmanjbmopar Newb with a view

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            Not necessarily I did a whole wright up on the fuel gauge itself the factory fuel gauge has the ability to to be calibrated using the adjusters on the back of the gauge self on standard gauges if the needle on the gauge doesn't go all the way to the full does it go all the way to the empty or past empty?
             
          • kiwigtx

            kiwigtx International Mod Staff Member FBBO Gold Member

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            Cool.
            Maybe we could include a list of Reputable Gauge rebuilders in this thread?
            I had a cluster sent to a guy in Tennessee a few years ago, but I don;t think he exists anymore. i had a Volt meter conversion done for a 68-70 B-Body rallye cluster.
             
          • wsutard

            wsutard Well-Known Member

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            I used JC Auto Restoration in the Seattle WA area. They had the dash done for me in a day. Did the ammeter conversion, fixed my needles, rebuilt the speedo, re calibrated all the gauges. Refaced my switch panel, fixed my wiper switch.
             
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            • CoronetDarter

              CoronetDarter Well-Known Member

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              The gas gauge on the Coronet reads about 50% - i.e. 1/2 full with full tank; empty when tank is really 1/2 full. Brand new sending unit when I restored the car last year; before installing I checked it and it was in spec throughout the float's arc. At the time I also refurbished the gauges - cleaned them up, swapped the ammeter for a voltmeter. Cleaned up the traces on the motherboard (standard dash) with 0000 steel wool. When replacing the gas tank, the new ground strap snapped, so using hose clamps I ran an 8 ga wire from the neck of the sending unit to the chassis.
              I've been dealing with it for a year, so yesterday it was time to troubleshoot. First, took off the sending unit wire, grounded it to chassis, and the gauge pegged full. Next, removed the sending unit, brought it up front and using test wires with alligator clips, connected the output stud on the sending unit to the input stud on the back of the fuel gauge. Grounded the sending unit to the dash. Turned the key on and ran the float through it's arc. With the arm pegged to the "full" tab, the gas gauge (slowly) crept up to just below the Full line and held steady. Moving the arm down, as expected, moved the needle. The gauge read "Empty" when the arm was about 3/4 down its' travel. I'm OK with that, figuring that's a margin of safety, allowing for another couple of gallons in the tank when reading empty. The gauge read 1/2 full when the arm was almost 1/2 way between the two tabs.
              So at this point, must be high resistance in the wiring? The OE sending unit wire under the trunk was weather-beaten with almost no elasticity. So I cut the wire about a foot into the trunk where the wire gained back it's elasticity, and crimped and soldered on a wire with a ring terminal at the end, secured to the sending unit with a star washer and nut. Filled up the tank - now the gauge reads a needle-width under 3/4 full - better than before but still not where it needs to be.
              So I'm thinking the next step is to run a dedicated wire from the sending unit directly to the input stud on the back of the gas gauge, leaving the original wire dormant.
              Before I take out the back seat and pull up the door sill and carpeting to lay the wire, I wanted to get member's input on my troubleshooting and the next plan of attack.
               
            • wsutard

              wsutard Well-Known Member

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              Take a wire from the sending unit to the driver side kick panel. There is a harness in there. Connect the wire up to the correct spot on the harness, then test. It's much easier to get to than the gauge stud and your wire from the harness to the gauge is probably fine.
               
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              • wsutard

                wsutard Well-Known Member

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                Sorry, just read your post again. The purpose of my suggestion is to get the sending unit in its proper location, with gas, to see what the gauge reads. If it reads proper then you know it's the wire run from the unit to the harness in the kick panel. If it reads bad still then you know it isn't the wire.

                For me it was a combo of two things: my float had a hole and had filled up with gas, and my gauge was not calibrated correctly.
                 
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                • tommytmopar

                  tommytmopar Member

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                  I've had the same readings full tank half on gauge. You must be on to something if the wire change gave you more gauge deflection. Why not just run a long jumper and see if that makes the gauge read correct? Then if that makes it read correct then pull the seat. I'm going to try jumping out my sending units wire.
                   
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                  • CoronetDarter

                    CoronetDarter Well-Known Member

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                    Its not the float, tested it in gasoline. I think we're all on the same page, isolating the wire run.
                    The reason I was thinking home run wire is because it could even be the pin connector on the MB. When the dash was out I had to tighten someof the connectors before reassembling.
                     
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                    • Qbrozen

                      Qbrozen Well-Known Member

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                      Hi,
                      Maybe I'm being obtuse, but there is one detail I can't seem to ascertain after reading countless fuel gauge threads: will the pulse from the voltage reducer come out of the cluster if the fuel gauge is bad? I have tested at the pin at the gauge cluster and got nothing.
                      Thanks.
                       
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