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Fuel Sending Unit / Fuel Gauge

Discussion in 'F-J-M: Volare, Diplomat, etc, 1977-89' started by Mopar_Gods, May 6, 2017.

  1. Mopar_Gods

    Mopar_Gods Member

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    The gauge is not showing a correct fuel level. With just around 8 gallons of fuel I am showing 3/4 on the gauge. The fuel sending unit is not even a year old and is the correct fuel sending unit for the tank not a after market re pop. On this type it has a one wire plug and there is only one fuel line that attaches to it. If I disconnect the wire the gauge will go to empty and if I re plug the wire it will go back to 3/4. I did a little wire test from the wire plug to a nice clean area for grounding and the fuel gauge now reads past the full mark on the gauge. It is almost like the gauge is reading backwards. I have never seen one do that before? Your thoughts. Thank You
     
  2. AllanC

    AllanC Well-Known Member

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    You just completed a successful test of the instrument panel fuel gauge. You verified that the gauge is functioning properly. Chrysler analog fuel gauges were setup to run with a constant voltage less than 12 volts at the instrument panel. This constant voltage would be applied and current would flow through the gauge and through wiring to the sending unit in the tank. As the float level rises in the liquid, the resistance in the circuit would decrease (voltage would increase) and the gauge pointer would move towards FULL.

    Conversely as the liquid level would drop and the sending unit float would drop, resistance would increase in the circuit (voltage decrease) and the gauge pointer would move towards EMPTY. With the wire disconnected at the sending unit there is no current flow (infinite resistance) and the gauge pointer is at EMPTY. Ground the sending unit wire (little resistance) and the gauge pointer is at FULL.

    You will have to remove the sending unit from the tank. There is a wound coil of resistance wire that the float arm contacts. Somewhere in the travel of the float arm, the resistance wire is shorted together and has the same resistance along the travel. Look for a frayed copper wire in the resistance coil.
     
    Bob Lincoln likes this.
  3. Mopar_Gods

    Mopar_Gods Member

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    I removed the sending unit from the tank. When moving the arm up and down I observed the arm tending to stick just a little. Bent the three tabs back and looks like the copper spring was bent down to much causing it not to ride smoother on the coil pad I pinched it just a hair bent the tabs back down and reinstalled. Now when I turn the ignition on we have 1/2 tank of fuel and when I turn the ignition off it goes right on E looks like I fixed it. Like I said it was a new NOS unit and just the coil spring was off it looked like. Little pinch closing seems to have fixed it. I will continue to observe it I am to old to walk because of running out of gas. Those days are over lol Thank You
     
    #3 Mopar_Gods, May 7, 2017
    Last edited: May 7, 2017
    Bob Lincoln likes this.

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