Gas gauge always full (1968 Barracuda)

Electrical and Ignition

  1. costan

    costan New Member

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    Hi folks,

    I've got a 68 Barracuda with some gauge issues:
    • Gas: Rises to full when key turns on, slowly falls to empty when key turns off
    • Temp: Always stays at C indicator
    • Oil Pressure: Always stays at low indication
    • Alternator: Always stays in the middle
    I'd like to get the gas gauge working (got aftermarket gauges for the rest).

    After poking around this forum, I suspect this is likely either a bad voltage limiter or possibly a short to ground between the sending unit and the gas gauge (as RedFish suggested here).

    Before I order a new voltage limiter and figure out how to pull the gauge cluster, does anyone know how I could rule out a short to ground or other issues?
     
  2. moparmat2000

    moparmat2000 FABO Gold Member FABO Gold Member

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    The voltage limiter or IVR (instrument voltage regulator) is inside the gas gage on a rallye dash. You can do 3 things, replace the gas gage with another used one, buy a repop one, or dissassemble the face off yours, dewire the voltage regulator part leaving the gage side intact and piggyback a homemade solid state regulator onto the back side of the instrument cluster. Much easier to do than it sounds.

    Your temp, oil psi, and fuel are all tied to the IVR which drops 12v to 5.5V to run those gages so if the IVR was acting up and sticking open they would all be at their lowest settings, if it stuck closed they would all go to their highest settings and would all do this at the same time.

    I suspect you have several issues going on. None of which are related to the IVR. I suspect a fuel gage sender short to ground as well. And the other 2 for oil pressure and temp are their own issues. Maybe the wiring is not connected to the senders, possible bad senders, possible broken wire internally. With these 2 an easy test will confirm the wire is ok. Jump the oil psi and temp sending unit wires straight to ground and turn the ignition key on. If those 2 gages go to the high end then the wiring is good and the senders are likely bad.

    Ammeter typically stays in the middle and just moves based on charge or discharge load. I recommend doing an ammeter bypass, convert the gage to volts an be done with it.

    Theres plenty of threads on bypassing the ammeter and bulkhead. I will post my schematics for this. Some people prefer to wire it differently. Theres other threads on here about converting ammeter to volts and what aftermarket voltage gage to use to do it. I used a 2" sunpro CP8215 volt gage for the guts to convert mine. They sell this gage as a white faced gage and a black faced one. Either one can be used since you will only be using the internal movement plastic grommets nuts n washers and throwing the rest away.

    1st pic, how its currently wired as stock
    2nd pic, the ammeter bypass
    3rd pic, homemade solid state IVR
    4th pic dewiring the gas gage, and what to leave intact
    5th pic solid state IVR. Shown with spades. One for rallye cluster uses ring terminals
    6th pic, how to jumper in the volt gage
    7th pic wiring in the Piggybacked solid state IVR

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    Last edited: May 22, 2019
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    • Rmoore

      Rmoore Well-Known Member

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      Is the fuel gauge wire plug in on both ends
       
    • Mattax

      Mattax Just the facts, ma'am FABO Gold Member

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      Not related to the other gages except by location.
      The main problem with the gage labeled alternator is the label. It's named kindof like the Batt terminal on the alternator.
      The gage is an ammeter indicating whether the battery is discharging or getting charged.
      Operational Check:
      Turn on the parking lights. The needle should move slightly toward Discharge.
      Turn on the headlights. The needle should move further toward Discharge. (That's about 12 amps.)
      Turn off lights.
      After the engine is started, running on fast idle, the needle should move to charge. Then gradually move to center.
      While driving, the alternator supplies power at rougly 14 volts to everything so no electricity flows through the ammeter (other than for recharging).

      Note: The ammeter connections are always hot. It doesn't matter if current is flowing or not. All the wires connected to the battery are at battery voltage and can flow if you accidently ground them.

      On the rally dash (your Barracuda) all of these get power when the key is on.
      The instrument voltage regulator provides a pulsing voltage. If you have an analog voltmeter, check what it shows between the temperature gage sending wire and ground. (It's the easiest to access).

      Then you could check the voltage at the fuel gage wire connection.

      It does sound like a short to ground is likely. Do those checks with the sending wire disconected and connected if the probe tip can be made to touch the the threaded section. Another way is to back probe in the bulkhead connector cavity.

      edit: quick first check could be to disconnect the fuel sender wire at the tank. Turn the key to run. If it the fuel gage shows full, there's a short and its not in the tank.
       
      Last edited: May 22, 2019
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      • moparmat2000

        moparmat2000 FABO Gold Member FABO Gold Member

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        Not sure what your asking. Fuel gage wire plugs in at sender. Fuel gage has 3 studs on it that go to circuit board. One stud is 12v in to the internal IVR, second stud is the 5.5V output that feeds oil psi gage and water temp gage, 3rd stud is for output to fuel sending unit.

        Quick way to check these 3 gages with power off is jumping a 9v battery across their terminals one direction they will rise and peg, other direction they wil go to the low side. This lets you know the gage part of the equation is working.

        All this plugs into the "gatlin gun" or cannon plug connector. Ine wire from there gives the 12v feed to the circuit board to gag gage, and 3 of those pins on the circuit board go to wires that go to the sending units.

        The gages not working is pretty simple to figure out, also make sure the cluster is grounding well to the dash frame this can also be a source of problems. Some guys will run an extra ground wire from the gage housing to a dedicated screw on the dash frame to make sure it is grounding.
         
        Last edited: May 22, 2019
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        • moparmat2000

          moparmat2000 FABO Gold Member FABO Gold Member

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          Note photo of back side of my dash. Printed circuit tracer w + id 12v in, the tracer w 6 is the 5 5-6V output to the 3 gages. The ivr i made attaches to red to the 12v stud, the yellow to the 6v output stud, and a black goes to cluster housing ground. The larger red and black jumper wires are from the volt gage i converted the ammeter to. Theres plastic grommets insulating them from touching the housing that is ground, they jumper to the 12v feed, and case ground. I have since removed the black wire from the volt gage, and have its leg ground to the housing where it bolts in. I reused the ammeter needle and face so it appears stock except when you shut the car off it goes to full discharge. There is a potentiometer or pot on the volt gage so you can fine tune the needle where you want it to sit at 13.5v after modding it to fit. I also used dimmable LED 5630 peel and stick strip lighting to line the gage housing for better illumination. Phone acting up. Will send pix later.
           
        • RedFish

          RedFish Well-Known Member

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          first thing I would do is disconnect the fuel sender at the tank. If at ign' switch on the fuel gauge needle remains at home, the short to ground is inside the sender. If the needle still races to full the sender signal is shorted elsewhere. Could be in the trunk or anywhere along left side of car. So pull the left kick panel and separate the rear harness connector. Ign' switch on again. If needle still races to full the short is somewhere between there and the gauge, maybe at the printed circuit board, maybe inside the gauge. Pull inst' panel. If circuit board pins, etc., seem fine, take a ohms meter to fuel gauge checking from post to post. Approx' readings expected , limiter is 50 ohms, gauge is 20 ohms, combined is 70 ohms. If the gauge winding is shorted to the beam, the expected 20 ohm reading will be wrong, 12 or less.
           
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          • costan

            costan New Member

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            Thanks, y'all! I'll poke around this weekend.
             
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