Fuel and Voltage Gauge Woes

Discussion in 'Workshop Forum' started by AKimball92, Jul 15, 2019.

  1. AKimball92

    AKimball92 True Classic

    By the title neither my fuel gauge and voltage gauge are not providing the values that boost confidence.

    The voltage gauge reads 16 volts with the red light lit both when running or lights on only. Car runs fine. Is this temporary as something gets too hot and wears out over time? I checked the voltage to the orange to black connections across the gauge and it showed the ~12V (engine off). The gauge shows 16V with engine off as well. With that I feel the gauge might need adjustment or replaced? Can they be rebuilt?

    The fuel gauge reads empty with the green light lit. The sending unit is brand new. I read in a different thread to test each connection on the sending unit by touching the leads to ground. The left, I believe the blue/yellow, killed the gauge to reading full I believe. The red/black connection to ground turned the light off. Zero resistant through the sending unit resister sounds like it should read full. Mine reads empty at all times. Before replacing the sending unit it read the same with a few flickers as you drove. I don't have any miles on the new unit to see. The old one's resister sounded like a box of rust. When the particles aligned I would get a reading, some reading at least.

    Both gauge's needles move when rotation the whole gauge cluster.
     
  2. Rod Midkiff

    Rod Midkiff True Classic

    Location:
    Eugene, OR
    I would start by checking grounds, (I know people ALWAY'S say that) but there is a reason. The first thing to check on any strange electrical issue is grounds.

    but if you have this kind of luck. Just put the tools down and walk away.

     
  3. Rod Midkiff

    Rod Midkiff True Classic

    Location:
    Eugene, OR
    Here is an example of how a floating ground will do strange things (at the 1:45 mark I said this was a floating ground issue).

     
  4. kmead

    kmead Old enough to know better

    Location:
    Michigan
    That guy needs to reconsider his choice of working with electricity. I work with line voltage all the time and have only managed to zap myself two or three times. He clearly needs to think more carefully about the nature of voltage and the propensity to arc. Too funny.
     
  5. Rod Midkiff

    Rod Midkiff True Classic

    Location:
    Eugene, OR
    I am fairly certain that He is doing it intentionally. (on the one where the is shocked and lands on the bed, if you look close enough you can see the plug (not plugged in)
     
  6. AKimball92

    AKimball92 True Classic

    I will try to diagnose any grounding issues with this. Any ground connections you think might be suspect? Grounding the fuel sender unit does the exact opposite as my car at its current condition as described above. Both idiot light and gauge read empty when grounded intentionally. The gauge is reading full at all times.
     
  7. Joe F

    Joe F Hi Miles, Lo Maintenance

    Location:
    New Hampshire
    According to the Electrical Diagnostic Manual, gauges are grounded at "G3". Probably a ground tree located under the dash to the left of the steering column. The fuel sender is grounded to the left of the engine, attached to the engine compartment wall.
     
    AKimball92 likes this.
  8. AKimball92

    AKimball92 True Classic

    Thank you. The whole gauge cluster is removed so now is the time to look.
     
  9. kmead

    kmead Old enough to know better

    Location:
    Michigan
    G3 is likely near your left knee when driving
     
  10. Rod Midkiff

    Rod Midkiff True Classic

    Location:
    Eugene, OR
    you can also test, ground a wire. and add that grounded wire to the ground terminal on the cluster (just be sure you pick the correct one, or you could see the traces on your cluster go up in smoke)
     
    AKimball92 likes this.
  11. AKimball92

    AKimball92 True Classic

    I thought about doing that. The black one out of 23 I believe is ground. This can also be confirmed by hopefully tracing the wire as well as using the picture of the back of the gauge Cluster that is floating around here somewhere. I found it the other day but not willing to search while on my phone.
     
  12. lookforjoe

    lookforjoe True Classic

    If the gauges are continually reading full, the cluster has a bad ground. The senders are being used as a path to ground for something else in the cluster.

    Credit to our factory wiring guru for this pic:

    [​IMG]
     
    Last edited: Jul 22, 2019
  13. AKimball92

    AKimball92 True Classic

    What should the fuel and voltage gauges show with key off? do the needles tend towards 0 V and 0 fuel or full?

    I believe I had the fuel sending unit wires backwards. I need to pull the sending unit out to verify its a full tank actually. Gauge is reading full but I don't want a false positive.
     
  14. Jimmy II

    Jimmy II Fair weather only

    Location:
    West End, NC
    In my 86 w key off every gauge reads zero. Mine works normally when the engine is running.
     
  15. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    I may easily be wrong without double checking my thoughts here, but I believe the wires to the fuel sender can go either way without difference? Isn't it just a loop to read resistance?
     
  16. Cratecruncher

    Cratecruncher True Classic

    Location:
    Austin
    Three wires total. One is a ground. One wire is the variable resister for the gauge. The higher the float, the shorter the variable resister length and the higher the voltage to the gauge. The third is for the reserve warning light. When the float reaches the bottom of it's travel it makes direct contact with ground.
     
    Dr.Jeff likes this.
  17. AKimball92

    AKimball92 True Classic

    I never provided an update. I've been driving the car a lot lately. All the gauges work great except for the voltage gauge. I didn't have any confidence in it working either because while the car was on I used the multimeter between the orange and black connector on the cluster and it read the usual 12ish. The gauge still reads 16V with light on. Are they rebuildable? Would something else be off on the circuit?
     
  18. Since it is reading high, you could always connect it to a voltage divider circuit. You could make one using a potentiometer and have a calibration knob for your voltmeter.
     
  19. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Just bend the needle till it says what you want and glue it there. :D
     
    AKimball92 likes this.

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