Fuel Guage / Sender Help.

Discussion in '1965 - 1973 Classic Mustangs -General/Talk-' started by j69302, Dec 28, 2013.


  1. j69302

    j69302 Active Member

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    Sorry for the long post, each little paragraph is a specific test I tried. I have been very thorough as you will see. Please take the time to read through it all.

    Been troubleshooting my fuel gauge, initially I grounded the sending unit lead to the tank and the gauge pegged full, so I changed the sending unit. Found out the float had flooded and was now an anchor.

    Replaced the sending unit, checked the resistances on it before hand, 8 ohm at full, 79 ohms at empty. Pretty close to Fords Spec (10 full 73 empty). And Also, it was very very close the the readings on my old sending unit with the bad float.

    Put fuel in 1 gal at a time, needle came up to the empty mark but never went above it. Checked again at 2, 3, 4, and 5 gal. Resistance on the float was changing, however the gauge indication never got above the empty line.

    So again, I grounded the lead to the tank and it pegged full. I also have 5 volts at the lead.

    I checked grounds from my fuel sender body (by the retaining ring) to the gas tank, the trunk floor, the leaf spring, and my exhaust pipes. All checked good

    Rechecked the same ground again but using the resistance scale, got 53 ohms to each one. Checking resistance though only the sender and also gave me 53 ohms. So my tanks seems to be perfectly grounded.

    I pulled my cluster out, checked resistance at the circuit board plug for the wire going to the sending unit and a dash ground. 53 ohms. Perfect! Right??

    Checked resistance for at the corresponding lead on the circuit board, though and to the far side of the gauge. Got 13 ohms. I also checked for shorts to ground and did not find any.

    Knowing that the water temp and oil pressure are the exact same gauges, I checked resistance across them. Got 12.8 and 12.9 ohms.

    I even made a short test lead with a bullet plug into the original lead, and a round terminal securely jam-nutted onto the threaded lead on the sender to guarantee a good connection. Check resistance from the instrument plug to ground, 53 ohms... Turned key on and needle read empty still.

    This next test is straight out of the ford manual:
    Now, to completely rule out the sending unit, even though is it obviously working as so far in spec, I got a potentiometer from radio shack, connected the wiper to one end and one of the legs to ground. Set it to 10 ohms. Verified 10 ohms from the instrument plug to dash ground, applied power, Gauge read full. Set the pot to 73 ohms. Gauge went to Empty.

    According to the Ford manual, my sending unit and gauge are perfectly within spec and should work.

    HOWEVER,
    When I set the resistance to 25 ohms, figured its around 3/4 tank, the needle stayed in line with empty.


    So I don't know what do do or check any more. I checked the entire system, with multiple grounds. Got 5 volts from my sender lead to just about every metal piece on the car.

    It appears the gauge doesn't want to work anywhere in between full and empty. The car sat for about 6 months. Gauge and old sender worked before.

    I also want to make note that when the ignition is turned on, the needle does move from below empty, up to empty, and results are the same in ACC position as well.

    All of my other gauges work great. My CVR is a solid state one from CJ pony parts and was putting out 5 volts which is what the Ford Spec is.

    I did check voltage through and after the sender and with the added resistance, it went down to 4 volts.
    Is 4 volts enough to move the needle?

    Tomorrow, I will see if I can check voltages at my water temp sender with the engine running, and see at what voltage makes the needle start to move.
     
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    Last edited: Dec 28, 2013
  2. geostang351

    geostang351 Member

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    Sign on for answer....
     
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  3. horse sence

    horse sence Please don't call me Mom SN Certified Technician

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    empty the tank ,pull the sender ,ground it to a good ground source .move the arm up and down slowly while someone watches the gauge .the arm could be hanging up in the tank some how.
     
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  4. robert912005

    robert912005 Member

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    Have you checked you wires from the gauge to the sender for a short?
     
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  5. iskwezm

    iskwezm Advanced Member

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    i tested my guage the same way and got similiar results. I gave up and just put in $20 bucks when i drive it.

    Other then replaceing the guage and sender again, im waiting to for a answer too
     
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  6. j69302

    j69302 Active Member

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    I ended up running a wire from the lead at the sending unit, to the driver seat. connected it to the unit, grounded to the dash, and watch the needle move in real time. worked fine... Also connected it through the water temp lead and watched if the sending unit responded any different to the water temp gauge and I got same results.

    Anyways, I got tired of messing with it and just filled it up. Took a total of 19.5 gallons and the needle stopped right at the full line.

    I didnt get to fill it up a little at a time and check the gauge... I will do this later if I feel the gauge is inaccurate at lower levels.

    after doing some math with the tank dimensions and converting it into gallons, and figuring out how many gallons it takes to raise the float an inch, I concluded that there are probably about 3 gallons of unusable gas. So at 5 gallons which is technically a 1/4 tank, is more closely to 1/8 tank actual usable amount.

    I figure with 5 gallons into a dry tank, there is probably not enough gas in the tank to make much much of a difference with the way I was testing it.

    I will drive around and see how the gauge travels as the fuel decreases and when it gets low enough to dump, I will see if I can barrow some most 5 gal cans so I can fill the whole tank up a little at a time, with known amounts of gas.

    I just want to know how its accuracy is at lower fuel levels.. I been keeping track of mileage and just fill up about every 150.
     
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  7. iskwezm

    iskwezm Advanced Member

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    heres one thing i noticed on mine a long while back, if i unscrew the cluster and pull it back away from the metal dash, it seems like the guage would move and register, if i screwed the cluster back in, the needle would drop to below empty.

    Anybody have ideas on that?
     
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  8. j69302

    j69302 Active Member

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    Not sure on that one... I dont know how it is on you year, but the 69's have the printed circuit board, and if the studs on the gauges touch anything metal other than the circuit board, the gauge grounds and will peg full.

    I supposed you can have 2 results here. Since electricity follows the path of least resistance, if the stud that is the gauge "INPUT" is grounded, it's possible all current will go straight to ground and completely bypass the gauge, basically making the the gauge and open circuit, and read nothing at all, not even reach the E on empty.

    If the second stud, OUTPUT is grounded, full current will go through the gauge and peg the high side.

    My best guess is that since yours drops below empty, the input connection is being shorted to ground somewhere when its installed.
     
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  9. iskwezm

    iskwezm Advanced Member

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    thats how my problems started, i removed the guages to put silver face overlays, when i put them back in the cluster they werent centered and when i turned the key, i saw magic smoke from the IVR.I built a new regulator and have the correct the voltage, but still no guage movement. Im ready to relace the whole thing(5 years later)
     
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  10. j69302

    j69302 Active Member

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    I remember when I took my cluster apart to clean it up, there were some insulated bushings (very similar in design to head bolt bushings) that go around the studs to prevent them from contacting the metal housing... Those crumbled to pieces and I don't know where to find them. I was just real careful at centering the studs when I reinstalled them.

    There are also plastic covers that clip on the the remaining parts of the studs that stick out, these won't prevent the studs from touching the housing, but can prevent anything behind the dash from coming in contact with the studs.

    The make these for 69 and up, not sure if you year has something similar. http://www.cjponyparts.com/gauge-stud-insulator-1969-1973/p/HW755/


    If you're like me and don't want to risk any reproduction crap, you can try and send your gauges to Tach man.. I have read nothing but good things. He can rebuild and re calibrate all the gauges. And probably even has the insulated bushings I mentioned.

    I think I'm going to send in my cluster anyways. I know my speedo needs the magnet to be re-magnetized. It reads MUCH lower than it should for the speedometer gear that I have in it.

    Plus, the error does not increase proportionally to speed. It is always a constant 10 mph low. Ex, if it is 10 mph low at 40, then it should be 20 mph low at 80.
     
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    Last edited: Dec 31, 2013

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