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Thread: Fuel Gauge only showing 3/4 tank when tank is full

  1. #1
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    Default Fuel Gauge only showing 3/4 tank when tank is full

    All:

    Trouble shooting a minor annoyance. The fuel gauge on my 1986 2a 88 is showing 3/4 of a tank when the tank is full. I've checked the sender and the float is pinned against the limiter and should be sending the "tank full" signal to the gauge.

    Not sure if it's the sender not sending enough current (even though it's pinned) or the gauge reading improperly.

    Any trouble shooters out there already "been there/done that" with this kind of an issue - interested in any tips to try next.

    Thanks!
    1968 Series IIa
    1997 Defender SW (Original Owner - Sold)

  2. #2
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    Quote Originally Posted by lumpydog View Post
    Not sure if it's the sender not sending enough current (even though it's pinned) or the gauge reading improperly.
    Hmm...I thought we just addressed this in another thread very, very recently. Maybe it was someone else with the exact same problem or on a different forum, but...

    The "sender" doesn't send anything. It's just a variable resistor which controls the amount of electrical flow from the voltage stabilizer, through the gauge and then through the "sender" then on to ground, based on the position of the float in the tank. The gauge just measures that electrical flow and displays the result in a way that means something to the driver.

    Troubleshooting the sender is a simple matter of comparing the resistance of the sender to known ohm values for different fuel states using the chart I made, at the bottom of this page (I presume you have a negative-ground sender).


    Was the ohm reading you took on the sender in line with the ohm reading you'd expect to get for the amount of fuel you had in the tank?


    In order for the fuel GAUGE to read accurately, the input voltage to the circuit must be correct. Any inaccuracy of the input voltage will affect the reading, since the current flowing through the circuit is mathematically related to the input voltage and the resistance of the circuit.

    Is the correct average voltage being supplied to the gauge by the voltage stabilizer?




    --Mark

    1973 SIII 109 RHD 2.5NA Diesel

    0-54mph in just under 11.5 minutes
    (9.7 minutes now that she's a 3-door).

  3. #3
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    Oct 2006
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    South River, On, Canada
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    A little offshoot. I have a 60 PU that has been converted from positive ground to negative, with a delco alternator. The fuel gauge is wonky at times especially after the car has been running for a while.
    There is no voltage stabilizer.
    Should I add one and is there any other parts to change ie the sender?

    Cheers John

  4. #4
    Join Date
    Oct 2006
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    1,199

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    You need the spiyda designs Fuel Gauge Wizard. I have one in the 2A. It has the original pos earth gauge and the sender is a neg earth S3. A little bit of tweaking and now the gauge reads perfectly. Low fuel light comes on at exactly the right spot too!

  5. #5
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    Quote Originally Posted by SafeAirOne View Post

    Was the ohm reading you took on the sender in line with the ohm reading you'd expect to get for the amount of fuel you had in the tank?


    In order for the fuel GAUGE to read accurately, the input voltage to the circuit must be correct. Any inaccuracy of the input voltage will affect the reading, since the current flowing through the circuit is mathematically related to the input voltage and the resistance of the circuit.

    Is the correct average voltage being supplied to the gauge by the voltage stabilizer?
    ohm readings were 248 Empty and 18 Full.

    Average voltage was 12V

    Tank is full but showing exactly 3/4 tank

    Thoughts?
    1968 Series IIa
    1997 Defender SW (Original Owner - Sold)

  6. #6
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    The Granite State (NH)
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    Quote Originally Posted by lumpydog View Post
    ohm readings were 248 Empty and 18 Full.

    Average voltage was 12V

    Tank is full but showing exactly 3/4 tank

    Thoughts?
    Sender = good

    Unless you used an analog voltmeter, it's going to be difficult to tell the average voltage the gauge sees from the voltage stabilizer because of the way the stabilizer works (it "vibrates" on and off rapidly in a calibrated way so that between the 13.5 battery voltage peak and the 0 volt low, it averages out to be about 10v, which is what the fuel gauge is calibrated to).

    Are you SURE the wires to the sender are on the right terminals? Are you SURE you have a good ground path from the sender back to the battery?

    It's only a 3-component system (4 if you count the wires themselves). The sender is good. presuming the wires are good and on the correct terminals on the sender and you have a good ground path, the next item on the list would be the voltage stabilizer, but only because it's cheap to replace. Certainly cheaper than the final component in the system, the gauge.
    --Mark

    1973 SIII 109 RHD 2.5NA Diesel

    0-54mph in just under 11.5 minutes
    (9.7 minutes now that she's a 3-door).

  7. #7
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    Quote Originally Posted by SafeAirOne View Post
    Sender = good

    Unless you used an analog voltmeter, it's going to be difficult to tell the average voltage the gauge sees from the voltage stabilizer because of the way the stabilizer works (it "vibrates" on and off rapidly in a calibrated way so that between the 13.5 battery voltage peak and the 0 volt low, it averages out to be about 10v, which is what the fuel gauge is calibrated to).

    Are you SURE the wires to the sender are on the right terminals? Are you SURE you have a good ground path from the sender back to the battery?

    It's only a 3-component system (4 if you count the wires themselves). The sender is good. presuming the wires are good and on the correct terminals on the sender and you have a good ground path, the next item on the list would be the voltage stabilizer, but only because it's cheap to replace. Certainly cheaper than the final component in the system, the gauge.
    Thank you for the quick response. A few additions.

    1) I'm unsure of the average voltage. My multimeter is digital and I now can't get it to give me a voltage reading off of a 12 volt battery, so something's not right there - not trusting it. I'm not sure my average voltage in my last post is accurate so strike that number from my last response

    2) There is only one wire terminal on the sender. So the wire to the sender has to be on the right terminal.

    3) I grounded (touched) the sender wire (at the tank) to the chasis and the fuel gauge easily moved up to "full". I also touched it to the top of the sender and the gauge went to full. So it seems like the sender is grounded.

    Very much appreciate the help here!

    Charlie


    Attachment 10233
    1968 Series IIa
    1997 Defender SW (Original Owner - Sold)

  8. #8
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    Hmm...That really seems to suggest that your float arm isn't making it all the way to the "full" position. Perhaps your float is iffy or the float is hitting the fuel uplift tube inside the tank or that the float arm just needs a bit of a "calibrated" tweak put in it.

    Before I did that, I'd still hold a jumper wire between the top of the sender and a good grounding spot and see if the gauge reads correctly first.
    --Mark

    1973 SIII 109 RHD 2.5NA Diesel

    0-54mph in just under 11.5 minutes
    (9.7 minutes now that she's a 3-door).

  9. #9
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    May 2014
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    Marblehead, MA
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    Mark:

    I'll try the jumper wire. It's a good thought and I suspect a poor tank ground as well at this point. Or - the stabilizer.

    The float arm is definitely clear and is pinned against the full position. As I've lowered the sender into the tank its floating high enough where it's pinned against the limiter just before I set it down flush. It will need a tweak/bend, as I've set it this way to make sure it is reading full as I trouble shoot.

    Edit: Just tried the jumper wire and no movement. Tank stayed at 3/4. Double checked and if I take the wire off the tank and touch it to the top of the sender or the frame - the gauge shoots to full.

    I'm going to pop out the sender tonight and ground it - then hook it to the gauge and manually swing the arm to see what happens.

    If that shows the same result, maybe the voltage stabilizer? Although, my water temp (fed off the same stabilizer) seems to read fine.

    Charlie
    1968 Series IIa
    1997 Defender SW (Original Owner - Sold)

  10. #10
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    Pulled the sender unit. Grounded it and swung the arm back/forth. Same result, full tank at 3/4 on the gauge. I'm going to try a new voltage stabilizer.

    Double checked resistance and it was confirmed 248 empty and 18 full.
    1968 Series IIa
    1997 Defender SW (Original Owner - Sold)

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