fuel gauge / fuel sending unit low power?

chowboys,

trying to troubleshoot/fix an issue with my fuel gauge. the gauge will rise after fill ups and fall as i drive, but it seems like it's always about an 1/8 behind/below what the actual fuel level is. for example: when full, the gauge shows nearly full (about an 1/8 below full), and when the tank is about half full, the gauge shows less than a half etc... i have been using the trip meter to know when i need to gas up -- an acceptable work around, but i'd love to have the gauge functioning properly.

here's what i've done so far to test:

1. remove sending unit and measure resistance:
attachment.php?attachmentid=3020&d=1436653158
SENDING UNIT RESISTANCE ("EMPTY")
- manual lists 105-110 ohms as acceptable range
- seems fine

attachment.php?attachmentid=3021&d=1436653163
SENDING UNIT RESISTANT (FULL)
- manual lists 2-5 ohms as acceptable range
- slightly above 5, but prolly close enough? seems fine here too


2. test voltage to sending unit
attachment.php?attachmentid=3019&d=1436653153

attachment.php?attachmentid=3018&d=1436653147

attachment.php?attachmentid=3017&d=1436653142

attachment.php?attachmentid=3016&d=1436653137
DOESN'T SEEM LIKE THE UNIT IS GETTING THE RIGHT POWER... IS THIS CORRECT?
- as you can see, i got a range of readings here...
- however, shouldn't the unit be receiving 12volts?


CONCLUSIONS (do these seem accurate?)
- the fuel sending unit itself is working fine.
- however, it seems like it's not getting enough power... (should the unit be getting 12volts?)
- potentially, because the power is low, this is leading to the low readings in the gauge


NEXT STEPS (which are these are more likely?)
if the voltage is not correct the manual lists three potential problems:
- blown no1 (10A) fuse in the dash fuse box
- an open in the YEL, YEL/WHT, BLK wire
- poor ground (G511, G551)

Comments

  • If you have it all the way up, during your test does the gauge actually go to full or it's still that 1/8th short?

    Voltage I believe should be 5-8V so you're fine there with the voltage going to the unit. Also are you sure it's fully filled up? I get about an extra gallon if I top off. Otherwise the gauge is a smidge below full on mine as well. Fully topped off, it's all the way up there.
  • thestzathestza Wagonist
    gtg,

    thanks for the response. i'll try to top it off tomorrow to see if i can push the gauge up over the F marker. i had a 91 civic hatch base model in the past, and i feel like the gauge would read at full (or over full with a top offf), but this gauge never seems to get there... worth a try nonetheless.

    is 5-8V standard for that circuit? or should i be expecting 12volts?



    will top it off like there's no tomorrow and send a pic of the gauge tomorrow.

    thanks again
  • Yeah that 5-8 is standard. That's what should be coming out of the regulator that goes over there, so that's fine. If you can pull it out of there and see if you raise it up all the way if it goes to full on the gauge itself it could honestly be something as simple as the float needing adjusted.
  • thestzathestza Wagonist
    hi again GTG,

    following up... i pulled out the sending the unit this afternoon and raised the arm all the way (to simulate a full tank). the gauge showed the desired result (slightly over full), but one that i can't seem to achieve normally during fill ups...
    attachment.php?attachmentid=3031&d=1437357620

    you mentioned maybe adjusting the float... what sort of adjustments can be done?

    thanks for helping to troubleshoot this... learning little by little
  • HaydzHaydz Moderator
    If I recall rightly, there is a float inside a tube and the tube can get grotty, causing intermittent (and sometimes full-time) fuel reading issues. Clean her up if you've confirmed the gauge and sender are OK.
  • thestzathestza Wagonist
    yo Haydz,

    thanks for helping out. where is the tube located? inside the body of the black float? inside the base of the unit (photo attached)? somewhere else entirely?
    attachment.php?attachmentid=3032&d=1437416903

    S
  • There's nothing in that black float it's just a float nothing else. Anything that needs to be cleaned up is in the electronics widgetry there. If that doesn't yield the results Haydz is talking about, I'd just caveman it and give that arm a downward bend. That's what I meant by adjusting the float. Just a bit of trial and error is needed there.

    Also when it's dead empty is it below the empty mark as much as it's below the full mark when full? If so then that should pretty well sort things out. If not, well then it might need more investigation electrical side with the sender itself.
  • thestzathestza Wagonist
    thanks GTG,

    yeah, when it's near empty and dead empty, the needle is about 1/8 below where i'd expect it (the same amount as below the full mark). i'll take a look inside the electronics widgetry this weekend... and try caveman-ing after that. when/if i go to bend, should i bend at the pivot closest to the float or in that middle section?

    s
  • Probably what I'd do is raise it up to the point where it currently shows when it's full. Then measure the distance between that and how far you need to move it to get it right at full. If you want to go that tick above full, that's fine, but I'd try to get it as close to right on the money as possible. Then just bend it down that distance. I'd just give it a bit of a twist really at that middle point. so the long leg with the float on it goes down a bit more.

    But yeah to me since it's reading as much off at empty as it is full it seems to me like it's just a matter of a float adjustment. If you were getting wildly different measurements, then I'd see it as something more electronic, so at least in my opinion this is just a mechanical issue to sort out.

    Definitely let us know the results. Hell mine does the exact same thing, I've just been too lazy to fix it myself. I have done this on my old volvo though which was experiencing similar issues.
  • Any news on this front?
  • unc25unc25 New Wagonist
    interesting read..just saw this thread

    I hate my fuel gauge as its never accurate and I figured I just had a shoddy sending unit.
  • thestzathestza Wagonist
    hey, sorry for the lack of updates... prepping to move and the fuel gauge has taken a back seat. GTG, to follow up on something you mentioned previously (about topping the tank off): last fill up, i topped the tank off like nobody's bidness -- micropumping every possible drop into the tank (squeezed out about an extra gallon as you mentioned), and the gauge still read below full...

    should have some time later this week to follow up on haydz comment and then try cave-manning... or replacing the sending unit.

    thanks again for the help
  • Or just bend the arm?
  • thestzathestza Wagonist
    had some time to work on this today, so i'm following up. wanted to try two things:
    1. investigate haydz's suggestion earlier about "... a float inside a tube and the tube can get grotty, causing intermittent (and sometimes full-time) fuel reading issues"

    2. try GTG's 'caveman' float / arm bending idea


    so i pulled out the sending unit to see if i could access the internals, but there wasn't an obvious way to get inside without some minor surgery (sorry the pics are kinda contrasty... warm california sun in the afternoon...):

    IMAGE OF SENDING UNIT (LOOKING FOR WAY TO ACCESS INTERNALS -- HAS ANYONE EVER OPENED ONE OF THESE?)
    attachment.php?attachmentid=3065&d=1439441813


    so i then went to GTG's plan of bending the arm -- following his suggestion: "...raise it up to the point where it currently shows when it's full. Then measure the distance between that and how far you need to move it to get it right at full. If you want to go that tick above full, that's fine, but I'd try to get it as close to right on the money as possible. Then just bend it down that distance. I'd just give it a bit of a twist really at that middle point. so the long leg with the float on it goes down a bit more."

    this took some trial and error, and i was amazed at how sensitive the unit is near the 'full' side. micro adjustments had a huge effect on the reading.

    INITIAL TEST AND EFFECT ON FUEL GAUGE
    attachment.php?attachmentid=3066&d=1439441818
    attachment.php?attachmentid=3067&d=1439441823


    eventually, i found an arm/float position that produced the result i'm used to seeing on a full tank:

    SENDING UNIT ARM/FLOAT POSITION AND FUEL GAUGE READING FOR NORMAL 'FULL'
    attachment.php?attachmentid=3068&d=1439441828
    attachment.php?attachmentid=3069&d=1439441833


    i measured the distance (as you can see, the arm is very near the max position), and then bent the arm to try and recreate the same distance:

    SENDING UNIT ARM/FLOAT WITH SLIGHT BEND
    attachment.php?attachmentid=3070&d=1439441837


    ***it's also important to mention that my tank was a little under half full at this point (which i knew from my trip meter). So, with that in mind, the ideal bend would produce a gauge reading of just under half.

    anyway, i reinstalled the unit and the reading was way off -- bent it too far!

    GAUGE READING WITH ARM BENT TOO FAR
    attachment.php?attachmentid=3071&d=1439441841


    ...took the unit back out, un-bent it a little and reinstalled... i did this about 8 times until i got the desired read:

    GAUGE READING WITH PROPER BEND (AT LEAST TEMPORARILY)
    attachment.php?attachmentid=3072&d=1439441847

    *** another important note, after bending the arm and reinstalling the unit, i was not getting super accurate reads by turning the key / activating the electrical alone. as soon as i would start the car and drive around, the reading would dip quite a bit (maybe indicating a deeper electrical issue). when turning on the electrical alone, i had overshoot my desired mark a bit... then turn the car on and drive around a wait for it to settle (hope that makes sense).

    after i got the desired reading, i went and filled up to check my work (without topping off). so far so good -- i got a reading right below absolute full! i'll follow up in a bit when my tank is at the half way point again to see if it all worked.

    anyway, the bending is not an exact science AT ALL and takes a shit ton of trial and error to get the right read -- and in my case, i might have a deeper electrical issue that this technique is simply a band aid for, but i'll take it for the time being.

    hope this is helpful to someone and thanks again to GTG and Haydz for chiming in and helping out.
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