Fuel gauge / sender calibration

Discussion in 'Bay Tech Clinic' started by steve_4802, May 2, 2017.

  1. steve_4802

    steve_4802 Active Member

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    Brisbane
    I've a 75 bay with iffy fuel tank reading. I'd like accurate readings for a long up coming road trip.

    My fuel gauge correctly reads full when full, but it reads empty by the time it's only about half empty. Thus for the entire second half of the fuel tank the reading says it's completely out but of course there's about 30L left.

    I had the sender out a while back and it seems to float correctly from empty to full so I expect it's a problem with the gauge, or is there a voltage regulator there too? I'm pretty poor with electrics but I read on another thread that there are some screws that you can adjust to calibrate the gauge? Is that what I should be tinkering with, and if so, can anyone tell me how, or provide a link to some good info on it?

    Bentley doesn't seem to say much on the topic of calibration, except send it to VDO at 116 Victor Ave. Detroit - helpful!

    Am I on the right track?
     
  2. Alpal

    Alpal Moderator Staff Member

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    Few threads covering fuel gauge.
    Try a search or someone might provide the link??
     
  3. steve_4802

    steve_4802 Active Member

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    Hi Alpal, yes lots of posts but nothing going into any detail that I could see, particularly with regard to calibration. Just the one speaking a little about adjusting screws but no description of process. Seemed a bit light on and was hoping to get a more conclusive thread for future reference. Maybe I missed something in what's already been posted?
     
  4. Barry

    Barry Well-Known Member

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  5. Barry

    Barry Well-Known Member

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  6. paul77

    paul77 Well-Known Member

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    The first check is sender. As per the threads on here, sender resistance rear of gauge to ground is about 10 to 70 ohms, full to empty I think. If you don't get these measurements then calibration may not be possible. I took my instrument cluster to ottos instruments in Salisbury qld. Rewind and calibration about $100 after I had fitted new stabiliser ($40 justkampers).
     
    Last edited: May 2, 2017
  7. Alpal

    Alpal Moderator Staff Member

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    Ohm range is 10 to 73. Half about 22?
     
  8. steve_4802

    steve_4802 Active Member

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    Thanks guys, the linked threads and sender info is most useful. I'll have a tinker.
     
  9. steve_4802

    steve_4802 Active Member

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    And what's the consensus on sealant / grease on the sender o-ring, when I get that far?
     
  10. 1500king

    1500king Well-Known Member

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    generally nothing... little silicone spray helps set a new one in place...while locking the sender.

    If your level drops off, the resistor wire on the sender is broken.
     
  11. Alpal

    Alpal Moderator Staff Member

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    Also calibrating gauge requires VERY sensitive adjustments. One adjustment is range and the other is to zero the gauge.
     
  12. steve_4802

    steve_4802 Active Member

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    Location:
    Brisbane
    Cheers 1500 and Alpal.

    Made a little progress today. Sender is cooked; I think lifting it out broke it completely as the the contact / connecter inside between the resistor and positive terminal was snapped, couldn't get any reading other than empty then. So ordering a new sender and o-ring today to fit later in the week.

    Some good and bad progress. Aside from sender the tank is looking pretty tired. I already had made a few repairs with knead-it (2 yrs ago) and had to do another two, so the tank is clearly on it's later legs. Going to look into getting the tank refurbished this week to extend the life a bit as any minor fuel leaks can really make the cabin smelly, and pulling it at short notice is anything but fun. Taking that mindset of 'while there' I used up some dynamat and hood-liner insulation that I had left-over from recently re-doing the top of the engine bay (can see in photos below). These leftovers I used to soundproof and insulate the bulkhead in front of the tank. Dynamat is of course usually put on the interior as it's obviously easier, but this I think is a rather elegant solution that means I don't have to rip up interior carpet and can also get a much thicker 3/4 inch insulation there - I took a couple of photos for reference. Yes, my orange bus used to be green!
     

    Attached Files:

    Last edited: May 7, 2017
  13. steve_4802

    steve_4802 Active Member

    Messages:
    269
    Location:
    Brisbane
    Alright, so I received the new sender today. I did some resistance tests on it and also hooked it up out of the tank (tank is off being cleaned / repaired).

    The resistance range was good, and showed a near perfect linear change in resistance as follows:
    Empty = 71 ohms
    1/4 = 57 ohms
    1/2 = 40 ohms
    3/4 = 23 ohms
    Full = 6 ohms

    So I'm happy the sender is working correctly. I then proceeded to hook up the gauge and battery, with ignition on but engine not started. Some interesting observations:

    • Empty and full readings correct
    • Readings are changing right throughout the range, and not maxing out or zero'ing out prematurely, but
    • Rate of change is quite a way off, considerably more non-linear than the gauge (which is slightly non linear I note)
    I took photos for each reading from empty to full, at each quarter to document what I had:


    Empty:
    0_Empty.jpg


    1/4:
    0.25_One Quarter.jpg


    1/2:
    0.5_Half.jpg


    3/4:
    0.75_Three Quarters.jpg


    1/1 (Full)"
    1_Full.jpg



    I don't want to tinker with the gauge adjustments unless I think I can correct the gauge. But from what I can tell a range & zero adjustment, if I understand it correctly, cannot correct what I'm getting because the range is already correct? I essentially need to flatten the fill profile which I don't see how I can do with those two adjustments? So then, begs the question:
    • Might it be the vibrator, or if that was faulty would it just not work at all? Any clue how to test this?
    • Might it be the gauge
    • Might I be completely wrong and it's adjustable to rectify, and likely already working correctly.
    With Surf Show and Shine in Mooloolaba coming up I'm pretty eager to get the tank back in and get the car cleaned up and running. As such, I need to get the sender back to the fuel tank guy for fitment (I'd rather he fit so that's it's pressure tested with sender in tank) in a hurry so car is all back together. So I think for the meantime at least I'll leave it as is and accept and just note what the real levels correspond to on the gauge. I can't see how it can be accurately calibrated once back in the tank as you'd forever be filling and emptying the tank to tank test readings?

    No last thoughts before I put back in the tank.

    Thanks for the assistance on this one. I must say, I've a new appreciation for guys who work on instruments. I was thinking about the calibration of this for hours and even started graphing the fill profile to best understand how it was responding. Geeking.

    Steve
     
  14. steve_4802

    steve_4802 Active Member

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    269
    Location:
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    Actually, upon further thought, I didn't think about the tab adjustments on the sender. I might be able to get a better set of readings yet.
     
  15. cbus

    cbus Well-Known Member

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    Wait for Al to make comment.
    His territory ;)
     
  16. Barry

    Barry Well-Known Member

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    there was reference to "Replacing the wire from the fuel gauge to the wire connector" in the first Samba link

    have you tried that?

    Earth?
     
  17. steve_4802

    steve_4802 Active Member

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    269
    Location:
    Brisbane
    Hi Barry, the earth looks in good nick, so only inspected it. I (rightly or wrongly) ruled out earth and power wire as it was reading empty and full correctly.
     
  18. steve_4802

    steve_4802 Active Member

    Messages:
    269
    Location:
    Brisbane
    OK, so I don't have the tank with me. As a long shot, does anyone happen to have a record of the fuel tank depth at the sender? Without this it's pretty hard to know were empty is, and I'm not confident my old sender necessarily showed the right travel to empty.
     
  19. Barry

    Barry Well-Known Member

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    Location:
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    not harping but is the connections nice and tight
    not slopping making a piece(sp) poor connection
     
  20. steve_4802

    steve_4802 Active Member

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    269
    Location:
    Brisbane
    Barry, I hooked a jumper lead to the neg wire just for you ;) . Same result.
     
    Barry likes this.

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