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  • 81 fuel sending unit
  • 81 fuel sending unit
  • 81 fuel sending unit
  • 81 fuel sending unit

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  1. #1
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    Default 81 fuel sending unit

    I just put in a new sending unit in a 81. All it shows is empty. I did the test with grounding out the sending wire, it went all the way pass full, plus if I just disconnect the sending wire it stays pass the fuel mark. I'm I missing something or just not getting it?

    I heard that the new sending units do not fit right in this tank because of the slant back, is this true or just another vette rumor, If true how do you adjust the sending unit so it works.

    A little back ground I replaced the old sending unit because it would read full and drop down like normal but it still had 1/2 tank of fuel left when it showed empty

    any help would be great thanks

  2. #2
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    Sounds like maybe the arm on the sender is hung up and not floating in the fuel. From your description, the gauge itself appears to be working. Tank ground is good or it'd be reading past full all the time. Other option would be to pull the wire off the tank and put a 12v test light between that wire and ground- the gauge should go someplace around the 1/2 mark. That will prove the gauge responds correctly.

  3. #3
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    Thanks, I tested it again using the 12v test light and it showed full again
    I plan on taking it out to check to see if it is stuck on something in the next couple of days

  4. #4
    81 fuel sending unit GerryLP's Avatar
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    I know basic information only. There are two styles or arrangement used, but the most common is that a voltage is applied to one terminal on the gauge, and at that terminal a cable cotimues to the sending unit. When the rheostat in the sending unit increases in resistance (when the fuel level in the tank increases, so does the resistance), the resistance causes the applied voltage to flow at the gauge's terminal to the left of the gauge, and this creates a magnetic field that pulls the bottom of the gauge needle to the left causing the needle to sweep right (or full mark). Reversely, the decrease of resistance in the sending unit (as the fuel is consumed) causes the voltage to increase across the terminal to the right of the gauge and bleeds away current from the terminal on the left (weakens the magnetic field on the ledt side) and that pulls the bottom of the needle towards it and in turn the needle sweeps left or towards the empty mark side.
    GerryLP

  5. #5
    Member black_81_vette's Avatar
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    Well I have sort of the same trouble as mine does not show empty when it is in fact empty. Mine is empty at 1/8 tank, from what Ive been told 80 t0 82 had a lined gas tank and the liner shrinks and kits the float arm and causes erronious readings. Take a good strong flashlight and take a look down there and see if it in fact has shrunk and moved up any.

  6. #6
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    I looked into my tank when it was empty (while doing a body off) and there wasn't a liner in mine! When the reading is incorrect (like mine did) but is working most of the time the resistant on the back of the gauge isn't the right one. Mine had a pink resistant but now I'm using a yellow one (I think) and it's working much better.

    Greetings Peter

  7. #7
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    I pulled the sending unit back out and it does not look like it was caught on anything. I will check to see about the liner. Another question while I had it out I hook it up to see if it worked outside the tank and it did not work, the power and ground was still going to the sending unit but no matter what I did the guage did not move. Can you check it this way? When it comes to electricity in the vette I'm not the brightest tool in the shed thanks for the help

  8. #8
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    First I want to thank everyone for there help. You can test the sending unit outside the tank just hook it up like normal and have a buddy move the float it will give you a general idea that the system is working properly. Someone (no idea who it could of done it) didn't notice that when I put the sending unit back in I bent the power strip into the filler neck so it was grounding out twice Ye who dosn't pay attention gets to do things over for stupid reasons.

  9. #9
    Moderator bill81vette's Avatar
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    I'm sure we've all done that at some point with our vettes!!!!!
    Bill and Gail...save the wave
    NCRS member 42415
    1981 St.Louis car #2735
    all matching numbers

    born Sept 18,1980
    Cruisefest 2008
    Cruisefestous 2009
    Cruisefest 2010
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    Cruisefest2012



    Without Trucks......
    You would be Hungry,Homeless and Naked!!!!!!



  10. #10
    81 fuel sending unit GerryLP's Avatar
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    Default 4-year old thread comes back again...

    Hello everyone,

    As Corvette owners, we suffer through the lack of space, and therefore, how easy a task is. You won't believe how pleasant my surprise was, when I realized that changing the fuel sending unit was going to be a snap.

    Gail's fuel sending unit went bad when I test drove her for the radiator repairs that were needed to get rid of this annoying heat-soak problem it had. Well, I solved one problem, to gain another. All throughout the test drive, I had to dead reckon the mileage with the trip meter, in order to avoid starving the engine off fuel, and getting stuck in the middle of no-where.

    I removed the fuel tank access door (or hatch > )



    A view to the left, or driver's side, reveals this...



    To the right (passenger side) you'll see this...



    You'll have to pry it up to relief the grip that this gasket has had on the sending unit (it dawn on me then that this unit is the original one that came from the factory in my vette; 34 years of service ain't bad

    Once you separate the sending unit's from the mounting on the tank, rotate unit 90 degrees counterclockwise. This should place the driver's side tube at 6 0'clock and the passenger's side tubes at high noon. Carefully lift the unit while slightly tilting it to let clear the hole's edges. Keep a good grip on the unit. If it slips from your hands, it is likely to scratch your paint finish. Continue pulling the sending unit until it clears the tank.





    I was surprised to find the fuel tank coated with zinc-chromate primer inside. This illustrates how innovative the Corvette engineers are.




    A new gasket comes with the new sending unit. By the way, I ohm'ed the old sending unit, and at full mark, it was registering 120-125 ohms. This explains why it was reading full for a while, and why when empty, it would read half a gallon. The new one read approximately 90 ohms at full tank.



    Now, after test driving the new fuel sending unit, I noticed that the engine may have a loose wire because it almost died on me twice...sighhhh...the fun continues

    I hope this helps you see that it is not that hard to do a fuel sending unit on an L81. Happy fixing!
    Last edited by GerryLP; 07-21-15 at 09:43 PM.
    GerryLP

  11. #11
    Moderator bill81vette's Avatar
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    Did your low fuel lite work before the fix???and does it still or not?????
    Bill and Gail...save the wave
    NCRS member 42415
    1981 St.Louis car #2735
    all matching numbers

    born Sept 18,1980
    Cruisefest 2008
    Cruisefestous 2009
    Cruisefest 2010
    Cruisefest 2011
    Cruisefest2012



    Without Trucks......
    You would be Hungry,Homeless and Naked!!!!!!



  12. #12
    81 fuel sending unit GerryLP's Avatar
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    Quote Originally Posted by bill81vette View Post
    Did your low fuel lite work before the fix???and does it still or not?????
    Bill,

    The low fuel module had stopped working long before; however, I had placed one of those low priced ones, but it never worked right. Perhaps it was a sign that the sending unit was begining to fail (perhaps a few ohms were creeping up and affecting the low fuel warning), but not the fuel gage reading???).

    I plan pan to test my old one, but I think it is likely bad. If so, I'll buy one of those remanufactured ones I see on eBay to try. The last NOS I had seen there, they were asking $150 for it....that's 3 tankfulls of gas. Hahahaha
    GerryLP
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