Fuel Tank Sender Readings by the Gallon
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Thread: Fuel Tank Sender Readings by the Gallon

  1. #1
    Just Some Dude in Jersey My location The Scifi Guy's Avatar
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    Fuel Tank Sender Readings by the Gallon

    Has anyone ever made a chart of the ohm readings from the fuel sender for each gallon added to the fuel tank of a GT?

    My next project is to go to the gas station a few blocks from my house, totally pump out all the fuel in the tank, then add one gallon at a time and record the ohm readings my digital dash tells me. I have to recalibrate how my digital fuel gauge reads my fuel tank levels and I figure it would be interesting to make a chart of what the fuel sender sees. Unless someone has done this already. In profile, a GT tank is triangular with a box at the top, so the fuel tank sender's readings are not linear.



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    2000 Post Club norbertone.gt371's Avatar
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    You have a custom tank

    So it need a own chart! Arbeit,Arbeit and Arbeit
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    Approx. 5-minutes to find using Google search. Probably for stock GT tank and not custom.
    http://www.opelgt.com/forums/1e-othe...tml#post167455
    Attached Files Attached Files
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    Just Some Dude in Jersey My location The Scifi Guy's Avatar
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    Lindsay The Amazing strikes again! Nice find!

    I just completed my little tank filling and ohm readings project, here are my results:

    My fuel tank and filler neck holds 11 usable gallons of gas. That's from empty, with as much gas as my electric fuel pump would pump out of the tank, to full, with gas right up to the top of the filler neck.

    The sender, however, only reads 10.25 of those 11 gallons.

    The first 1/2 gallon and the last 1/4 gallon are unreadable by the sender.

    The fuel tank sender is brand new and the fuel gauge has digital accuracy.

    My custom aluminum tank is as close to a copy of the original tank as they could make. There is no spaquetti pot or any other empty chambers at the top.

    My fuel filler neck sticks out of the tail panel, not the rear deck as with a normal GT. I therefore lose a teensy bit of vertical filler neck that could be filled with gas.


    These are my ohm readings:

    90 ohms empty

    90 ohms with 1/2 gallon added(at this point the ohms start to go down as additional gas is added)

    63 ohms with one gallon added on top of the first unreadable 1/2 gallon

    51 ohms at two gallons

    44 ohms at three gallons

    38 ohms at four gallons

    32 ohms at five gallons

    28 ohms at six gallons

    17 ohms at eight gallons

    12 ohms at nine gallons

    8 ohms at ten gallons

    7 ohms at ten and 1/4 gallons(I can add 1/4 gallon of gas at this point to fill it to the top of the filler neck. The ohms will not go lower than 7 ohms.)



    Time to go to bed. I'll expect fully developed graphs and charts of my results from you guys when I wake up tomorrow morning.

    Dan, time to spring into action!


    Last edited by The Scifi Guy; 03-17-2017 at 09:01 PM.

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    7,000 Post Club My location wrench459's Avatar
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    Quote Originally Posted by The Scifi Guy View Post
    These are my ohm readings:

    90 ohms empty

    90 ohms with 1/2 gallon added(at this point the ohms start to go down as additional gas is added)
    Looks like you got only a few miles left on reserve when the gauge hits FIND A GAS STATION SOON.
    Have you heard of gas buddy?

    Si vis pacem, para bellum "If you want peace, prepare for war"

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    Just Some Dude in Jersey My location The Scifi Guy's Avatar
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    This advisory was stated in my Dakota Digital dash cluster set up booklet:

    "Fuel level sensors are generally not precise---------and you should expect to see some error. For instance, a GM 0-90 ohm sensor may display 5 ohms Empty and 97 ohms Full. If you are within 10% of expected results, this is within specification."

    GT's use the reverse 90-0 ohm type of fuel sender. My 7 ohm reading when full is within the 10% margin of error.

    For my own personal records:

    Ohm data needed to program the DD gauge:

    1/3 full = 42 ohms
    2/3 full = 24 ohms
    99% full = 8 ohms
    Full = 7 ohms
    Last edited by The Scifi Guy; 03-18-2017 at 09:51 AM.

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    2000 Post Club norbertone.gt371's Avatar
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    Quote Originally Posted by wrench459 View Post
    Looks like you got only a few miles left on reserve when the gauge hits FIND A GAS STATION SOON.
    Have you heard of gas buddy?
    That does no matter!
    There are lot of Police Officers they want to help with gas!
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  10. #8
    Just Some Dude in Jersey My location The Scifi Guy's Avatar
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    AArrgghh!

    What a PIA custom programming the DD fuel gauge is. The dash cluster's programming unit has about 8 pre-programmed types already in it's memory, but the 90-0 ohm type isn't one of them(0-90 is). So, I have to custom program the unit. The only way offered to do it is to:

    A) Remove your fuel sender from the tank and manually move it's sensing device at 1/3 intervals and at 99% and 100%, pressing a button to enter the resistance value at each step.

    B) Drain your tank and add fuel one gallon at a time until you reach each of the intervals and press the button.


    That's all fine and dandy if you already know precisely how much gas your tank holds(in my case 10.25 gallons with an additional, unsensible, 1/2 gallon at the beginning) and the precise range of ohms that the dash unit will sense coming from the sender(in my case 7-90 ohms). Also, if you want your gas gauge to give you a perfectly linear exact accounting of how much gas you have at the 4 quarter tank intervals displayed on the instrument cluster, you need to know the sender voltage readings at 1/3, 2/3, 99%, and 100% of your actual capacity, due to irregularities of fuel tank shape. Well, I didn't know all those ranges until I did the pump out of the tank, the adding of the individual gallons, and I took all the various voltage readings. Because I hit 100%(7 ohms), before I knew that 8 ohms was 99%, the programming unit wouldn't accept my calibration because it didn't see a number less than 7 ohms when I pressed the button for 100%. Confusing I know, but what this all means is that I now need to repeat the whole darn process of draining and refilling the tank one gallon at a time.........

    ......OR I could use the spare fuel sender I have in the basement, hook the wires from the programming unit to it, and teeter and totter it until I get the voltage values I need to enter. Great, right? No. I volt metered my relatively newish fuel sender that used to be in the Red Baron and only had 3000 miles on it and it's voltage range will only go from 70-5 ohms. No matter how much I banged it on the table, the float or the resistance wouldn't drop far enough to give me a number closer to 90 ohms.

    Dammit!

    Urgh! I'm thinking of going to Radio Shack to see if I can find a potentiometer that I can dial fairly accurately in the 0-90 ohm range.


  11. #9
    Just Some Dude in Jersey My location The Scifi Guy's Avatar
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    Here's the same chart with the capacity and calculated voltage levels at the various points:





    As you can see from comparing the DESTEC graph, my graph, and taking into account nuances of fuel tank shapes, and disparities in voltage readings between senders and other intangibles, these charts are just rough guidelines of what other GTers might measure if they analyzed their own fuel capacity, fuel gauge readings, and fuel sender voltage readings. Assuming that the sender has perfectly linear resistance, notice that a little more than the whole first half of the ohm or movement range of the sender is used up just putting the first 1/3 of the tank's capacity in the tank(3.3 gallons @ 42 ohms)
    Last edited by The Scifi Guy; 03-18-2017 at 05:53 PM.

  12. #10
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    Quote Originally Posted by The Scifi Guy View Post



    This advisory was stated in my Dakota Digital dash cluster set up booklet:

    "Fuel level sensors are generally not precise---------and you should expect to see some error. For instance, a GM 0-90 ohm sensor may display 5 ohms Empty and 97 ohms Full. If you are within 10% of expected results, this is within specification."

    GT's use the reverse 90-0 ohm type of fuel sender. My 7 ohm reading when full is within the 10% margin of error.

    For my own personal records:

    Ohm data needed to program the DD gauge:

    1/3 full = 42 ohms
    2/3 full = 24 ohms
    99% full = 8 ohms
    Full = 7 ohms
    While all of that may be true, your plotted value for 2 gallons is 68 but your measured was 63. Is one of those a typo? Just wondering?

    Aside from that (and the little wiggle at 28 ohms), it is amazingly linear (especially considering a 10% variation of the gauge) if you curve fit a straight line from 2 gallons (51 ohms) to 10 gallons (8 ohms). It is even closer if you use the 3 gallon and 9 gallon values! I'll try a table below:

    .................Measured.........Calculated...... ...Calculated
    Gallons..........Ohms............Ohms 2-10.........Ohms 3-9
    1 ................63.....................56.4....... ...........54.7
    2 ................51.....................51.0....... ...........49.3
    3 ................44.....................45.6....... ...........44.0
    4 ................38.....................40.3....... ...........38.7
    5 ................32.....................34.9....... ...........33.3
    6 ................28.....................29.5....... ...........28.0
    7 ................22.....................24.1....... ...........22.7
    8 ................17.....................18.8....... ...........17.3
    9 ................12.....................13.4....... ...........12.0
    10.................8......................8.0..... ...............6.7

    Looking at your later post, I would hazard a guess that the 1/3, 2/3, etc part is an attempt at picking up non-linearity in the tank volume. This provides for 3+ straight line segments rather than a single line. An interesting way to approach a known problem.

    HTH -- Doug
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  13. #11
    Über Genius My location First opel 1981's Avatar
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    The little wiggle at 28 ohms is, likely, due to the seams in the tank being a little more voluminous than the sides of the tank.
    Opel GTs are not GM products
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  14. #12
    Just Some Dude in Jersey My location The Scifi Guy's Avatar
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    Thanks for finding my screw up! I deleted the previous "final" graph and replaced it with this revised one:





    I recalibrated my system again. I pulled the fuel line off the filter I have near the tank and let all 11 gallons drain out. While I was at it I replaced my fuel filter. Wow, it went WAY faster than having the fuel pump pump out all 11 gallons. 10 minutes for the gravity drain, probably 1/2 hour if I had used the fuel pump to do the job. Armed with the knowledge gained from the first calibration exercise, this time I poured in the gas using my calculated 1/3 of capacity ohm readings and it went PERFECTLY. I ended up pouring in almost exactly the same amount of fuel at the 3 set points with no gas left over. Theoretically, my gas gauge will now display my gas usage perfectly linearly.


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  15. #13
    2000 Post Club norbertone.gt371's Avatar
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    Also in right direction:Full to empty

  16. #14
    Just Some Dude in Jersey My location The Scifi Guy's Avatar
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    Ha! Yes, that was how the gauge read before I had done any calibration using the "Custom Fuel Sender" option in the programming. Previously, when you and I had our problem with me running out of gas, I had used the pre-programmed 0-90 ohm sender choice. That type read backwards: Full when empty, Empty when full. Dakota Digital has since added the 90-0 ohm type to their list of pre-programmed choices in their dash clusters. However, if I had used one of the pre-programmed sender choices, my gauge would read inaccurately due to the changing shape of the GT fuel tank. So, it is actually better that I did the custom set up, noting the ohm readings and capacity the first time, and then redoing the process with the knowledge gained.


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