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250 fuel tank sending unit ohms?

Discussion in 'Vintage (thru 365 GTC4)' started by Mattie69, Oct 15, 2017.

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  1. Mattie69

    Mattie69 Karting

    Aug 26, 2016
    147
    #1 Mattie69, Oct 15, 2017
    hi
    I read the 250 and 330 used a 0-100 ohm fuel tank sending unit,if that is correct should I have an issue using a sending unit that is 0-90ohms?
    Matt
     
  2. John Vardanian

    John Vardanian F1 Rookie

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    #2 John Vardanian, Oct 15, 2017
    The 250 coil typically reads 95 ohms at full peg. Your alternative would work, though it would not be as accurate. It's probably more important that the unit has the second terminal for the warning light.

    john
     
  3. Mattie69

    Mattie69 Karting

    Aug 26, 2016
    147
    #3 Mattie69, Oct 16, 2017
    Thank you very much for the info
     
  4. Mattie69

    Mattie69 Karting

    Aug 26, 2016
    147
    #4 Mattie69, Oct 16, 2017
    My sending unit currently has two terminals but is the incorrect ohms , I assume one goes to the gauge and one to ground ? Does the warning light run its own Wire?
    Matt
     
  5. John Vardanian

    John Vardanian F1 Rookie

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    #5 John Vardanian, Oct 16, 2017
    Last edited: Oct 16, 2017
    Matt, there should be two identifiers on the lid, "T" and "W", going to the gauge and the warning light, respectively. In this picture the terminal on the right goes to the light. The ohm numbers of this terminal is of no value. The one on the left is the one that moves your gauge needle. The unit is self-grounding. Hope this helps

    john

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  6. Mattie69

    Mattie69 Karting

    Aug 26, 2016
    147
    #6 Mattie69, Oct 17, 2017
    Thank you
     
  7. Mattie69

    Mattie69 Karting

    Aug 26, 2016
    147
    #7 Mattie69, Oct 17, 2017
    Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login I am having a tough time getting the gauge to work properly, I assumed the fuel level wire from the sending unit goes to the -VE ( green)and 12v power goes to +VE(red),the terminal above those ( black )is for ground or for the low fuel warning wire from the sender? I assume the small red and white wire are power for the low fuel warning light , I have bench tested a 0-90ohm sending unit and it reads backwards /empty when float is in the full position , the sweep seems fine with the 0-90 ohms , I am unsure which terminal on gauge the warning light wire from the sender is used,but I don't think that will make a difference in terms of the fuel level reading backwards/opposite level , the VDO sending unit only allows the float to be attached and pivot one way/direction or I would reverse it and the gauge would read accurately , my car has a custom built tank or I would use an original sending unit ...any info would be much appreitated
     
  8. jimmyr

    jimmyr Formula Junior

    Oct 10, 2004
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    #8 jimmyr, Oct 17, 2017
    Please note the cork float will not hold up well with the modern gas, and will after awhile soak up the gas and sink. Using a plastic float from a later car can solve that problem.
     
  9. John Vardanian

    John Vardanian F1 Rookie

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    #9 John Vardanian, Oct 18, 2017
    Matt, two things would make your needle peg backward, having the wires swapped at the gauge and lack of ground to the gauge. The chassis of the gauge must be grounded. Note the brown wire at the sending unit and at the gauge. The warning light wire starts out green then it becomes black/red. The orange wire is just illumination. When I take things apart I photograph and take notes. I was having a similar problem recently and so I'm glad I took these. Hope these will help you.

    john

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  10. John Vardanian

    John Vardanian F1 Rookie

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    #10 John Vardanian, Oct 18, 2017
    Yes, I am just as surprised that the varnish on the cork has lasted 50+ years and still gives accurate readings.

    john
     
  11. John Vardanian

    John Vardanian F1 Rookie

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    #11 John Vardanian, Oct 18, 2017
    Matt, apologies, disregard the first picture in my earlier post--the wrong gauge. I couldn't remove it, so here is the fuel gauge. You can see the green wire going to the warning light which corresponds to the green on the sending unit.

    john

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  12. Mattie69

    Mattie69 Karting

    Aug 26, 2016
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    #12 Mattie69, Oct 18, 2017
    So on my gauge the green terminal is warning light lead? And the black is fuel level wire ? and red is power, red& white wires are constant power and ground for warning light?...or do the fuel level and fuel warning wires from the sender go to same terminal on gauge ? I have this on bench with a battery box so I know it's getting grounded tried many configurations , it's seem if the ohms on the sender were 90-0 it would be fine ,has anybody used an after market universal sending unit with on of these veglia gauges?
     
  13. John Vardanian

    John Vardanian F1 Rookie

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    #13 John Vardanian, Oct 18, 2017
    Matt, I am guessing that your gauge was modernized. The can is different from original and the bullet connectors are replaced with spades. I suspect the middle spade is a dedicated ground and the ones on either side are power and warning light feed. I am guessing the two loose wires go to an LED inside the gauge. Check for continuity between the middle spade and the chassis, this might confirm if the middle spade is ground.

    john
     
  14. Mattie69

    Mattie69 Karting

    Aug 26, 2016
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    #14 Mattie69, Oct 19, 2017
    Thanks for all your info John, The gauge was purchased thru gto engineering ,they have someone who reproduces them I believe in the U.K. they are built to use all 250 senders and cables....yes on terminal (black )is for ground , red is for power, green is for the fuel level , the wire from the warning light terminal on the sender goes to the red wire for the warning light, the funny thing is when I when I hook up power and ground only to the gauge it shows empty ,when I run a wire from the fuel level terminal ( green) on the gauge to the ground terminal it moves the needle to full, which tell me instead of being 0-100 ohms , it's 100-0 ohms ,am I off on this ...I think that's why it's reading opposite with my 0-90 ohm VDO sending unit , maybe they made a mistake when they built this gauge ? Or I am backwards on it ? I asked gto engineering and they said they have ran these gauges plenty of times but never use the warning light part of it, I am sure they are probably using a 250 sending unit
     
  15. John Vardanian

    John Vardanian F1 Rookie

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    #15 John Vardanian, Oct 20, 2017
    Last edited: Oct 20, 2017
    Matt, think of it this way, the gauge is essentially a DC voltmeter and the sending unit is a variable resistor (or a rheostat). When you feed 12 volts to the gauge it will peg one direction and if you reverse that feed it will peg the opposite direction. The two units are in series, so what would keep the gauge from pegging is the 20-100 ohm variable resistor in between; i.e., the sending unit. It might help if you look at the single-line diagram in the 250GT owners manual. All car gauges of that period work on that principal, so you might also consult Google. Let us know how it goes.

    john
     
  16. drew1295

    drew1295 Rookie

    Nov 28, 2017
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    #16 drew1295, Dec 14, 2017
    Hey guys, i don't mean to thread hijack here but does anyone know where i can find a replacement fuel tank sending unit? The one in our car isn't reading properly and when i checked with a multi-meter after removal, the ohms were not consistent at all.
     
  17. John Vardanian

    John Vardanian F1 Rookie

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    #17 John Vardanian, Dec 15, 2017

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