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Technical Damping a fuel gauge

Discussion in 'The Hokey Ass Message Board' started by 1great40, May 27, 2016.

  1. 1great40
    Joined: Jan 1, 2008
    Posts: 455

    1great40
    Member
    from Walpole MA

    I'm running older Dolphin gauges in my 40 Ford pickup. I had never bothered to hook up the gas gauge since I determined my swap meet gauge purchase netted me a mis-matched sender and gauge.
    I recently installed the correct Auto Meter sender in a new tank and now, although it seems to read right, the fluctuations in the needle are crazy. I don't have the option of building any baffles in the tank to smooth things out but I'm wondering if there's an electrical solution that I can build and wire into the fuel gauge circuit to damp the fluctuations. I tried the search here but came up with nothing. Any help would be appreciated.
     
  2. d2_willys
    Joined: Sep 8, 2007
    Posts: 4,110

    d2_willys
    Member
    from Kansas

    A capacitor from sender unit input to ground will help dampen the needle movement.
     
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 47,158

    squirrel
    Member

    What size capacitor? My wild guess would be somewhere in the 1 to 100 uF range?
     
  4. d2_willys
    Joined: Sep 8, 2007
    Posts: 4,110

    d2_willys
    Member
    from Kansas

    If you have a 30 -90 ohm sender unit, a 100uf capacitor would damp at a 1-3 millesecond rate, which will probably work ok. If still not enough damping, then add another 100uf capacitor in parallel with the first cap. Make sure that if you are using a negative ground electrical system that the + side of the cap(s) are on the sender unit lead, with the other lead grounded. If positive ground, then reverse the polarity of the caps.
     

  5. 1great40
    Joined: Jan 1, 2008
    Posts: 455

    1great40
    Member
    from Walpole MA

    The sender is from Auto Meter and it's 30 ohms full to about 260 ohms empty, Should I still start with 50-100 microfarads?
     
  6. squirrel
    Joined: Sep 23, 2004
    Posts: 47,158

    squirrel
    Member

    I'd give that a try...
     
  7. d2_willys
    Joined: Sep 8, 2007
    Posts: 4,110

    d2_willys
    Member
    from Kansas

    Try a 47uf first, if not enough damping, put another 47uf across the first cap. Make sure polarity of each cap is matched up, + to +, - to -. One thing to remember, when filling up full, the gas gauge will SLOWLY rise, due to the capacitors.
     
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  8. David Gersic
    Joined: Feb 15, 2015
    Posts: 2,302

    David Gersic
    Member
    from DeKalb, IL

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  9. G-son
    Joined: Dec 19, 2012
    Posts: 689

    G-son
    Member
    from Sweden

    The capacitor should also work if connected parallell to the gauge. In case that's an easier place to work with.
     
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  10. acme30
    Joined: Jun 13, 2011
    Posts: 175

    acme30
    Member
    from Australia

    Hi electrical gurus

    I have the same issue with gauge bounce on my fuel gauge due to poor baffles in the fabricated tank.

    Whilst I have read this post and the link to the other post with David's diagram I am afraid I am still electrically challenged and don't rally understand the diagram. Whilst that probably means I shouldn't mess with it I do want to learn.

    I have classic gauges, 12 volts negative earth

    In that spirit of learning can I ask / clarify..............

    does the + side of the capacitor get spliced into the sender wire so that the the sender wire remains unbroken to the gauge but the - side of the capacitor goes to any ground source?

    Or as an alternative can I run the capacitor across the back of the gauge with one the + end to the sender terminal and the other to the - terminal on the gauge?

    Thanks for sharing your knowledge
     
  11. David Gersic
    Joined: Feb 15, 2015
    Posts: 2,302

    David Gersic
    Member
    from DeKalb, IL

    I messed around with this and a few other ideas, none satisfactory. In the end, I bought a “Fuel Link” from Classic Instruments and I’m happy with it.

    https://www.speedwaymotors.com/Classic-Instruments-SN34-Fuel-Link-Interface,230181.html




    Sent from my iPad using The H.A.M.B. mobile app
     
  12. squirrel
    Joined: Sep 23, 2004
    Posts: 47,158

    squirrel
    Member

    Yes,

    Yes.

    I haven't tried using a capacitor on a fuel gage, but what you describe is two ways it could be wired, for the desired effect.
     
  13. acme30
    Joined: Jun 13, 2011
    Posts: 175

    acme30
    Member
    from Australia

    Thanks David and Squirrel for taking the time to reply and for sharing your experience.

    I think the classic interface is probably good value if your sender unit and gauge don't match but mine are matched from classic already so it is only the gauge bounce I was trying to solve. But also the way the Aussie $ has tanked against the US dollar it works out just under $200 AUS by the time I would get one shipped and taxed with the poor exchange rate.

    So even if I only get a partial result with the capacitor (or something similar) then I would be happy with that. My time isn't worth much so I don't mind spending some time rather than $ to try to fix. Also my dash panel comes out with only 3 screws so its no real drama to take it out to experiment.

    Thanks again guys
     
  14. David Gersic
    Joined: Feb 15, 2015
    Posts: 2,302

    David Gersic
    Member
    from DeKalb, IL

    My sender and gauge are matched (VDO), but with an unbaffled tank, the gauge was essentially useless while driving, with the needle oscillating wildly from almost empty to almost full.

    $200 is a lot. I’d probably try a few other things first, too.

    If you do some research, Ford modern cars mostly use a little plug in board on the back of the gauge cluster to provide fuel gauge damping. When they fail, the gauge goes full-on or full-off. There are a bunch of references to bypassing this module to get a working, if now bouncy, gauge again.

    I found one guy that was working on a little foreign roadster and retro fitting one of these modules to it. I don’t know that he ever got it working.

    I started off on the path of reverse engineering the Ford module. A junkyard run netted me a dozen modules to play with, $5 each. I eventually gave up on this idea, and sold them on eBay for $15-20 each, which funded buying the Fuel Link box.

    Good luck. Let us know if you get something working.



    Sent from my iPad using The H.A.M.B. mobile app
     
  15. Blues4U
    Joined: Oct 1, 2015
    Posts: 5,015

    Blues4U
    Member
    from So Cal

    I'd be interested in hearing about your results, please post back here after you've had a chance to work on it and let us know how it goes. I personally don't think a cap in that position is going to help, but it's worth a try. I think an R/C network, using some resistance in series with the sending unit line along with the capacitance in parallel to it, would be more effective, but deriving the correct values would be tricky. I'd just go about it by trial and error.
     
  16. nochop
    Joined: Nov 13, 2005
    Posts: 1,837

    nochop
    Member
    from norcal

    I usually get gas when mine stops bouncing
     
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  18. David Gersic
    Joined: Feb 15, 2015
    Posts: 2,302

    David Gersic
    Member
    from DeKalb, IL

    I’m not a fan of pushing my car to the gas station.


    Sent from my iPad using The H.A.M.B. mobile app
     
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  19. nochop
    Joined: Nov 13, 2005
    Posts: 1,837

    nochop
    Member
    from norcal

    Not a fan of getting run over when trying to push a high boy.....ask me
     
  20. acme30
    Joined: Jun 13, 2011
    Posts: 175

    acme30
    Member
    from Australia

    Blues4U and David I think you guys called this correctly and I don't think this solution works. It is definitely a case of monkey see and do in my case but l couldn't get it happening.

    So others know what I tried.

    This is what I did .............as I have a 33-240 ohms sender and gauge I went for a 220 uf capacitor. I wired it directly across the sender input on the gauge (+ side to sender) and - side to earth but it made zero difference in reducing needle bounce.

    So it appears that more is required than just a capacitor.

    Cheers
     
  21. I stop when my back side tells me it’s time. HaHa
     
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  22. acme30
    Joined: Jun 13, 2011
    Posts: 175

    acme30
    Member
    from Australia

    Hi guys

    Just reporting back - I gave up and purchased one of these 225mm KUS Fuel Water Sending Unit Boat Truck Car Level Gauge Sensor 240-33ohms on ebay.

    Gauge now reads as it should with minimal bounce. Whilst the product is Chinese it is good quality, well made, stainless steel and worked out about a quarter of the cost delivered than the tanks item or the classic instruments converter.
     
  23. Warpspeed
    Joined: Nov 4, 2008
    Posts: 532

    Warpspeed
    Member

    Most serious race cars and all aircraft use foam filled tanks. The foam only loses you about 1% of the tank volume, but it completely eliminates the problem of fuel sloshing around. Its really just like a giant sponge.

    I have zero personal experience with fuel tank foam, but it might be something worth researching.
    It may then be just a case of cutting out a well just large enough for the tank float to move freely over its full range of travel.

    This may be a solution, or it may be hopelessly impractical for us. I really do not know. But its just an idea that suddenly came to mind.

    The capacitor idea would work, but you need a time constant of many seconds, not milliseconds. 100uF and a typical average fuel sender resistance of 100 ohms is only 10 milliseconds time constant, and is far too short to do anything useful.

    Now you can but "ultracapacitors" with capacitance of one Farad or more, but they have a voltage rating that will require several in series. One Farad and 100 ohms will produce a time constant of 100 seconds, which is more like what you need to give a good steady long term average of the violent sloshing around in the tank.

    The picture shows a 1.5 Farad 5 volt ultracapacitor. Three of those in placed series might be worth a try.
     

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  24. David Gersic
    Joined: Feb 15, 2015
    Posts: 2,302

    David Gersic
    Member
    from DeKalb, IL

    I looked in to using foam. Everything I read seemed negative, like it was going to be more trouble in the long run. Probably could make it work initially, but then the foam breaks down and starts clogging things.

    If it’s used in aircraft, that seems like it must be a solved problem, but most of us car drivers won’t do aircraft level maintenance and inspection schedules.

    I have a box of 1F 5V super caps here for other purposes, but no current need to see if they would work.



    Sent from my iPad using The H.A.M.B. mobile app
     
  25. Warpspeed
    Joined: Nov 4, 2008
    Posts: 532

    Warpspeed
    Member

    I am pretty much in the same boat as far as the ultracaps go. Have the caps here, but do not have any wildly fluctuating fuel gauge problem on which to try them out.
     
  26. Beanscoot
    Joined: May 14, 2008
    Posts: 1,693

    Beanscoot
    Member

    The foam will make it hard for the fuel sender arm and float to go up and down.

    I believe aircraft gasoline aka Avgas does not have all the stuff in it that modern auto gasoline does, so it is more stable and doesn't go rancid near as bad as reformulated gasoline. So perhaps that's why the foam works okay in aircraft fuel tanks.
     
  27. Budget36
    Joined: Nov 29, 2014
    Posts: 4,839

    Budget36
    Member

    I've B&S engines with plastic mesh in the tanks, of course no fuel gauge.
     

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