Intermittent fuel level sender

Discussion in 'General Electronics Chat' started by wml52, Apr 27, 2016.

  1. wml52

    Thread Starter New Member

    May 8, 2014
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    Fairly new to the forum so hello to all and my apologies if this subject does not apply to this forum but I believe it might be better suited here then elsewhere.

    I have a John Deere tractor, which uses a fuel sender assy inside the fuel tank. The sender consists of a float, which moves up and down vertically on a potentiometer and some basic components i.e. resistors, diodes etc.

    The problem I am having is when the fuel sender unit is mounted in the fuel tank I get no reading on the fuel gauge regardless of how much fuel is in the tank, however if I remove the sender unit and move the float manually it will read from empty to full depending on where I position the float. If I place the sender in a container of water and vary the level of water in the container I will get a reading on the fuel gauge from empty to full depending on the water level. Once I place the sender back into the fuel tank I get no reading at all.

    I am hoping someone may have a suggestion as to why the fuel sender appears to work perfectly when I move the float manually and also when it is submerged in a container of water but does not work once installed back in the fuel tank.

    Any help would be appreciated as this has baffled me for some time now.



    I thank you all
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Those things can be touchy. I'm speculating that the spring pressure holding the wiper arm onto the resistance wires is just a hair too small. Something about the angle of the dangle when installed in the gas tank is causing the wiper to come off the wires.
     
  3. #12

    Expert

    Nov 30, 2010
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    And I'm thinking the fuel tank might be grounding out the signal.
    Look for any metal on the tank that might be connecting to the fuel sensor.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Could be. I considered that too but when I pictured in my mind the last one I worked on (a Buick Regal), I couldn't imagine how that would happen. Bad connector pins maybe?
     
  5. bwilliams60

    Active Member

    Nov 18, 2012
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    What is the voltage on the wire going into the tank? Is it a single wire (case ground) or double wire unit. Usually the potentiometer type range up to 270 ohms and vary voltage going to the gauge. The gauge has a coil in it and it becomes like a voltage divider circuit. the resulting magnetism pulls the needle one way or the other. Measure the voltage going in. It may not be grounding properly.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That would be my first guess too. Corrosion, oil, animal crap etc all take their toll on a farm tractor.
    Check also the wire to the sender. It may be fractured internally as a result of vibration, or a crimped-on terminal may be making a poor connection.
     
    bwilliams60 likes this.
  7. crutschow

    Expert

    Mar 14, 2008
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    This is a long shot, but the float may have lost some of its buoyancy so that it still floats in water but not in gasoline (which is less dense than water).
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Nice out-of-the-box thinking. Easily checked.
     
  9. #12

    Expert

    Nov 30, 2010
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    Here's another idea: When you measure out of the tank, you're using an ohm meter. When you measure in the tank, you're connecting to the wiring harness? If that's true, everything except the sender comes into question.
     
    shortbus likes this.
  10. bwilliams60

    Active Member

    Nov 18, 2012
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    Is the OP still here? If so, can you post a picture of the sending unit please. There are many different styles from rheostats to sonar being used on equipment right now and John Deere is at the forefront in the Ag business. A simple voltage test should give us the answer depending on which style he has.
     
  11. wml52

    Thread Starter New Member

    May 8, 2014
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    Sorry for the delayed response. I haven't had much time to work on the sender but this is what I have so far with regard to answering some of the questions.

    It's a 2 wire setup.
    When I checked the sender out of the tank I was using the wire harness not an ohm meter.
    Wiring appears to be fine no intermittent readings while checking continuity.
    With regard to Crutchow's suggestion I did not think to check the float using fuel I was thinking safety over SG of fuel vs water. I will be sure to check over the next few days, great suggestion!
    I will take some photos of the sender and circuitry as well and post them.

    Thanks to all who responded I will let you know what I find.
     
  12. bwilliams60

    Active Member

    Nov 18, 2012
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    I am starting to think that there is not enough tension on the wiper portion of the potentiometer. When you have it out and in your hand, you typically would have it laying in your hand somewhat and exerting a slight pressure on the arm which will result in more pressure on the wiper. Take it out and move it around while trying it and see if it changes things. For example, try it one way and then flip it 180 degrees and try it again.
     
  13. #12

    Expert

    Nov 30, 2010
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    This isn't just a great imagination. I have seen carburetor floats that sank...in the 1960's:D
    Nobody uses carburetors now, so most people have lost the old knowledge.
    I just ran into that yesterday in this thread from my "fixin your car" site.
    http://www.explorerforum.com/forums/showthread.php?t=322730
     
  14. bwilliams60

    Active Member

    Nov 18, 2012
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    What's a carburetor? JK. I rebuilt hundreds of them in my lifetime and even into the seventies and eighties we had this problem. It is not so common now and I find the OP's problem kind of unique as JD does not build simple tractors. They are one of the most complex in the market place right now, that is why I would like to see a picture. Generally if they use a potentioemeter, there is no circuitry on the sender unit, it is done in the cluster. Curious.
     
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  15. wml52

    Thread Starter New Member

    May 8, 2014
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    OK, so I finally got a change to do a little more work on the fuel sender and take a few photos. I'm really sure how this sender works but it appears that the float which is housed in a cylindrical aluminum tube has 2 small magnets on either end that change resistance across the 2 wires that exit the sender. When the float is at the bottom of its travel (Empty position) I was getting a reading of about 450 ohms, when at the top of its travel (Full position) I was getting around 35 ohms. The float seemed to be as buoyant in water as it is in fuel so I don't believe the float to be the cause of the failure.

    As you can see from the photos the circuit board has resistors and something that is encased in a clear sealed almost shrink like tubing which I cannot identify, I tried to take a close up of it but I could not get the detail I was looking for so my apologies. The circuit board also appears to me to have extensive discoloration and as you can see from the photo and some type of conformal coating that looks to be lifting in several places.



    Float inside housing.jpg Float On end view.jpg PCB Full view.jpg Float inside housing.jpg Float On end view.jpg PCB Full view.jpg PCB Close-up.jpg PCB Close-up.jpg
     
  16. BR-549

    Well-Known Member

    Sep 22, 2013
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    What the hell is that? A reed switch that the magnet opens and closes?

    So in theory the gauge can only show 6 positions?

    Not to mention the shot board.
     
    Last edited: May 4, 2016
  17. wayneh

    Expert

    Sep 9, 2010
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    Never saw one like that!
     
  18. Lestraveled

    Well-Known Member

    May 19, 2014
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    I use that kind of fuel level sensor in my dune buggy. Works great!
     
  19. BR-549

    Well-Known Member

    Sep 22, 2013
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    So those are reed switches. We are making physical electric contact in a gas tank.

    Shocking.
     
  20. BR-549

    Well-Known Member

    Sep 22, 2013
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    I would be looking for fuel contamination or bogus fuel additive. Something corroded that board.

    JD would have protected it from regular fuel.

    Just a thought.
     
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