Question about Ford E-350

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When gauge reads full, it is empty? and when it is empty, it is full?

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  • mmbowler Nov 08, 2010

    i have the sending unit out, it doesn't make a different. you put a new ground it doesn't change a thing.

  • mmbowler Nov 09, 2010

    the readings correct buy the way you say. but i have tried a new gauge and a new sending unit. but i still have the same problem. even tried adding new ground wire and replaced instrument cluster voltage regulator. it doesn't make any sense.

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  • Ford Master
  • 17,970 Answers

After the sending unit has been removed from the tank, check to make sure the float is present, intact and still floats and that there is no binding with the float arm. Reconnect the gauge wiring to the unit and ground the unit. Move the float arm up and down. If the gauge reads full in the full up position and empty in the full down position, then the sending unit is good. Verify by hooking up an ohmmeter to it and making sure it reads 8-12 ohms full and 70-73 ohms empty. If not, the sending unit needs repaired or replaced. You can easily disassemble the head of the sending unit to clean and/or repair.

NOTE: You MUST make sure your ohmmeter is capable of reading the range needed...many inexpensive hardware-store variety units are not designed to measure that fine a range. My little analog ohmmeter only had one range for reading resistance, which was 1-1000?. This scale is MUCH too great for reading the slight variations needed here, and didn't even result in the meter's needle moving at all. You need a tester having a 1-100? or 1-200? range. Having the right equipment makes all the difference.

A fuel tank sending unit is nothing more than a variable resistor, and the fuel gauge is simply measuring the resistance between the hot wire and the ground. The fuel tank sending units used on all Ford products up through 1986 have a resistance range of approximately 8-12 ohms full and 70-73 ohms empty. (I tested my factory 25-gallon in-frame auxiliary tank's sending unit, however, and got readings of 20? full and 85? empty.)

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Posted on Nov 09, 2010

  • ZJ Limited
    ZJ Limited Nov 09, 2010

    If the resistive element's action is opposite to what you need
    (resistance goes up with decreased fuel level but gauge level goes up,
    or vice versa) then there's three options - install upside down as per
    the above, remove the resistive element as re-mount it 180' in it's
    housing or build an out-board translator circuit.

    There isn't enough info here to sort it out. Your meter (tank unit) and
    gauge (dash unit) may be compatible, compatible with modifications, or
    incompatible. With components from two different vehicles or sources (?)
    like this it's basically dumb luck to arrive at full compatibility as
    there are many types of setups. The meter may register from say, 10 ohms
    at full to 90 ohms at empty, or vice versa. Or 20-150 ohms, 40-270
    ohms, etc, ad infinitum. There are nearly innumerable calibrations and
    the meter and gauge must be matched to attain any accuracy, naturally.
    Also, there are voltage-stabilized gauges that operate at voltages other
    than 6V or 12V through a stabilizer or constant voltage regulator. Both
    Smiths and AC supplied across the industry so based on the limited info
    here, you could have most anything. You may even have a magnetic gauge
    unit though it's unlikely. You say decades old but that's not very old
    anymore. Heck, that's the '80s; how time flies.

    Also, it is not
    terribly reasonable to simply assume that both your components are
    functioning normally to begin with. I would start here: remove your tank
    unit, attach an ohmmeter across the terminals, and record the
    resistance while sweeping the float arm from empty to full (making sure
    you note which is which) while also watching for dropouts through the
    range. Next, you can do a basic test of the dash unit by supplying power
    and ground. With the wire to the sender open, the gauge should go fully
    full or totally empty, and with the wire grounded it should go
    opposite. From there you can sort it out. One easy way to proceed: take
    the gauge and the meter specs you recorded to an instrument shop and
    tell them to calibrate the gauge to the meter. If it's possible in your
    case and they know their stuff, they'll know just how to do it -- though
    it may be more expensive than simply buying new, compatible components.


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  • Master
  • 3,917 Answers

Hi

It could be the float stuck to the end of tank for which it is showing empty, if it's not, then it is the sender unit because often fuel that contains too much foreign chemicals can mess up a fuel sending unit in the tank. However there is also a possibility of improper connection at the sender which means bad connection in the wire between the gauge and the sender. A bad ground at the sender will do the same. It is suggested to calibrate the sender and gauge together. You need to siphon all the gas from the tank and get an ohm reading and then top the tank off and get another ohm reading, then first make sure you have the correct gauge and the range of the gauge is correct by using resistors, then connect it all up with a ground connected to the gauge and also one from the frame (good ground spot) to one of the nuts that hold the sender on just to be sure of a good ground.

Let us know if you have something more to ask or else please accept the suggestion.

Thanks for contacting fixya.com

Posted on Nov 08, 2010

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