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Thread: Testing Fuel Gauge

  1. #1
    Join Date
    Apr 2010
    Posts
    148

    Default Testing Fuel Gauge

    My fuel gauge has been acting funny - reading full when full but empty when there's still fuel in it. So, after checking the sender and the voltage stabilizer, I tested the gauge today. The results were off so I want to make sure my test was reliable before I order a new one. Does this sound right:

    I removed the gauge and connected the terminal nearest the F side to the positive terminal of a reliable 10v power source. I connected the E terminal to one side of an adjustable resistor (to simulate the sender) and the other side of the resistor to the ground of the power source. I ran 10v through the gauge at various resistances from 0 to 260. I got those numbers from a chart in another post. Below are the results I got. All the numbers are rough. I estimated what I think the gauge reading should have been on the percentages in the chart and a 12gal tank.

    Ohms Gauge Should Be
    260 Below E E
    207 E ~1/4
    135 ~1/8 1/2
    72 ~3/8 3/4
    42 ~5/8 7/8
    10 F+ F
    0 F+

    So, assuming my test setup was OK, the next question is whether the gauge can be fixed (it's a Smith - probably original).
    1970 Series IIA
    1964 Series IIA [sold]

  2. #2
    Join Date
    May 2014
    Location
    Marblehead, MA
    Posts
    383

    Default

    Mark!
    1968 Series IIa
    1997 Defender SW (Original Owner - Sold)

  3. #3
    Join Date
    Oct 2011
    Location
    Mountains of Western Pennsy.
    Posts
    592

    Default

    Did you check the sending unit for OHMs? Or make sure the float itself actually, floats in a bucket of water? It may well be the sending unit that's at fault.

  4. #4
    Join Date
    Apr 2008
    Location
    The Granite State (NH)
    Posts
    3,435

    Default

    Quote Originally Posted by lumpydog View Post
    Mark!

    I only have a ohms vs. temp reading for the temp gauge, I think.

    Since the variable resistance of the sender is linear, if you know the ohm range of the fuel sender, you can just plot this on a graph (or just do the simple math) and figure out how many ohms= 1/4, 1/2, 3/4, etc. on the gauge.
    --Mark

    1973 SIII 109 RHD 2.5NA Diesel

    0-54mph in just under 11.5 minutes
    (9.7 minutes now that she's a 3-door).

  5. #5
    Join Date
    Apr 2008
    Location
    The Granite State (NH)
    Posts
    3,435

    Default

    Oh...Make sure the + and ground wires are on the correct terminals on the sender. Reversing them will give you wonky readings. If you only have 1 wire/terminal on your sender, make sure the sender is properly bonded to the tank and/or body/chassis and that the tank is properly grounded to the chassis
    --Mark

    1973 SIII 109 RHD 2.5NA Diesel

    0-54mph in just under 11.5 minutes
    (9.7 minutes now that she's a 3-door).

  6. #6
    Join Date
    May 2014
    Location
    Marblehead, MA
    Posts
    383

    Default

    Quote Originally Posted by SafeAirOne View Post
    I only have a ohms vs. temp reading for the temp gauge, I think.
    Found it - see Mark's reply here.

    This was the first (of many) time that Mark bailed me out where I got stuck.
    1968 Series IIa
    1997 Defender SW (Original Owner - Sold)

  7. #7
    Join Date
    Apr 2010
    Posts
    148

    Default

    Thanks. I've seen Mark's post and tried all the other suggestions. I think the gauge is the only place the problem could be.
    1970 Series IIA
    1964 Series IIA [sold]

  8. #8
    Join Date
    Apr 2008
    Location
    The Granite State (NH)
    Posts
    3,435

    Default

    Quote Originally Posted by lumpydog View Post
    Found it - see Mark's reply here.
    Huh...I really don't remember making that graph, though it seems like something I'd do if I had a brand new sender in-hand (like I did when I installed my long-range aluminum fuel tank a few years ago).
    --Mark

    1973 SIII 109 RHD 2.5NA Diesel

    0-54mph in just under 11.5 minutes
    (9.7 minutes now that she's a 3-door).

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