Fuel Gauge Does Not Work
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  1. #1
    Senior Member Old Okie's Avatar
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    Fuel Gauge Does Not Work

    I know there is a thread on this subject but I have not found it.
    The fuel gauge and warning light do not work on my CX500tc.
    Point me to that thread, please.
    1979 CX500C (Marshall's bike)

    1981 Yamaha XS650 (New Adventure)

    1979 Honda CB400T II (Rescued Bike)

    1972 Suzuki RV90(for the grandkids)Ole Smokey

  2. #2
    Senior Member dutchturbo's Avatar
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    Most likely it's the 7 volts regulator. It's mounted above the headlight on the 500T. I have replaced it on my 650T with the LM317. See schedule. You can leave out the diodes, they are for this use not necessary. The temperature meter works also on this 7 volts so it should be dead also.

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    Last edited by dutchturbo; 09-29-2016 at 01:27 PM.

  3. #3
    Senior Member Old Okie's Avatar
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    That is what puzzled me, the temp gauge works OK.
    The manual is not much help, "replace as a unit", "unit" not available.
    I will dig deeper into this in a day or so.

    Thanks
    1979 CX500C (Marshall's bike)

    1981 Yamaha XS650 (New Adventure)

    1979 Honda CB400T II (Rescued Bike)

    1972 Suzuki RV90(for the grandkids)Ole Smokey

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  5. #4
    Senior Member George in Indiana's Avatar
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    The reserve light is operated off a separate thermister and not related to the gauge.
    If both are dead, it may be the electrical plug connector from the sending unit which I believe is under the seat.
    Last edited by George in Indiana; 09-30-2016 at 07:20 AM.
    CX500 TURBO
    CX650 TURBO
    GSX1300R Hayabusa

  6. #5
    Senior Member Old Okie's Avatar
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    I will check the plug today.
    How does the reserve light work???
    I will check the bulb for the reserve light also.
    1979 CX500C (Marshall's bike)

    1981 Yamaha XS650 (New Adventure)

    1979 Honda CB400T II (Rescued Bike)

    1972 Suzuki RV90(for the grandkids)Ole Smokey

  7. #6
    Senior Member George in Indiana's Avatar
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    There is a little capsule with holes in it. Inside the capsule is a thermistor that changes resistance depending on whether it's being cooled down with fuel or not. When no fuel is touching it, it closes the circuit allowing the light to illuminate.
    It's part of the sending unit assembly but does not interact with the gauge portion. It's also not available separately as a repair part.

    If the thermistor goes bad, it's a pretty easy repair. The little metal capsule can be pried apart and a new thermistor soldered in place. I think I bought mine from Newark and they cost next to nothing.

    Edit:
    Went through some old files of mine and this is what I bought back in 2003 and it's still working fine today...
    http://www.newark.com/amphenol-advan...stor/dp/20F735

    Use any of this advice at your own risk. Keep in mind you're dealing with a potential gas bomb in front of your crotch! :bom:
    Last edited by George in Indiana; 09-30-2016 at 04:38 PM.
    CX500 TURBO
    CX650 TURBO
    GSX1300R Hayabusa

  8. #7
    Senior Member Old Okie's Avatar
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    Since I have the tank off and it is nearly empty.....more work.
    If the warning light works I may not care about the gauge so much.
    What makes the fuel gauge important is that I can not reset the trip meter
    so I never know how far I have been on a tank of gas.
    I do not know why I worry, I have ridden the bike 400 miles in 4 years.
    Thanks for the help.
    1979 CX500C (Marshall's bike)

    1981 Yamaha XS650 (New Adventure)

    1979 Honda CB400T II (Rescued Bike)

    1972 Suzuki RV90(for the grandkids)Ole Smokey

  9. #8
    Senior Member George in Indiana's Avatar
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    Both are simple to diagnose and get working. If the connector from the sending unit is plugged in, you probably have two separate problems.

    For the gauge, simply cleaning the coil area and slightly bending the sweeper arm for better contact almost always cures any problems.

    Now my 500T had a really odd problem that had me losing my mind. The gauge always read nearly empty. Remove the sending unit, move the arm and the gauge works perfect. Put it back together and it's dead again. Remove it and it works fine! Put the arm/float in a bucket of water and it works perfect. After about the 5th time, I decide to remove the float and drop it in a jar of gas...it sank almost all the way to the bottom! For some reason the float lost buoyancy in gas but was fine in water.
    New sending units were already discontinued so I went to the performance aisle at Autozone and bought a similar size carburetor float which was easy to retrofit. Even if the sending unit was available, I wouldn't have paid Honda's price vs. a $4.00 float.

    My 650T had the bad thermistor and also needed the sweep arm tweaked a bit.
    CX500 TURBO
    CX650 TURBO
    GSX1300R Hayabusa

  10. #9
    Senior Member Old Okie's Avatar
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    You have told me what I needed to know to get this thing to work.
    Probably work on it on Monday. The fuel gauge worked for a while then quit.
    I think I can make this thing work.

    Thanks for the help.

    Edit: The suggested thermister is bare, is that OK or should it be coated????
    Last edited by Old Okie; 10-04-2016 at 09:23 PM.
    1979 CX500C (Marshall's bike)

    1981 Yamaha XS650 (New Adventure)

    1979 Honda CB400T II (Rescued Bike)

    1972 Suzuki RV90(for the grandkids)Ole Smokey

  11. #10
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    Hi! I recently bought the 82 turbo which had sat a long time and very cruddy in the tank, etc. The fuel petcock is all plugged, etc. The fuel line just past the petcock is plugged solid also! I found the float assembly lying in the tank, when i took the sending unit out is was all built up with crud and took some cleaning. I took the cap off where the float assembly goes into the unit and there was a needle with a built in nut on top and a spring. I have no idea how the wire part of the float attaches to the needle(the wire is smooth and round as is the tiny hole in the nut on top of the needle! If i could get this back together how is it calibrated? Thanks, John

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