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Thread: Fuel light always on

  1. #1
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    My Van(s):
    Toyota Townace YR20 1985
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    Fuel light always on

    The fuel light in my 1985 YR20 Townace is always on ever since after I stripped most of the interior, welded several patches and repainted everything - oh and I replaced pretty much all of the gauge cluster lights with LEDs.
    The fuel gauge works fine, it's just that the yellow fuel warning light is always on. All other warning lights seem to behave normally.

    I am now wondering if there might be a fuel door sensor that I broke (or maybe prevented from closing contact properly) during my painting efforts?

    Any idea if that light should only be on when fuel is low or if it might also double as an indicator for an open fuel door?

    Any other thoughts on why it may be always on?

    Thank you.

  2. #2
    Van Obsessed
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    88 4WD DLX 5spd
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    Re: Fuel light always on

    Did the fuel light stay on before doing all this work?
    Most times, new problems that occur right after a repair are usually self inflicted.
    You need to go back over everything you did and make sure all is as it should be, maybe you pinched a wire, maybe something isnt hooked up correctly.

    LED's cause all sorts of issues and are quite possibly contributing.
    You will need to remove them all, install the proper bulbs and continue the diagnosis from there.
    Once the issues are resolved then you can go back to making "improvements"

    As to gas door, I think you may be reaching, even if there was a "gas door open" switch, I would think it would illuminate a "door open warning" not a
    "low fuel" warning??

    Good luck

  3. #3
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    Interested in buying '85 van cargo
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    Re: Fuel light always on

    I think it is very simple. It is the low fuel light itself.
    There is a thermistor mounted low in the fuel tank.

    I copy pasted this from an ebay listing:

    The thermistor senders work in a rather unusual way, so to test them it is important to understand how they work.
    Usually, they are connected with the case to ground, and the lead connected to a 3 watt bulb, the other terminal of the bulb goes to +12V

    When the thermistor is submerged in fuel, the fuel keeps it cool, and its resistance stays high, 1000, to 2000, Ohms.
    a very small amount of current will flow through the thermistor from the bulb, but because the resistance of the thermistor is high, it is a tiny amount and will not light the bulb.

    When the thermistor is removed from the fuel and in air, things start to change very slowly, that small current heats the thermistor itself very slightly, the increase in temperature causes the thermistor to reduce its resistance.
    Because the resistance is reduced, the current flowing through the bulb, through the thermistor to ground increases a little.
    As the current increases, the heating effect increases and the thermistor reduces resistance even more, so the temperature of the thermistor and the current flowing increases more... and so on !

    This current and temperature keep increasing until there is sufficient current to light the bulb.

    At this stage, the thermistor is hot but still has a few Ohms resitance, The bulb is on, but not quite as bright as if it was connected directly to ground.

    Then fuel is added , the thermistor cools and the bulb goes off !


    Sooo, if your led uses way less power than the incandescent lightbulb, it will always be on.

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