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Fuel level warning lamp circuit

Discussion in 'General Electronics Discussion' started by gjscott, Mar 24, 2013.

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  1. gjscott


    Mar 24, 2013

    I have a circuit that turns a lamp on and off dependent of fuel level.


    Without capacitors for better view:



    I made this layout (I cannot guarantee it's accuracy):


    The switch is a reed switch with a float and magnet acting on it, when the switch closes the low fuel lamp on the dash should light up.

    The fault is the fuel level low light didn't work, I disassembled the sensor, dug out the board that had been fully potted.

    Action taken:

    Replaced the 2B transistor with a 2FZ because I pulled the 2B off when cleaning the board. I was assured the 2FZ would be comparable.
    Replaced the two electrolytic capacitors which smelled of gasoline.
    Replaced the LM393 comparator chip.

    Now the low fuel light is always on and will not go off regardless of the fuel level switch state.

    The markings came off a couple of the components, hence the ?'s

    Can anyone suggest what the problem may be and what I should check? I have a meter and can test for continuity and resistance etc

    Last edited: Mar 24, 2013
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