Circuit help

Discussion in 'The Projects Forum' started by gjscott, Mar 23, 2013.

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  1. gjscott

    Thread Starter New Member

    Mar 23, 2013
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    Hi, this is the circuit diagram I made (may not be 100% accurate) from a Ducati fuel level sensor.
    The problem I have is the fuel level low light didn't work, so I disassembled the sensor, dug out the board that had been fully potted.
    I replaced the 2B transistor as I pulled it off when cleaning the board, the two electrolytic capacitors which smelled of gasoline,
    and 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 any electronics gurus help me out with this as I'm a mechanical engineer with basic electronic troubleshooting skills ?

    [​IMG]

    [​IMG]
     
    Last edited: Mar 23, 2013
  2. gjscott

    Thread Starter New Member

    Mar 23, 2013
    2
    0
    Without capacitors for better view:

    [​IMG]
    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

    Thanks
     
  3. absf

    Senior Member

    Dec 29, 2010
    1,492
    371
    I believe one or both of your capacitors' polarities are reversed. If you measure the trace where you connected to the negative side of your caps with the +12 using an ohm meter, what is the resistance you get?

    The 2B transistor is an PNP equivalent to 2n2907 so is 2F(z). But the Base of the transistor should not be connected to the input of the comparator (P3). I redraw your circuit with proteus and I guess it is much easier to understand.

    Allen
     
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  4. Georacer

    Moderator

    Nov 25, 2009
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