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I am replacing the instrument panels on my monster s4 and i am trying to understand how the switch works. Schematics looks simple. Low level and it sends a signal. I guess 12v signal. The problem is that if I try to measure it up its no voltage coming from the switch. Even connecting a LED to the right pins makes nothing. Connecting the original instrument and works and it indicates low fuel. How does the sensor work? Is it a high-tech hall effect sensor? Am i missing something here?

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maybe yours is the old style - float moves, and connects circuit.

there is a time delay too - 20 seconds or so, which stops the light flickering with fuel sloshing. plus they often need a little jiggling, like the vibration of the running engine, to make them connect.
 

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it's a thyristor (?). no fuel around it to cool it allows it to heat up and pass current. ? fucked if i understand it either.
Explains why it is difficult to measure it. I guess thats its connected to the PCB and some kind of magic makes it working. I preferred the good old 851 floater that alerted you 150km before empty during heavy braking.

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Explains why it is difficult to measure it. I guess thats its connected to the PCB and some kind of magic makes it working. I preferred the good old 851 floater that alerted you 150km before empty during heavy braking.

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yeah, mine does that too. well, it did 12 years ago last time i rode it.

the circuitry is inside the black tower.
 

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it's a thyristor (?). no fuel around it to cool it allows it to heat up and pass current. ? fucked if i understand it either.
Thermistor. It's a variable resistor whose value changes with temperature. low temp = high resistance = (almost) no current. Your explanation is exactly correct. What's nice about them vs. floats is that it's a damped switch which requires a prolonged period (minutes) of being out of the fuel before it turns on, i.e. when you hammer the brakes or lean over for a short time it does not make the light come on. Curious how they'd do in the winter time, whether the low fuel light would ever come on...
 
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