Archaeology expedition - or WTF is inside a fuel level sensor, anyway?

[Speeddog]

Known dead fuel sensor gave up it's carcass for science.
This is a carbie unit, for whatever that's worth.

If there's anyone 'sparky' enough to understand how this type of sensor works, feel free to chime in.
Nothing obvious to me as to why this unit quit working, other than it's been submerged in gasoline for a decade.

The slightly ravaged condition is due to removing the potting.

[greenmonster]

Somekind of variable resistance thingy attached to float.
Believe it develops/wears bad connection where mechanic ends meet electrics,
resistance becomes irregular or fixed - stops working.

Same as in cars fr 50-60s I learned fr my father who worked as a car electrician then.
Many basics still the same, batteries, charging etc, still learning from him.

[battlecry]

More like a variable capacitance, looks to me.  Many electrolytic capacitors (the little can gizmo) are known to fail.  Bad batches abound.  I saved a 65" Mitsu TV with $11 of replacement capacitors in the power board.  If the shiny end of the can is slightly bulged out, it gave up the smoke.  The other parts, resistors and transistors should be reliable.  Pity.

My old '59 alfa has a variable resistance fuel sender.  Wrapped many turns of wire on that sucker.

[abby normal]

also an analog filter to limit response to rapid changes (e.g. waves moving around in the tank).

[Jarvicious]

Yep, bad capacitor.  If you were so inclined, you could probably just solder a new cap on there and it would work fine.  Doesn't look like there's really anything else on there that would go bad, at least not with the minimal current moving through that thing.

[Speeddog]

All of the components were potted inside the plastic sleeve (that I cut in half), so it was basically impossible to access anything without ruining it.

What I did figure out, or at least strongly suspect...Is that you can't check one of these for function with a VOM set to 'continuity'.

I'll know better when I get my hands on the brand-new replacement,

[battlecry]

Yes, it looks like it needs juice before you can get anything out of it.

[jerryz]

Speeddog ..excellent thread , i have had two of these units fail over the years on M750 and S4 and tried many things to clean and ressurrect them so it intersting to see an Autopsy and understand the ''cause of death''   would it not be possible to change capacitor and use some kind of epoxy to reseal the unit as they are damned expensive .  i kept the last dud one  off the S4 so have a spare to play with .

[Speeddog]

Perhaps an electronics specialist could fix it, but it would be very hard to get the components out of the epoxy potting without ruining them.

[Timmy Tucker]

Capacitor gets my vote. Electrolytics have a finite life span. With the exception of bad vacuum tubes, they have been the #1 culprit in every guitar amp I've repaired over the years.

[greenmonster]

Yup, sure looks like capacitor.
I just guessed (sorry! ) it was the same as the one in my 907,
which my father repaired & is resistance based.

[gjscott]

Hi,

I have what  looks a newer version of this sensor from a 2001 Monster M750 which I've dissembled and am in the process of trying to repair:





I've made a circuit diagram that maybe somebody here can troubleshoot. these are the symptoms I had and what I've tried so far:

The fuel level low light didn't work and was permanently off. 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



If/when it's working I'm planning to relocate the circuit board external to the tank to help preserve it.

[JoeB]

To me that sounds like the diode is buggered. (the leftmost component in your picture).

check the diode isn't leaking current in the other direction. should only have flow one way (i.e. very little resistance in one direction and almost infinite in the other)

[jerryz]

and whats worse Ducati want ridiculous money for them ,, in UK£96   ( $150),,, shameful profiteering  !!!!!!!!! for something that costs $10 to make

[scaramanga]

good job reverse engineering the circuit diagram, but i have a doubt about how the transistor is configured.
Its been a while since i've analyzed a diagram but here goes.
The theory is the fuel sensor/switch sends a low level signal to the input of an operational amplifier(lm339 op amp).
This voltage is compared to a predetermined voltage set by a voltage divider(couple of resistors). When the level switch's voltage reaches or surpasses the predetermined voltage the output of the op amp changes states. btw as you can see there are 2 op amps per lm339. Normally the outputs of these op amps are not used to directly drive hardware (relays,solenoids, lights) so this is where the transistor comes in.There are 2 types of transistors PnP and NpN. A transistor has 3 legs, Base, Collector and an Emitter. In this case you can think of them as a switch where current will flow from the collector to the emitter. The way they basically work is you apply a positive voltage to the base(PnP type) or a negative voltage (NpN type) to get the current to flow from the collector to the emitter.
The output of the op amp drives the base of the transistor. The transistor will now be used to drive the light(flow current through the collector to the emitter and then the light). Now this is where the diagram confuses me. One leg of the transistor(i'm assuming collector) is connected to the supply 12v, so far so good. The base of the transistor should be connected to the output of the op amp and its not. This is the part of your diagram you should maybe re-check.
A shorted diode will have the light on all the time so an easy check would be to just lift it out but remember that they have a polarity and must be put back with the same orientation.Your Transistor and capacitors also need to be oriented correctly. When soldering a transistor be very, very carefull not to apply to much heat, this will destroy them.

What I did figure out, or at least strongly suspect...Is that you can't check one of these for function with a VOM set to 'continuity'.

poking around the circuit board with a VOM in Resistance measurement mode is a bad idea. The vom sends a voltage through its probes(in resistance measurement mode) in order for it to calculate a resistance to display.This voltage can easily fry your op amps or transistor.
Sorry for rambling on.......