'95 M900, testing the fuel sensor...

[Macflurry]

In my head this is a stupid question, as this seems like it should be obvious but I've tested three of them and I dont get the fuel light on.

I noticed while working on my bike that there was no fuel light when the tank was almost empty. If I unplug the connector and short the terminals then the warning light comes straight on so the circuit is good.

I have a spare tank with a sensor, so in my head if I plug in the spare sensor, without it being in the tank, then there is no fuel and the light should activate, but it didn't.


So I grabbed a used, tested working, sensor off of eBay and plugged it in but nothing.

From testing the wires, it should be plenty of fuel breaks the circuit, low fuel makes it and so no fuel should be well and truly made. I even tried blowing in the little holes as the top in case something was stuck.

Am I on the right track and have another broken sensor (3 of them) or is it that I don''t really get how they work and one of the gurus on here can explain a better way to test them?:confused:

[mickj]

Have you tried a multimeter on the sensor to see if you get a change of resistance when you move the float and a short when in the lowest position?

[Macflurry]

It's one of the older sealed metal ones with no visible moving parts. Maybe I could try to submerge it while measuring the outputs to see if they change but as it's not a fuel level sensor, just a warning light i would expect it to be on/off.

https://i.ebayimg.com/images/g/c1sAAOSwBgZbyMJ4/s-l1600.jpg

[Luddite]

I think that's a thermistor type sensor where the resistance of the unit changes depending on the temperature difference between full and empty. When the sensor is covered with fuel, the temperature is lower than when uncovered and the resistance is high. When empty, the sensor is exposed to the air and the temperature will be lower, reducing the resistance.

I don't know what the readings should be, but if you attach a multimeter switched to kOhms and then immerse the sensor in fuel, you should see an increase in resistance if the sensor is working.

[Mr Gazza]

Well I didn't know they worked on temperature, but a quick think leads me to believe that a dunking test won't work.

It can't work on a nominal temperature threshold, because the fuel temp will vary by several degrees depending on whether it's a frosty Scottish morning or standing in the Mediterranean Sun, with a similar range for the air within the tank.

It must work on the differential between the top and bottom of the sensor? (I'm guessing here.)
So dunking the whole thing in and out will not produce a differential.

Probably better to dip the tip into (say) cold water whilst holding the bottom with a warm hand (Oo er missus!)

Warning.. Do not take the test suggestion literally. I knew someone who badly scolded their feet whilst preparing a can of soup.
The instructions said .."Stand in boiling water for 5 minutes".

[Capo]

There is a float, it has nothing to do with temperature

[Luddite]

We need an auto-electrician here as I'm relying on my old college physics lessons here!

I think the thermistor is driven by a the battery voltage, which generates a self-heating effect. When the thermistor is immersed in petrol, its temperature, and therefore its resistance, will remain relatively constant; the fuel acting as a heat sink. If the fuel level drops, however, the thermistor is exposed, the heat sink effect of the liquid disappears, the temperature rises, and the resistance increases.

That being the case, you would probably need to energize the thermistor before trying the immersion test if you want to get accurate readings. If you try that, be careful connecting the voltage source - electrical sparks and petrol vapour are not a good combination!

[Luddite]

There is a float, it has nothing to do with temperature

I know Ducati have used various designs for their fuel level sensors over the years, including float types like this Multistrada one.

https://i.postimg.cc/02GTtFSN/1-multistrada-1200-my2013ing-57-638.jpg (https://postimages.org/)

But that picture Macflurry posted looks like a thermistor type to me.

[Capo]

The float does not have any switches, it uses some form of capacitance/ induction
I tore one to bits and have photos

[Luddite]

The float does not have any switches, it uses some form of capacitance/ induction
I tore one to bits and have photos

I stand corrected!

If you still have the photos, can you post them up please? I'd be fascinated to learn how they work (sad I know!).

[Macflurry]

Looks like this one turned into a bit of a can of worms!! I'm glad I'm not the only one scratching my head on this one!!:D

[mickj]

If you are going to test it in liquid just use water not petrol, it's safer :D
You are just 30 odd miles from me if you fancy a ride out bring one with you and we can have a look at it together, I have a multimeter and plenty of water, even got coffee.

[Darren69]

Yea, it should work in coffee. That's probably what they use to test them anyway. :)

[Flip]

Yea, it should work in coffee. That's probably what they use to test them anyway. :)

Illy or Lavazza?

[Darren69]

Got to be Lavazza

[Flip]

Ok back to the topic-

So you know the bulb and fuse are good yes?

So the next thing to try is with the ignition on check to see if you have voltage on the blue/black wire on the sensor connector. If there is none you need to check the wiring between the connector and the fuse box for open circuits or poor connections.

[Capo]

https://i.postimg.cc/ZRXTQDW2/P1000694.jpg

https://i.postimg.cc/WprG9qT8/IMG-2469.jpg

https://i.postimg.cc/bwRwvT5S/5449377608-beeda975df-b.jpg

https://i.postimg.cc/xjbjmY72/P1000697.jpg

[Luddite]

Thanks, Capo. Very interesting.

That's one complicated unit!

[Flip]

Okay, I've just re-read this thread (I replied earlier using my phone and couldn't be bothered to read it all).

So you have 12v at the connector and the light comes on when you short the terminals suggesting the circuit there is good.

I think the photo Capo posted is from a later bike with an electric fuel pump hence the additional wiring and circuit board whereas the early bikes used the float only to make or break the circuit.

So a silly question- If you shake it does it rattle?

I know it seems unlikely but if you have three sensors all used and of the same rough age is it possible they could all be gummed up with years of crappy fuel.

I know Ducati have had quite a lot of stick over the years about dodgy sender units but personally my twenty one year old bike is still on the original but maybe it's an exception as it's also on the original Regulator/Rectifier too.

[Macflurry]

Hi guys and thank you for all the responses.

If you are going to test it in liquid just use water not petrol, it's safer :D
You are just 30 odd miles from me if you fancy a ride out bring one with you and we can have a look at it together, I have a multimeter and plenty of water, even got coffee.

Thank you for the offer, and I would have happily taken you up on it but my bike currently needs MOT'ing.


So you have 12v at the connector and the light comes on when you short the terminals suggesting the circuit there is good.

I think the photo Capo posted is from a later bike with an electric fuel pump hence the additional wiring and circuit board whereas the early bikes used the float only to make or break the circuit.

So a silly question- If you shake it does it rattle?

I know it seems unlikely but if you have three sensors all used and of the same rough age is it possible they could all be gummed up with years of crappy fuel.


Yes, the circuit appears good and appeared to be a simple make or break circuit, which is why I wanted to check how these older ones work as there's no obvious moving parts for a float.

I managed to find the Monsters wiring diagram to check and confirm it was a simple make/break circuit, so I'm happy bike side is good but could check that it's definitely getting the full 12V.

No, it doesn't rattle. There's no sounds of anything moving in there which brings me back to why I posted this, as I don't know what's inside or how it's supposed to work, I needed to check I wasn't causing more damage to any components by trying to clear stuff out or if this is simply how it's supposed to be.

I know mine is for sure knackered, the one on the spare tank is of unknown history so I assume that one is knackered but number three is supposed to be known working... but as it is used I need to know how they're supposed to work to rule it out properly. A jammed float was what I suspected at first but as they don't appear that you can strip them I'm back to needing to know how they work before I can try to fix it properly. If it was something other than a float, and isn't gravity activated as suggested earlier, then I may need a different approach.

I can try leaving one to soak in something to try and clear out some gunk.

The float does not have any switches, it uses some form of capacitance/ induction
I tore one to bits and have photos

The older sensor like mine should be just make or break. The newer ones, if they show a fuel level , will need either a resister ladder or variable resistor on the float to give different readings for the levels. The capacitors on the board will most likely act like a rectifier to smooth out the values that are fed out so the fuel gauge doesn't constantly go up and down with every bump in the road.

[Flip]

A couple of years back during one of my ‘refreshes’ where I replaced the fuel pipes, vacuum tap and pump bits I had the fuel sender out to swap the prone to cracking plastic nut for a nice alloy one.

While it was out of the tank I am 99% certain it rattled as the float ball moved inside.

I’ll look back through my photos to see if it looks the same as yours. It might not help at all but it might give a little clue on what to check next.

[Macflurry]

A couple of years back during one of my ‘refreshes’ where I replaced the fuel pipes, vacuum tap and pump bits I had the fuel sender out to swap the prone to cracking plastic nut for a nice alloy one.

While it was out of the tank I am 99% certain it rattled as the float ball moved inside.

I’ll look back through my photos to see if it looks the same as yours. It might not help at all but it might give a little clue on what to check next.

Thank you, but if its a pain don't worry about it. I'll find something too soak mine in and see how it goes.:thumbsup:

[Tosh]

Sorry to jump in but, if you have a known bad un, and have a spare, why not cut one open and have a look inside ?

[Macflurry]

Sorry to jump in but, if you have a known bad un, and have a spare, why not cut one open and have a look inside ?

Trust me, it has crossed my mind. But with the price of these used being ridiculous I wanted to confirm if they were knackered before hacking them apart. I can see where they are put together using a circlip at the end but it's ddefinitely not designed to come apart gracefully.