Low fuel warning sender?

[Mr Gazza]

Okay, so I bought a nearly new sender some years ago from Drumnagorrach, installed it and tested the range with a ready can tucked in a pannier.

The light came on at about 120 miles if I recall, but that was the last and only time I ever saw the light on. I have since had the tank empty and I don't think the light has come on. I have assumed that it doesn't work.

So I've just acquired a new one and thought I had better just test to see if it isn't the bulb gone before I get the new sender wet. Yes the bulb is good, I can light it by feeding the correct pins in the instrument nacelle socket. And I can also light it by poking a "staple" of wire in the socket at the sender end, so I have cable continuity as far as there at least.

But if I plug the new sender in, external to the tank, I get nothing, surely it should light the bulb if it's out of the tank and bone dry? Or is this not how they work? Obviously the pump is not connected for this test, but that's a completely different circuit, besides I lit the bulb by bridging only the sender cables.

I have also tested the continuity of the signal wire either side of the plug/socket to make sure there is contact. Come to think of it I didn't test the live, so that's the next thing.

I also need to drain the tank with the ignition on to see if it lights with the old sender, just to make sure.

Am I missing something here? Does the sender have to be in the tank in petrol?

[Darren69]

Which type are fitted to the 900Sie? Some of them the later ones are thermistors and as per my understanding of such they are just thermal resistors so when they are in petrol they are cold and as the petrol level drops the temp rises and when they get to a certain temp they trip the circuit and light the bulb.

[Darren69]

That probably wasn't so helpful but the Sie was always a bit of a bitza special? I would have thought though with the sensor in the air it would light up, so maybe you have an earthing or live feed issue?

You probably don't need me to tell you that Ducati have been making electricians and mechanics of motorcyclists since the 1970's? Mine actually works by black magic but it works so I'm leaving it well alone!

[Mr Gazza]

Okay first. A bit less of the bitza and more of the special if you don't mind me laddio!.

You have got me thinking with the thermal aspect of the sensor though. If it is thermal it must work on differential of temp between the top and bottom and not a specific temperature. Since the temperature of the petrol and for that matter the air could have quite a range given all circumstances, but if the sensor is fully submerged at least it will be all the same temp until the tip is exposed, when a difference might occur.

Looks like I'm going to have to get it wet..

[Darren69]

Yea, it's a factory special like the S4 Foggy, and like the Foggy it was probably made off the main manufacturing line so whatever bits they had went on! I'm not sure myself exactly how the fuel sender works besides the theory or how accurate it is, maybe there are holes for the fuel to sit in? Dunno. I can check the wiring diagram I have for the '01 S4 but my guess is that there is a live feed of a certain colour from ignition or one of the relays that are live with the ignition on (fuel pump etc) and then there will be an earth (black maybe). will check and let you know, maybe if you're lucky your wires will match mine?

[Mr Gazza]

I think you'll find that the 2000 M900Sie is a millennium special with all the best bits they could find. evidenced in the sudden drop in spec of an S soon afterwards.

Darren, the cables are white/brown and black and the continuity is good all through. I can put the warning light on by bridging the two cables where the sender plugs in, just a bit mystified why the sender won't put the light on.

[Darren69]

On my fuel sender wiring there are 4 wires but 2 are for the pump and the other 2 are presumably for the fuel sender.

The 2 black wires are both earth and appear to be shown joined together, so maybe they are. Goes without saying they are the negative.

The B/W (Blue/White) which comes from the Dashboard (maybe the lamp?)

Then there is a Bn/W which I cannot find on the key but assume it to be Brown/White which comes from the injection relay (so the switched live) so you should have 12V there when the ign is on I'm guessing.

The key actually lists Br as brown and there is no mention of Bn so your guess is as good as mine and like I say that is from the '01 S4 schematic in the workshop manual.

[Darren69]

Originally Posted by Mr Gazza

I think you'll find that the 2000 M900Sie is a millennium special with all the best bits they could find. evidenced in the sudden drop in spec of an S soon afterwards.

Darren, the cables are white/brown and black and the continuity is good all through. I can put the warning light on by bridging the two cables where the sender plugs in, just a bit mystified why the sender won't put the light on.

Yea, they had a side room where they made the S4 Foggy's apparently which had all the Ducati Performance bits. they just had the wrong year DP ECU's in there at the time!

[Darren69]

maybe you have a bad sender unit, but I know they are stupidly expensive new from Ducati (over £100 iirc)?

[Darren69]

It's interesting as I know my fuel level is actually below the reserve limit but it does take a few minutes for the light to come on sometimes.

[Mr Gazza]

Stop worrying everybody... ..And calm down Darren!

Sooo, only one thing for it, I decide to fit the new sender and try it wet.
I have a very nifty tool for syphoning tanks which is merely a tube with a ball valve at one end. You just poke the valve end into the tank under the fuel and jiggle it up and down. Very soon a column of fuel rises up the tube which is then lowered into the ready can and presto, out it all flows. So as planned I had the ignition on as the tank drained and would you believe it? The fuel light came on! It's always worked.

So I do now have a brand new fuel sender going spare if anyone has a need. Pm me for the price (It won't be the new price!).

So with the trip at 92 miles from last fill up (I'd run it low on purpose), I drained about 5 litres I would say. I have repeatedly recorded 68mpg from the old bird and always tried to fill up after no more than 100 miles, but it seems I have much more range than I thought.
I usually start to sweat at 120 miles thinking the fuel light doesn't work, but I must re-think this. Maybe I will take a ready can for a ride and see just when the light comes on and how far I go after that... Interesting.

[Luddite]

Mr G, the fuel level sensor in my Evo is a thermistor type and, when I had the pump assembly out to change the filter, I had a little play around with it. (I know, I should get out more!)



As already stated, the thermistor does operate on a temperature basis. The way it works is that it draws a small current from the bike, which heats it up to a certain temperature. When it's immersed in petrol the fuel acts as a heat sink and keeps the thermistor at a constant, lower, temperature regardless of the ambient temperature. At the lower temperature, the resistance is high but, when the fuel level drops below the thermistor, it heats up, which reduces the resistance causing the fuel light to come on.

Be aware if you're testing it, that it may take 30 seconds or more for the thermistor to heat up enough to switch the light on.

Hope that helps.

[Mr Gazza]

Thank you once again Vince. That explains everything nicely.

It explains why I got so much fuel out before the light came on and also why the new one didn't put the light on, as I expected it to be instantaneous.

So to conclude my experiments I not only need to do the ready can trip, but also just pop that new sender on to the loom again and give it some more time. I don't want to be selling a dud!.. and perhaps before I sell it, I ought to test the one on the SS too.

[Luddite]

Sounds like a plan!

If you touch the thermistor before you turn it on and then again after half a minute or so, you should be able to feel the heat of it.

[Darren69]

If the one on your bike has the plastic nut and the new spare one has a metal one it'd be worth switching to the metal one. The plastic ones are known to crack/leak, presumably when the plastic gets brittle.

I don't know when they made the switch over but I checked my '02 and it had a metal nut, may have been original or a later replacement, I don't know.