Fuel gauge - free petrol!

[Norumph]

Set off for a ride, fuel showing about half, which thinking about when I last filled up seemed about right, 94 miles range showing on instrument display. After about 25 miles I stopped for an hour or so, and on starting up again the gauge showed full! After a while riding hoping it would settle down I stopped and started up again, but no change. The reading and range were going down slowly, but from the full reading.

Is this more likely to be the float sensor, or an instrument fault?
2012 Tiger 800

[awjdthumper]

It sounds like the float sensor - all the ECU does (I believe) is to convert the sensor output voltage into a fuel level with a bit of averaging thrown in to account a bit of sensor output variation due to fuel sloshing about inside the tanks as you ride along.

I am sure you can display the float sensor output voltage using a diagnostics setup to check whether it corresponds with the amount of fuel you think is in the tank and with what the fuel gauge is showing. I would have thought the sensor calibration was fixed and there would be no way this could be reset inadvertently to end up showing a half-full tank corresponding to a full tank on the dash display

[K1W1]

The logical thing to do would have been to refill the tank and keep track of the distance since full from then (reset trip 2 to zero when you refill). It may simply be that the float was stuck for some reason and a fill might unstick it.

[awjdthumper]

From reading around the subject, the sender unit appears to be just a simple potentiometer and, in series with a fixed resistor inside the ECU, produces a voltage that the ECU uses to determines fuel level. The potentiometer varies between about 10 Ohm when tank is full to about 125 Ohm when the tank is empty, the former corresponding to a voltage of the order of 0.5 V and the latter to a larger voltage, perhaps, the order of 5 V.  TuneECU can display the voltage.

In principle, the sender unit functioning can be checked via the voltage displayed by TuneECU (I assume TigerTool can do the same) or by measuring the resistance at the sender unit connector. If the problem does not right itself after filling the tank up again then the only way of sorting it may be to remove the sender unit from the tank and to test it.

[Norumph]

Thanks for the replies.
For some reason I was fixated on the half full reading being correct, but it should have been obvious to me that the full reading was more likely to be the true one if the float might have been sticking.
I've just filled the tank today and it took 6 litres which would be about right for the current gauge reading, though the indicated range remaining of 114 miles seems a bit low. But if the range is up to when the fuel warning light comes on it would be correct, I think.
Is an intermittently sticking float just a Tiger thing, or is it a sign that the sensor is on its way out?

By the way, I have TigerTool and it works with my lead (on port COM 6) but I don't think it shows fuel sensor voltage. I tried TuneECU but it says "cannot open port COM 6", I don't know why.

[awjdthumper]

Here's what the sender looks like - just a potentiometer with the float attached. I would have thought that the only way the float can stick is if the potentiometer is failing, in which case, you would have to replace it - a new one costs about £50 + VAT. The sender is clipped to a bracket inside the petrol tank and you would have to empty the tank and remove the pump to get at it.