Modern Vespa

How could I explain/solve this problem on a GTS250 ie from 2010, the one with the analogue pointers for temperature and fuel level, not the digital ones:

While driving, the engine warms up as it should do, the indicator is nicely in the middle, and when I loosen the throttle, the temperature indicator also goes down completely, position 0. When accelerating again, the pointer also returns to the center whitin a second. Coolant fluid level is ok, plug on the sensor is good. For a moment I thought the charging current drops, but that is not the case.
I have the impression it has something to do with the injector, that one is switched 'off' when relaising the throttle, as long as the rear wheel makes the engine go round. Once the clutch releases the rear wheel, the injection takes over to keep the engine running, and the pointer goes up again. Where should I start looking?

That is really funny.
I think your analysis is correct that it probably is linked in some way to the injector.

But then the injector and the temperature gauge have to be on the same circuit.
Do you have an electric scheme of your scooter?
If not maybe someone here on the forum has.

One other question:
Your GTS is from 2010.
How long do you have it? What is the mileage? When did the problem pop up? Did you do quite some mileage before without the issue?

I don't think the injector is fully switched off as fuel needs to be supplied to the engine to keep it idling.
The wiring diagram does not show any connection between the fuel injector circuit and the temp gauge circuit.
One thought is a loose connection possibly on the connector to the ECU. I would pull this connector off and inspect the pins. You may want to plug it in and out a couple times to wipe the connections.

I think the injector is fully switched off when slowing down as long as the engine is running above idle.

But the important thing is that you state the wiring diagram does not show any connection between the injector circuit and the temp gauge circuit.
That means the relation LoMax experiences between the functioning of the injector and the temperature gauge must be indirectly.

So maybe indeed there is something wrong with the connections on the ECU.
Is there any connection between the temperature gauge and the ECU?

I don't know about that. I would think if the injector is fully switched off no fuel would flow into the engine and it would stall. The temperature sensor contains 2 sensing units. One goes to the instrument cluster and the other to the ECU. I would try checking the connection at the sensor also

as wbdvt said, the injector does not turn off at idle.

one thought I do have is battery / regulator voltage. if it's low than either the temperature sensor or the temperature gauge itself could be struggling

Does it happen with the bike on the stand?

The only diagram I can find, is the one with the digital dash, not the analogue one. I just bought the GTS, it has 12000 km on it.

The injector is cut off when the rear wheel makes the engine run at higher rpm as the 1200 idle rpm. The connections are in good condition, as are the wires.

There is a good 13,8 volt when cruising, and it drops to 13,4 volt when idling, that are good values...

With the bike on the stand, everything goes fine, and also with the GTS on its wheels and the motor idling.

There is a connection between the sensor and the ECU, but I cannot find out if the gauge gets its signal from the ECU, or directly from the sensor (the 'analogue' wiring diagram would help big time)

14.2-14.5 is normal for a GTS. So replace your battery, it could be the cause of your troubles (yes you can try charging it first to see if you can get over 14)

I understand that except for this strange issue the scooter is running fine?
If it does then there is plenty of time to find out what causes that strange issue. And no need to start replacing parts or sensors right now.

There is a connection between the temp sensor and the ECU, and that is logical, but it is not sure if the gauge gets its input directly from the sensor or indirectly from the ECU.

I would first try to get hold of a wiring diagram of your 2010 GTS250. I am sure someone here has that.

Maybe wbdvt has one:
He explained that:

That only supports your observation that as long as the injector works, meaning is operational and injects fuel, the issue is not there.

That is indeed how it is supposed to work.

Now, is your statement based on "how it is supposed to work" or is it based on a measurement?

It's probably something loose that's wiggling the wrong way under deceleration.
I'd be pulling the headset and randomly fiddling with connectors to see what's up.

Attached is a wiring diagram

You are right, it is just a matter of fixing ghost-errors, a challenge...

There is a new battery in the GTS, and every week she gets the Optimate attached to it for a couple of houres. First I thought in that direction too, but the gauge is cut of in an instant, not going down slowly with the voltage dropping...

Based on how it is supposed to work, my IAWMIU diagnostic program is installed on my laptop, not easy to put on my lap while driving and decelerating on the GTS.

Thanks, the Haynes is new to me. The other one is for the digital instruments, the Haynes is not very specific about digital or analogue. I am afraid the analogue is only built for a short period. But as you can see, the value that goes to the instrument cluster is - (earth/negative), with changing resitance. I suppose that will be the case with any kind of instrument. Now I have to find out if the injector is triggered by negative (earth) pulse, or if +12V is used, pulsing or permanent. But I am getting closer...

I don't think earth (-) and resistance, going to the gauge, is influenced by the on/off function of the injector...

After a new testdrive (checking charging voltage at different speeds), I noticed the gauge did not dropped at very high speed/high vibration. And after cutting the engine, the GTS would not start again, no reaction at all. Fiddling with the kill-switch, and it started. Kill switch to the 100% ON position, not starting. Switch in 97% ON position, starting like a brandnew GTS. And as you can guess, the temp-gauge worked fine after that.
Conclusion: bad killswitch, I should have known, Italians and electricity, pff...