Modern Vespa

Hi all,

Today I have replaced some parts on my Yourban. Thermostat and thermostat housing are replaced because of a coolant leak on the bleeding nipple. Luckily that is solved, no more leaking coolant.

But another issue that was already there is that the motor is overheating and the radiator fan is not turning on. Even when the temperature gauge on the dash is fully in the red!?

I tried to unplug the temp sensor on the cilinder head, then i see the temp gauge needle drop, when i put it back it goes back up. So i assume that is not the issue.

Also all the fuses i checked and they look okay.

Does anyone know what could be the issue?

Below is a video:

https://youtube.com/shorts/SI5sCKoeHIM?feature=shared

air in the system or faulty thermostaat

Thanks for the fast reply Maksor!!

Today I have replaced the themostat and the thermostat housing for brand new ones. I have bled the system once after filling up the coolant again.

Do you think it needs more bleeding?

And do you think it is related to the radiator fan not activating even though the gauge is in the red?

The sensor can be faulty. There are two sensors in the one screw-in package - one is just for the gauge, the other goes to the ECU which in turn switches on the fan if there is overheating. The gauge one might be faulty, it is not that uncommon.


Sometimes the fan can seize (it doesn't come on that often) and that'll have blown the relevant fuse.

The sensor for the gauge you mean the green one next to the thermostat housing? It goes straight into the engine? If so, that one I haven't replaced.

Can you please tell me where the other sensor to the ECU is located? By the way I checked the fuses and all look good. Just to be sure I will be replacing them all today or tomorrow.

Is there a way to check if the fan has seized?

Both sensors are in the same unit. They cannot be separated. However, they are electrically distinct. If one fails, the whole unit has to be replaced.

Just to be sure, you mean this unit right:

Yes.

Do you know if I have to drain the entire coolant again to remove the temperature sensor? Or can I just removen and replace the old sensor with the new one without draining the coolant.

Just be quick. I'm not even sure it touches the coolant - others might be more certain on this?

Today I have replaced the temperature sensor with a brand new one. (For those who are interested, I did not drain the coolant and it also did not drip from the sensor, just unscrew and screw in the new one without any issues)

Unfortunately it still does not switch the fan on, even when hitting the red zone on the dash.

I would like to see if the fan itself is not dead, can I just put 2 wires in the fan connextor and connect them to a 12v external battery to see if the fan starts moving?

Shame - but... If otherwise completely disconnected, yes, apply 12V to the fan to check it runs. If it's seized, it'll have blown a fuse.

Next I'd be checking the dashboard temperature gauge using external resistors to match what the sensor should be sending.

I checked al the fuses today and none of them seem to be broken.

Tomorrow I will try with 12v directly on the fan.

That last check you are mentioning, not sure on how to perform that exactly…

I have tried to feed the fan with 12v directly from a separate battery. This did not trigger the fan to rotate. Next to that I have also started the motor and put my multimeter on the other side of the connector (not the fan side) and measured the voltage from the connector. In the beginning it does not measure any voltage but as soon as the temperature gauge goes over 90 degrees I can see that its sending around 12/13v trough.

So my conclusion for now is the fan is dead. Any way to fix the fan or is it not worth it and should I buy another (used?) fan?

First, check the 12V that you measure can actually provide current - use a lightbulb and a. that it lights up and b. the voltage doesn't drop. If so - replace the fan. I've had to replace two in my time.

If the voltage drops right down - look for a high resistance somewhere in the circuit.

Just to update everyone on here and maybe be of help for others: it was solved by replacing the fan itself. Now it works like it is supposed to, little over 90 degrees Celsius the fan turns on.

You can recognize a faulty fan by checking the below:

- turn it around with your hand, it should spin freely and smooth. (Mine spun but with a lot of resistance)
- measure it with a multimeter, disconnect the the connector of the fan and then measure the fan side for resistance and the cable harness side for 12v when temp goes over 90 degrees on dash

Fan should turn when engine temp reaches 103-105C and turn off when engine reaches about 95C

How do you unclip the coolant connector? I am trying to replace one on a Gilera Fuoco (same parts) but access is poor and I don't want to break the housing

Lift the tab gently and then pull the other side of the connector.