Technical Seicento overheat light on but no overheating

[Rioleo]

Hi,
I'm a new user to the site so apologies if this is in the wrong place. My car is a 1999 Seicento Suite 1.1 Spi. Mileage 135,000 of which I have covered 95,000. I had to have a new alternator fitted recently and following that the coolant overheating light came on for the first time, I have never seen it on before. It came on dimly at first then after a few miles got to full brightness. There are no signs of overheating and I have put on a new water pump, thermostat and temperature sensor and changed the coolant as well as bleeding the system. The heater gets hot too. Radiator fan cuts in and out as it always has. On starting the engine cold the light is off.
I have followed a number of similar queries but none of those I have found has ever said how this problem was fixed.
Maybe the new alternator is a coincidence but I am left thinking that this is some form of electrical problem, possibly a short or poor earth that the new higher output from the alternator is showing up?
Any ideas on sorting this out would be appreciated as it is now driving me mad!
Thanks All.

 

[rallycinq]

Did the heater work properly before?

Cheers

SPD

 

[Rioleo]

Sorry,
Getting used to the site so this reply may arrive twice. Yes, the heater has always worked fine and I should have said that I have bled the system both hot / running and when cold.

Thanks.

 

[s3ccw]

I'm having this exact problem. Can anyone help?
Ice had new thermostat and changed all 3 sensors to do with coolant

 

[DRSIK]

The Cinq and the Sei have a different sensor (and it is stored wrong on a lot of Auto factors computer systems as well).

Putting the Cinq one in a Sei (different resistance) causes it to read as overheat when it is actually saying "engine warmed up".

Safest option is to fit a temperature gauge.

(see the guides for this)

If you dont have time / energy you need to check the temp sender on the head and fit the correct one.

I currently have the wrong one in my buggy (head rework co fitted the wrong one) - so I use the light as a guide to when I can REALLY use the accelerator pedal

 

[DRSIK]

Here you go

https://www.fiatforum.com/cinquecento-seicento/26286-temperature-gauge-issues-2.html