Yesterday a very strange thing happened to me on my 1.9 Croma.
The alternator warning light turned on when I turned the ingition key to start the engine.
"Oh no! I said! The alternator...again! I changed it only 1 year and 5 months ago!"
I turned off the engine and then immediately started it again: the warning light then was off.
The light hadn't lit until I turned the engine off and then on another time.
It was annoying, because, if the warning light was on when I started the engine, it didn't turn off until I turned off the engine.
Instead...if I turned the engine on and the warning light didn't appear immediately, It didn't appear ever more during that "session" of engine on!
So if warning light turned on during starting: ever on
If not: ever off.
Very strange!
Finally I tested the voltage with a tester and it was good! 13,8V both with the warning light on or off!
I found it was the wire from the alternator to the dasboard that was broken. Totally broken! Near the connector to the alternator. Probably cause to vibrations. It's a very little and short wire.
The strange thing was the warning light, with a totally broken wire....something on and something off.

Mine does that too!

It's been constantly on for 2 years now, and like yours, the alternator actually works fine. I've always put it down to a faulty sensor.

I might have a look at it before I sell mine.