Hi everybody. After three weeks of search this morning I've found that the "Masse - red light" come from Yanmar engine.
When engine is off "Masse -" light is on, when engine is running both "Masse -" AND "Masse +" are ON.
I've checked external wiring of 24V alternator and all cables are well wired.
If I disconnect the 12V battery the light don't turn off (I think it's from 24V batteries).
Thanks a lot and sorry for my bad Enlish
Lorenzo SV "Red Passion" currently in Elba island, Italy.