"Masse -" and "Masse +" (24V current leak)


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).
Any ideas?
Thanks a lot and sorry for my bad Enlish

Lorenzo SV "Red Passion" currently in Elba island, Italy.

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