thanks...
I've already changed the wire connectors, and cleaned everything... I'll check the signal wire, but I'm not sure I clearly understood (infact, I do not unstand "peg"

) : my needle is already "sticked" at the top of the gauge; when i'll put the signal to the ground, will it go to the bottom or stay at the top??
furthermore, it sometimes works again; is it a clue about the fact that the guilty may be the gauge ?