THANK YOU GUYS
I THINK I DID NOT MAKE MYSELF CLEAR : THE SENDING UNIT IT SHOW THE RIGHT QUANTITY OF DIESEL FUEL BUT BECAUSE VIBRATION IN THE ROUTE THE RESERVE LIGHT TURN ON AND OF VERY OFTEN
I DID CLEAN THE FILTER AT THE BOTOM OF THE TANK BEFORE - NO ALGAE SO I THINK IT IS SOME ELECTRIC CONECTION BECAUSE OF VIBRATION
THANKS AGAIN