|90% of Guzzi Charging Problems Solved Here
1) Clean all contacts. Take them all apart and hit them with 400 grit sandpaper and spray with wd40 and reassemble.
2) Check resistance across the alternator rotor with the stator off. It should be a few ohms. If there's infinite resistance, your rotor is bad.
3) Clean your rotor contacts with steel wool/sandpaper and finish up with carb cleaner.
4) Make sure that your diode board has a ground wire running from the ground side to the engine/battery.
5) Make sure that the wires are all connected correctly (duh).
6) Check the individual diodes on the diode board. They should have high resistance in one direction and low resistance in the other. If you lose even a single doide, you are down 1/3 of the capacity of the charging system. Motorrad Elektric sells rebuilt Bosch diode boards that will probably last forever for about $100
These issues take care of 90% of charging problems.