If you need help understanding electrics. Have a look at this excellent article over at Greg Benders site. It shows the common problems and helps to explain the various parts of the electrical system.
When trouble shooting you need to know what should be happening or the stages of the charge so that you can find out which part is not working. On mine as it was intermittent this made it hard to find and as I had more than one issue also difficult. Use the forums and I hope you get yours fixed quickly.
I have owned my Moto Guzzi California a while and in that time the charging has been intermittent. It is only today that I think I have corrected the problems and have it charging.
When I started the bike most of the time the charge light on the dash would show. The bike would start and a rev of the engine past a few thousand would kick in the charging. The meter would move just over into the green.
I hope I can pass on some tips and help you to trouble shoot your Moto Guzzi although as the title when I spoke to Charles of aientam classic he joked with me that Guzzis dont have the same problem as Ducatis. I modified my Ducati 916 with a moffat regulator rectifier which has helped it to be so reliable compared to the old set up.
In the pictures you will see a diode Bosch 0 197 002 003 020 14v 22a. This was faulty on mine. Once replaced and a second hand one bought from gutsibits it improved. The day I bought and replaced mine. The charge light was out. This should have given me warning signs.
Baldrick helped me out by supplying a diode to change over with mine.
Check that before you start the bike the charge light is lit or that you have a alternative resistor in parrallel.
Make sure that the carbon brushes are new or that they are held on the copper under tension
Check all connections and wires. Are they in good condition. All wire strands in good condition
Check with a good meter all the wires from one end to the other
Make sure that you have a good earth from the diode to earth. Check with a meter.
Clean the brush pick up with a fine emery
If like mine the dash light was faulty. I could not source a new one and swapped for one of the other lights in the dash.
As the display shows the voltage