I looked up 'alternator' for 1984 Moto Guzzi. It appears to be identical to the Bosch alternator BMW airheads used for several years. I have documentation from Rick Jones at Motorrad Elektrik that discusses the lamp status and potential electrical problems I can send you if you think it'd be useful. When working correctly, does the light come on when ignition is switched on, then go out once started. Is the alternator outputting power?