|SPIII Charging Solutions
Having just been through problems on my '91 SPIII, here are my findings:
The wire which connects to the voltmeter also supplies the headlight and front brake switch. I put relays in to supply the headlight direct, as I was getting a true reading of not much more than 10 volts at the bulb. This helped. The Veglia voltmeter on my bike is accurate, in that it does give the correct voltage on the end of the bit of wire, but this does not reflect the voltage at the battery.
I also fitted a new regulator, as the later ones have a higher regultor setting, at 14v. The earlier ones would not let the voltage go above 13.5 tops.
I finally discovered that the rotor had lost most of its magetism, so I got a new (SH) alternator, and have not looked back since.
I now get almost 14v on the Veglia meter with all lights on. It even balances the ignition on tickover.