was it an original ducati or "correct" replacement reg, they have a voltage reference from the battery that goes through the loom and if it is low due to age and all that crap it will give a misleadingly low voltage at the reg and so the reg puts out enough voltage to make that reference what it wants.
it's not a reg issue if that is the case. fitting a reg without the reference wire does bypass it though, like the later fitment sh579b.
from my charging system write up:
If you use an original style or Electrex or other single phase regulator that uses a reference voltage input (the two wire flat connector has one wire to run the light, the other is a reference) then you need to make sure the reference is accurate. This reference is switched through the ignition switch and runs through the loom. If the reference is low, due to the resistance that naturally builds up in an old loom, the regulator will think the battery voltage is lower than it actually is and it will overcharge the battery. I think itís the white/red wire (or maybe pink?).
You can either check the voltage in this wire or (better) run a jump wire from the battery positive to the connector. I pull the connector apart enough that itís still connected, but you can touch the terminal. Then, with the engine running and while checking the battery voltage, touch the jump wire to the terminal. If the battery voltage drops instantly (and it will if itís overcharging) the easiest fix is to cut the reference wire on the loom side of the two wire flat connector, fit a relay running battery power direct to this wire into the regulator and switch the relay on with the loom side of the reference wire coming from the ignition switch. 906 Paso have done this for years, and Iíve seen a few Monsters and SS do it.
The SH579 3 phase regulator doesnít use a reference, so fitting one does away with this potential issue.
know all wanker