Something is grounding the field brush that connects to the voltage regulator (using the green wire).
> Test this one first. Engine off, key off, set the multimeter to resistance. Backprobe the field connection with one probe and connect the other with ground. The alternator housing or a mounting bolt is fine.
+> It should be open, infinate resistance, no connection.
- > If there is low or some resistance then disconnect the connector with the green wire. Restest by measuring resistance between the field terminal and ground. Then measure resistance between the green wire terminal and ground.
The "isolated field" alternator and regulator work by controlling the current flow to ground. There's a diagram in this post about 1/2 way down. So if there is a connection to ground before the regulator, the rotor will draw as much current as it wants without regulation.