Put one of your Volt meter leads on the battery negative post (on the battery). With the other lead, check to see if you have voltage at every place on the vehicle that you think is a ground. In other words, you should have "0" volts at the engine, frame, firewall and dash grounds.
There are alternators that are sold as "One Wire Alternators". You are using the term, "single wire". Are you sure of that we're talking about the same thing? Do you have a wire from the #2 pin to the Bat post or is yours suppose to work without it.
Speaking of terms.
Please use terms like
"Key Off" ,or
"Key On, Engine Off" , or
"Key On, Engine, On"
terms like "on start" or "motor start", make me wonder if you mean during "Key In Start, Engine cranking", but just aren't saying that.
If the grounds are good, then the internal regulator is probably bad.
