As stated - the regulator will adjust the field current until the voltage it sees is right - if there is a bad or open connection on either the voltage sense wire or the regulator earth, it will tell the alternator to up the voltage. 
So while the regulator may be faulty, a bad connection or broken wire is more likely. I had the same thing years ago when the 110 was left parked at Tin Can Bay for two months - came good after a few kilometres. I suspect a bad connection that came good with vibration after the isuzu had been running for a while!
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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