Engine Alternator/Voltage regulator charging problem

It does sound like the voltage regulator has went south on you. I am not familiar with diesel engine alternators, but they should be the same. They do the same function, charge the battery. It is quite possible that the voltage regulator is inside of it. I haven't had a vehicle with an external voltage regulator in some time now. If it has an external regulator you should be able to trace the wiring from the alternator to the voltage regulator.