Hello Bus429
Sorry, some delay in replying , Drive "C" suddenly developed some bad sectors.Had to re-format and re-install Windows etc., what a pain, took all afternoon.
You are probable correct, if the AUX terminal was connected to a a.c. voltmeter or an oscillascope and the other side to earth, then if either a winding shorted to earth or a diode short circuited then a reading of a.c. volts would be obtained.
It is another test that could be applied to a suspect faulty alternator.
Regards