What was found
WHILE OPERATING THE APU EN ROUTE, WITH THE LEFT GENERATOR PLACARDED INOPERATIVE. THE APU FAULT LIGHT ILLUMINATED AND UTILITY BUSSES AUTOMATICALLY SHED LOAD. THE AIRCRAFT WAS RETURNED TO SEA WHERE IT LANDED WITHOUT INCIDENT. THE APU WAS LOADED WITH THREE ELECTRIC HYDRAULIC PUMPS AND BOTH PACKS. THE LOAD WAS SUCCESSFULLY HELD FOR APPROXIMATELY 20 MINUTES. THE FAILURE COULD NOT BE DUPLICATED. IN ADDITION, THE NR 1 IDG WAS REMOVED AND REPLACED. LEAK AND OPERATIONAL CHECKS WERE GOOD. THE GENERATOR PLACARD WAS REMOVED.