What was found
(CAN) IN CRUISE LT AC BUS LIGHT, GENERATOR OFF LIGHT, AC BUS LIGHT AND AC CROSS-TIE LIGHT ON. AIRCRAFT GROUND RUN CROSS-TIE CHECK TRIED, LOCK-OUT LIGHT ON, GENERATOR WOULD NOT COME ON LINE AND AC CROSS-TIE LOCKOUT LIGHT STAYED ON. AUDIBLE CLACKING FROM OVERHEAD HEARD BEFORE GENERATOR FELL OFF-LINE. APU AND LT ENGINE VOLTAGE REG INTERCHANGED, NO FIX. REPLACED LT GENERATOR.