What was found
DIVERSION & EMERGENCY DECLARED. L AC BUSS OFF LIGHT IN FLIGHT. FOLLOWED PROCEEDURE IN QRH. RESETTING GEN CONT SWITCH HAD NO EFFECT. UPON STARTING APU AND RUNNING CYCLED L BUS TIE SWITCH WHICH RESTORED POWER TO ACFT. DEFERRED LEFT GENERATOR IAW MEL 24-00-1. DMP C/W. LEFT GEN DRIVE DISCONNECT ACCOMPLISHED. TROUBLESHOT PER FIM 24-20-00-815-801, NO FAULTS NOTED ON LEFT GEN PWR SYSTEM, BUT EXTERNAL PWR SYS SHOWED: REPLACE BPCU" ON ALL FLGT LEGS. INSPECTED WING FROM LEFT ENGINE IDG TO FIREWALL, NO DEFECTS NOTED. REPL BPCU IAW MM 24-41-03. REVERSED DMP & PERFORMED ENG RUN. OPS CKS GOOD. THIS CLEARS REMOV PLAC AND UPDATED DI LOG. IPC 24-41-03-01, ITEM 10