What was found
LHR - FLT 121 - LEFT GENERATOR FAILED 10 MINUTES AFTER TAKEOFF, UNABLE TO RESET. STARTED APU RETURNED TO LHR TAXI TIME PRIOR TO TAKEOFF WAS 18 MIN. LEFT IDG/GCU WIRING TROUBLESHOOT IAW B767 FIM 24-20-00-1. ALL FOUND TO BE OK. LEFT IDG ENGINE HARNESS INSPECTED FOR CHAFING. ALL OK. IDG HARNESS FOUND TO HAVE OIL INGRESS FROM PREVIOUS OIL LEAK. IDG HARNESS AND C/P CLEANED ALL OIL REMOVED. C/P RECONNECTED. LEFT ENGINE GROUND RUN FOR 40 MINUTES WITH NUMEROUS ELECTRICAL LOAD CHANGES. ALL OPS AND IND NORMAL. NO REPEAT OF FAULT ON THE GROUND. (M)