What was found
EICAS MSG LT GEN DRIVE DISPLAYED.GEN DRIVE LIGHT ILLUMINATED. GEN DRIVE DISCONNECTED. FOUND IDG TO BE LOW ON OIL. HAS HISTORY. IDG REPLACED 2 TIMES. FUEL OIL COOLER 1 TIME. LEFT IDG TOOK 64 STROKES UNTIL OIL OVERFLOWED INTO DRAIN BOTTLE. OPENED REVERSER AND CHECKED FOR EVIDENCE OF EXTERNAL LEAKS FROM AIR OIL COOLER ON DOWN AND NONE FOUND. SUSPECT INTERNAL LEAKAGE OF FUEL OIL COOLER BYPASS VALVE. REPLACED FUEL OIL COOLER BYPASS VALVE PER MM. RAN ENGINE FOR 10 MINUTES WITH AC ELECT ON LEFT IDG. NO LEAKS OR ABNORMAL INDICATIONS NOTED. RESERVICED IDG AFTER LEAK CHECK. REPLACED THE AIR-OIL HEAT EXCHANGER (PRECAUTIONARY).