What was found
ELEC GEN SYS L EICAS STATUS IDG HAS DISCONNECTED OR A PROTECTIVE TRIP OF THE GENERATOR CONTR OL BREAKER HAS OCCURRED OR A PROTECTIVE TRIP OF THE BTB HAS OCCU RRED. FIM TASK REF : FIM TASK REFERENCE NOT FOUND CMC MSG# 2431761 FLIGHT STATUS: L INTERMITTENCE CODE: M PH: CRUISE GENERATOR CONTROL UNIT (L) REF L/P 8170313 FOR CORRECTIVE ACTION. ELEC GEN SYS L EICAS STATUS IDG HAS DISCONNECTED OR A PROTECTIVE TRIP OF THE GENERATOR CONTROL BREAKER HAS OCCURRED OR A PROTECTIVE TRIP OF THE BTB HAS OCCU RRED. RESET PER ECL RESET SUCCESSFUL DEBRIEFED FLT CREW LEFT IDG DISCONNECTED 2 HRS INTO FLIGHT ACCOMPLISHED RESET LASTED 20 MINUTES DICONNECTED AGAIN AIRCRAFT RETURNED TO EWR PERFORMED FIM TASK 24-21 TASK 817 IDG OPEN CIRCUIT PROBLEM. ALL WIRING CHECKS ARE GOOD. REMOVED AND REPLACED LEFT GCU PER AMM TASK 24-22-02-400-801.NEED ENGINE RUN DUE TO HISTORY. ELEC GEN SYS L MESSAGE WAS THERE BEFORE REPLACE THE LEFT GCU BUT UNIT, GENERATOR OFF 25-2422-9-0002 0617 ON 25-2422-9-0002 0719 UNIT, GENERATOR OFF 25-2422-9-0002 0617 ON 0000 DISAPPEARED. ROTABLE PARTS CHANGE WAS INDICATED - TT # 1011973 SN OFF 0617 PERFORMED LEFT GCU SOFTWARE LOAD IAW AMM 24-22-02-400-802 AND LEFT GCU INSTALLATION TEST IAW AMM 24-22-02-710-802. TEST PASSED. LEFT ENGINE RUN WAS PERFORMED AND NO FAULTS NOTED.