What was found
3 HRS 45 MIN. INTO FLIGHT AT FL 380 THE #2 ENG DRIVEN GENERATOR LIGHT ILLUMINATED THE DRIVE DISCONNECT SWITCH WAS USED TO DISCONNECT THE GENERATURE AND THE APU WAS STARTED AND PUT ON THE BUS. GROUNDED - YES. PERFORMED B737-900 TASK 24-11-11-080-801 FOUND CONTAMINATION IN IDG FILTER. FLUSHED PLUMING LINES IN IDG EXTERNAL OIL COOLING CIRCUIT IAW B737-900 AMM 24-11-41. REMOVED AND REPLACED #2 ENG IDG IAW 24-11-11 UP TO STEP (I). IDG INSTALLATION TEST REQUIRED PER AMM 24-11-11, IDG AIL/OIL COOLER AND IDG OIL COOLER REQUIRE REPLACEMENT A&P TSAR174M.