What was found
APU WILL NOT START. BATTERY REPLACED FOR LOW VOLTAGE AND STARTER REPLACED. STILL NO START. FOUND STARTER GEAR (P/N 3860823-1) WORN AND ELECTRIC CLUTCH PACK DAMAGED. ADDITIONAL, DAMAGE FOUND IN AREA OF CLUTCH HOUSING DURING CLUTCH REMOVAL RESULTED IN APU REPLACEMENT. *S/D* PREVIOUS OCCURANCES TRACED TO STARTER ENGAGEMENT DURING APU OPERATION AS A RESULT OF OPENING CERTAIN CIRCUIT BREAKERS WHILE PERFORMING OTHER MAINTENANCE. BOEING IS PLANNING ON RELEASING A SERVICE BULLETIN TO PREVENT STARTER ENGAGEMENT WHEN AFFECTED CIRCUIT BREAKERS ARE OPEN.