What was found
LEFT AMBER PACK LIGHT ILLUMINATED ON TAKE OFF ROLL AT 100KTS, AFTER TAKE OFF, QRH RUN. PACK RESET OK. GROUNDED - YES. PERFORMED MULTIPLE PZTC BITES OF LT AND RT PZTC REF FIM 21-62 TASK 801 WITH NO FAULTS FOUND. PERFORMED GVI OF LT RAM AIR SYSTEM AND HEAT EXCHANGERS WITH NO DEFECTS NOTED. WHILE PERFORMING STEP 3, FOUND LT PLENUM FAN BYPASS CHECK VALVE AND HINGE PIN MISSING REF AMM 21-51-03. REMOVED AND REPLACED LT PLENUM DIFFUSER ASSEMBLY. REF AMM 21-51-03. OP CHECK GOOD. NO LEAKS FOUND.