What was found
QRF TO ORIGIN AIRPORT DUE TO LEFT PACK TRIP OFF LIGHT ILLUMINATES FOLLOWED OF CABIN CLIMB OF 3000 FT/MIN UNCONTROLLABLE. PILOT DECIDED TO PERFORM IMMEDIATE DESCENT. CABIN ALTITUDE WARNING SHOWED. LEFT DUCT NOT WORKING PROPERLY. WAS CLEANED TURBINE INLET OVERHEAT SW ACM LT SIDE AND PERFORMED AIR CYCLE SYSTEM THERMAL SW TEST. ALSO BLEED AIR CONTROL SYSTEM OPERATIONAL TEST WAS PERFORMED. FINALLY PACK VALVE WAS REMOVED AND REPLACED.