What was found
CREW REPORTED SMOKE AND ELECTRICAL SMELL IN THE COCKPIT COMING FROM LEFT SIDE FLOOR VENTS DURING CRUISE APPROXIMATELY TWENTY MINUTES INTO THE FLIGHT. CREW DECLARED AN EMERGENCY, BEGAN AN EMERGENCY DESCENT, AND REMOVED ELECTRICAL POWER TO NON-ESSENTIAL CABIN ITEMS (WHICH SMOKE THEN DISSIPATED). CHECKLIST FOLLOWED, UNSCHEDULED LANDING PERFORMED AND LANDED WITHOUT INCIDENT. MAINTENANCE EVALUATION FOUND FAULTY ENVIRONMENTAL CONTROL SYSTEM (ECS) PACK DURING APU RUNS. LEFT AIR CYCLE MACHINE WAS SEIZED CAUSING LEFT PACK TEMP HIGH CAS MESSAGE. LEFT ECS PACK PLACED ON MEL, AIRCRAFT REPOSITIONED TO MAINTENANCE SERVICE CENTER WHERE LEFT ECS PACK IS BEING REPLACED AND CLEARING MEL.