What was found
APU RUNING AT GATE, STARTED TO PUMP AIR CONDITIONING SYSTEM SMOKE INTO CABIN, LEFT PACK DUCT TEMP SHOT UP TO 99 AND WAS UNABLE TO CONTROL TEMP IN BOTH AUTO AND MANUAL MODES. RIGHT PACK WORKS FINE AND SMOKE CLEARED. NO EICAS MESSAGE DISPLAYED. REMOVED AND REPLACED THE LEFT ACM IAW AMM 21-51-08. PERFORMED AN OPS TEST IAW AMM 21-51-08. OPS CHECKED GOOD, NO OTHER DEFECTS NOTED.