What was found
SHORTLY AFTER TAKEOFF, THE LT PACK VALVE WENT TO FULL HOT IN AUTO, THE CONTROL WAS SWITCHED TO MANUAL AND MODULATED TO COLD. THE PACK VALVE DID NOT RESPOND AND THE CABIN AND COCKPIT BEGAN TO FILL WITH SMOKE. THE PACK AUTO SHUTDOWN, BUT THE SMOKE CONTINUED. THE AIRCRAFT WAS RETURNED TO MDW WHERE IT LANDED WITHOUT INCIDENT. MAINTENANCE FOUND THE LT PACK TEMP CONTROLLER AT FAULT. THE TEMP CONTROLLER WAS REMOVED AND REPLACED, OPERATIONAL CHECKS WERE GOOD WITH NO SMOKE NOTED.