What was found
CREW REPORTED, BTB DUE TO SMOKE IN FLIGHT DECK. RAN ENG 1 AT A TIME, ISOLATED PROBLEM TO LT PACK AND DEFERRED PACK PER MEL. REMOVED AND REPLACED LT AIR CYCLE MACHINE PER MM 21-51-12 PG 401. OPS CHECK OFF APU AND ALL ENGS DURING TAXI, NO DEFECTS NOTED.