What was found
PHX - AIR TURN BACK - FLT 0014 - AFTER TAKEOFF FROM PHX EN ROUTE TO JFK, MAXIMUM OPERATING LIMIT SPEED (VMO/MMO) ALERT WOULD SOUND AND REMAIN ON IN ACCESS OF 240 KNOTS, AND EXTINGUISH AT 230 KNOTS. NR 1 AIR DATA INERTIAL REFERENCE UNIT (ADIRU) DISPLAYED ERRONEOUS VMO INDICATION ON PRIMARY FLIGHT DISPLAY. TURNED BACK TO PHX. MAINTENANCE REMOVED AND REPLACED NR 1 ADIRU IAW AWA T/C R2A34304, OPERATIONAL CHECK IS GOOD. VENDOR BENCH TEST OF ADIRU S/N 3419 CONFIRMED FAILURE OF UNIT. REPLACED AIR DATA RESOLVER PCB ASSEMBLY, CALIBRATED AND TESTED IAW CMM TP1011 REV 22.