What was found
ATB, IN FLIGHT ALT DISAGREE? EICAS MESSAGE DISPLAYED. 1ST OFFICER AIR DATA DETERMINED TO BE AT FAULT, ALT AIR DATA SELECTED PERFORMED T/S ON PITOT & STATIC SYS AS PREVIOUS TESTS SHOWED AN ALTITUDE DIFFRENCE OF 100 FEET BETWEEN CAPT AND F/O SIDE. CHECKED ALL CONNECTIONS OF ALL PITOT - STATIC DEVICES DOWN TO ADCS. NO FAULTS OR LEAKS NOTED. TO ISOLATE TOO MANY POSSIBLE FAULTS SOURCES, INSTALLED TEST SET UP ON CPT & F/O SIDE SEPERATELY. PERFORMED TEST IAW AMM: 34-11-00-862-045, REV: 140. DIFFERENCE BETWEEN CPT AND F/O IS AT MAXIMUM TO THE TESTSET IS MAX 30 FT AND DIFFRENCE OF CAPTAIN TO F/O SIDE IS MAX 20 FT. LEAK TEST OF SYSTEM SATIS & OPS TEST OF FLIGHT DECK ALTITUDE INDICATION WITHIN LIMITS. A/C OK FOR SERVICE.