What was found
NR 3 ENGINE N1 GAUGE FAILED ON DEPARTURE ROLL ALONG WITH AUTOTHROTTLE OFF - "VIA 1 FAIL" AND "VIA 2 FAIL" LEVEL 1 ALERTS. VIA 1 FAULT HISTORY SHOWED NR 3 ENGINE N1 DISAGREE AND VIA 2 SHOWED NR 3 ENGINE N1 INTERMITTANT. FCC'S 1 & 2 LIST NR 3 ENG TACH FAULT ALSO. RAN NR 3 ENGINE IAW JRH CHAP 6 AND N1 BLANK THEN STARTED TO WORK. SWAPPED VIA 1 AND VIA 3 FOR TROUBLESHOOTING. OPS CHECKED GOOD ON RTS IAW AMM 31-43-01-5. DURING ENGINE RUN, SHOOK DOWN WIRING FRON THE N1 SENSOR TO TERMINAL BOARD S3-800, NO FLUXUATIONS. SWAPPED N1 TACH FROM NR 1 ENG TO NR3 ENG AND OP CHECKED GOOD ON ENGINE RUN. RETURNED TACH GENERATORS TO ORIGINAL POSITIONS. REF WDM 77-11-01. R & R NR 3 ENGINE N1 FAN SPEED SENSOR T2-69 & OP CHECKED GOOD ON ENGINE RUN IAW JRH CHAP 6. REF AMM 77-11-01-2-0. FAILED OPS CHECK & REPLACED WITH SERVICABLE UNIT.