What was found
ENGINE FAIL CAUTION LIGHT ILLUMINATED DURING TAKE OFF DUE TO NR 1 ENGINE N1 TACH GIVING ERRATIC INDICATIONS. (REJECTED TAKE OFF AT 70 KNOTS.) REMOVED AND REPLACED NR 1 ENGINE FAN SPEED SENSOR , RAN ENGINE NR 1 UP TO 85 PERCENT NR 2, FOUND NO ERRATIC FLUCTUATION ON N2 INDICATOR, BOTH NR 1 AND NR 2 STABILIZED ON INDICATORS. OPS CHECK NORMAL.