What was found
(CAN) A DEFECT WAS REPORTED OF A POSSIBLE FAULTY TACH WHICH WAS CAUSING AN OVERSPEED INDICATION, TACH CABLE WAS DISCONNECTED AT BOTH ENDS TO CHECK CABLE SMOOTHNESS AND NO FAULT FOUND. ACFT GROUND RUN SEVERAL TIMES AND AT SEVERAL RPM SETTINGS WITH NO FAULT FOUND, ALSO TACH ACCURACY CHECKED WITHIN LIMITS IAW CAR625 APPENDIX "C". ACFT RETURNED TO SERVICE. 17.3 HRS LATER, THE ACFT WAS AGAIN SNAGGED, THIS TIME THE PILOT REPORTED THAT WHEN SET UP IN CRUISE, AND THE TACH INDICATING CLOSE TO MAX RPM, THE NEEDLE WOULD JUMP INTO THE 3300 -3400 RPM RANGE, BUT THE ENGINE WAS NOT FOLLOWING THE RPM INCREASE. NEW TACH WAS INSTALLED.