As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
90 DEGREE TACH DRIVE ADAPTER PUT ON AIRCRAFT. WHEN ADAPTER FAILS, THERE IS NO RPM INDICATION TO TACH. PROBLEM APPARENTLY VIBRATION INDUCED. SOME PROVISION SHOULD BE MADE TO REINFORCE UNIT OR RELOCATE IT TO AN AREA NOT AS PRONE TO VIBRATIONS.