As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) WHILE CARRYING OUT ENGINE RUN-UP TO DETECT SOURCE OF TRANSMISSION OIL LEAK, A FLUCTUATION OF ROTOR RPM WAS NOTICED FOLLOWED BY A TOTAL LOSS OF ROTOR RPM. IT WAS LATER DISCOVERED THAT THE TACH GEN CABLE HAD FRACTURED CAUSING THIS LOSS OF INDICATION OF ROTOR RPM.