On July 19, 1996, at 1830 hours Pacific daylight time, a Hiller UH-12E, N124HA, collided with trees after the pilot entered autorotation after reacting to an erroneous rotor/engine tachometer reading near Healdsburg, California. The helicopter was substantially damaged. The certificated private pilot and passenger were not injured. The helicopter was being operated as a personal flight by the pilot/owner when the accident occurred. The flight originated from a private ranch at 1825, and was destined for the Healdsburg Municipal Airport. Visual meteorological conditions prevailed at the time. The pilot said he was flying at 1,400 feet msl and 70 knots when the needles on the rotor/engine tachometer instrument split. The engine decelerated to idle. The pilot entered an autorotation over unsuitable steep terrain and struck trees during the emergency descent. According to the Federal Aviation Administration (FAA), the helicopter had a previous rotor-tach generator failure. The failure was attributed to a loose connection and was allegedly repaired by the previous owner. The rotor-tach generator electrical connection was examined by an airframe and powerplant mechanic. The connection produced intermittent and inaccurate instrument readings when the wire bundle was moved or flexed. The mechanic attributed the erroneous instrument reading to the poor condition of potting material within the electrical connector. There was no evidence of mechanical failure or malfunction found with the helicopter's powerplant after the accident. The pilot indicated on his accident report his total aeronautical experience consisted of 485 hours, of which 157 were accrued in helicopters.