The helicopter collided with a telephone pole during an autorotation forced landing after an engine malfunction. The aircraft was cruising at 500 feet when there was an audible decrease in main rotor and engine RPM. The rotor RPM was between 80-85 percent when the instructor pilot took control from the student. During an autorotative landing, the instructor maneuvered to avoid wires while landing on a downhill street, away from automobile traffic. The autorotation was almost complete when the rotor struck a telephone pole and rolled the aircraft on its side. The low RPM warning horn and caution light did not work as programmed. The battery terminals were found corroded and the battery voltage was found to be low. Electrical wire #19 was found disconnected from the alternator switch where the wire had been soldered to the switch. An examination revealed that the solder adhesion was good, but the wire had not been passed through the terminal post and wrapped before it was soldered. No other preimpact malfunctions or failures were found.