[Targalid]

The yellow wire provides the power for the starter solenoid. It is the switch that turns on the starter. When you turn the key to the start position the solenoid should be energized and close the circuit to the starter motor. If the voltage is too low the solenoid will not work. When you turn the key and there is no sound at all, the solenoid is not operating. If you hear a click, the solenoid has been activated but the starter motor has not engaged. I would think that at 9 volts the solenoid would not activate for its full travel and operate the starter motor. Low voltage is most often due to a bad ground. There is the connection of the battery negative ground cable to the car body and the car body to the transmission ground strap...and, don't forget, the battery cable connections at the battery. These connections should be sanded until they are shiny, then reassembled with protective grease to prevent oxidation.