The starter motor engages, but it cannot turn the engine. If nothing happens when you close the starter switch, there is a failure in the electrical system. The failure could be an open circuit caused by broken connections or burned out components. Test the circuit continuity to make sure the relay closes and the battery provides sufficient voltage and current to the starter circuit. If the circuit is complete, there may be resistance through faulty battery connections.
Considerable current is needed to operate the solenoid and starter motor.
If the starter runs without engaging, it will produce a distinctive hum or whine. The lack of engagement is usually caused by dirt or corrosion, which keeps the solenoid or Bendix gears from operating properly. If the starter motor engages the flywheel ring gear but is not able to turn the engine or cannot turn it quickly enough to obtain starting speed, the cause may be lack of battery power or, more likely, a mechanical problem. If the engine can be barred over, there is excessive friction in the meshing of the starter pinion and the ring gear. Either the teeth are burred, or the starter pinion is out of alignment.