Starter motor

1. Main Housing (yoke)
2. Overrunning clutch
3. Armature
4. Field coils
5. Brushes
6. Solenoid

When the start switch is closed, the coils  in the solenoid are

energized, creating a magnetic field. The field pulls the plunger

inward, which causes the shift lever to push the drive assembly

into mesh with the ring gear on the engine flywheel. Once the

pinion is in mesh, the plunger pushes the contacts closed, and

closes the circuit between the battery and the motor.

The current passes through the field coil then through the brushes to the armature commutator. The current forms interacting

magnetic fields around the field coil pole pieces and the armature lamination pack, and causes the armature to turn. The armature turns the drive pinion, which turns the ring gear, cranking the

engine.

When the engine starts, the start switch is released. This causes

the magnetic field in the solenoid to collapse, and a return

spring forces the plunger outwards. This opens the contacts, and

then disengages the drive assembly from the ring gear.