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Starting and Charging SystemStarting and Charging System Components > Starter Motor 

Function: The starter motor cranks the engine when engaged with the flywheel. The solenoid uses electromagnetic force to power the starter motor and flywheel.

Signs of Wear: Sporadic engine cranking, poor starting qualities.

The starter motor is comprised of a starter gear with an overrun clutch, an armature, field coils, brush holder with brushes, and a starter solenoid.

When the ignition key is turned, current flows through the starter solenoid windings, pulling the solenoid plunger and one end of the starter drive gear lever inwards. This forces the starter gear to move outwards and make contact with the flywheel of the engine.

When the solenoid plunger moves, it also closes a switch which allows current to flow through the field coils and through the brushes to the armature. The field coils create a magnetic field that cause the armature, an electromagnet, to rotate. The armature is connected to gear which drives the flywheel.

When the starter drive gear becomes worn, the plunger is unable to pull the lever, resulting in a no start condition as the pinion gear is unable to come in contact with the flywheel. When this failure occurs, the "whirring" noise from the starter can be heard, but the engine does not turn over. 

Another common failure of the starter is worn starter solenoid contacts. With repeated use, the contacts can wear away creating high resistance for the current passing through the solenoid. If the resistance is great enough, the current flowing through the solenoid will be insufficient to drive the starter motor and, as a result, the engine will not start.

The starter motor drives the engine until it can run on its own power. 

A worn starter drive gear can stop the starter gear from running the engine. 

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