How an Engine Starter Works

An engine starter is constructed as a high torque, high amperage 12 volt motor fitted with an electrical solenoid and Bendix gear which is used to rotate an engine between 85 and 150 rpm's to begin the combustion process (start the engine). A starter motor's components include a main outer housing which contains positive and negative pole magnets, an armature that contains windings, and a brush set which is used to contact the armature and transfer electrical current and a solenoid which acts as the high amperage switch. Most starter motors are mounted underneath the engine, either on the left or right side, some are mounted under the intake manifold.

The electrical device that controls the starter motor is the solenoid. In the image below you can see how the starter and solenoid is wired to the battery and ignition switch. The top lug is where the positive battery cable is connected, the trigger terminal is sent from the ignition switch, the lower larger terminal is connected to the starter motor when the solenoid is activated.

Here is an image of the starter armature which is the internal part of the starter that spins to create the starter's purpose. The arrows show the roller bearings on either side to support the unit while inside the starter housing.

A brush set rides against the armature using spring pressure to produce a connection for the electrical power to travel through, this is what makes the armature spin.

The bendix gear is designed to extend from the starter motor when in use, and retract back into the housing once the engine starts. This small gear meshes with engine via the flywheel (manual transmission) or flex plate (automatic transmission) and is designed with a one way clutch which enables the starter motor to "freewheel" as the engine starts while forcing the gear back into the starter motor housing and away from the flywheel.

Questions?

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