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A starter motor is an electrical device used to start an internal combustion engine. Typically a very low-geared device, this motor is able to crank over the much larger engine by virtue of its extreme gear reduction. The starter motor is a part of a starting system consisting of the starter, a starter solenoid and the battery. As the ignition switch is turned, it sends an electrical charge to the starter solenoid. This, in turn, sends the charge to the motor that cranks the engine until it starts. Once the engine fires to life, the starter motor clicks off and disengages the starter ring.
The typical starter motor is composed of heavy copper wire wound around an armature. This device is placed inside of a heavy steel or aluminum case equipped with electrical brushes that contact the armature and pass the electrical charge to the heavy wire. As the electricity flows through the wire, it causes the armature to spin. A small gear is attached to the output shaft of the armature and moved in and out by means of a Bendix. The Bendix engages the small gear with the starter ring, which turns the engine as the armature turns.
Typical problem areas with a starter motor are the Bendix and the brushes. Often, the brushes become burned and fail to transfer the electrical charge to the armature. The Bendix can also become sticky and fail to engage the starter gears. A bench test, which is accomplished by running an electrical charge through the starter while it is out of the engine bay, will usually determine the problem. A starter that fails to run when a charge is placed to it typically implies a brush defect. A starter that spins but does not send the starter gear outward is often suffering from a broken or defective Bendix.
When replacing a starter motor, it is almost always necessary to send the old unit back to the parts store as a core, as the old starters are rebuilt and sold as re-manufactured starter units. These can often cost only a fraction of the price of a new starter, and most re-manufactured starters come with a warranty. In the case of a diesel engine starter motor, the starter can often cost as much as several gasoline engine starter motors. Before replacing a motor, it is important to note that many starter problems are actually electrical problems caused by faulty wiring and dirty wiring connections. Often, cleaning the wires and making better connections can cure a faulty starter or difficult starting traits.
Starter motors are incredibly durable devices, but they can be a real problem when they fail. If one does break, don't let anyone tell you it can be repaired -- get a new one installed instead because a bad motor is very difficult to smack back into working shape.