A starter motor also  know as self starter, self  is an electric motor, pneumatic motor, hydraulic motor, an internal-combustion engine in case of very large engines or other device used for rotating an internal-combustion engine so as to initiate the engine’s operation under its own power.

Function of a Starter Motor

The function of the starter motor is to start up the combustion engine. An electric motor forms the basis of the starter motor. When the starter switch is turned on, the starter relay turns on the electric motor. This motor drives the starter gear ring via the pinion gear.

If testing the starter circuit  indicates a fault in the starter motor, disconnect the battery and remove the motor from the car – complete with its attached solenoid if it is a pre-engaged type of motor.c

To trace the fault and carry out repairs, you will nearly always have to strip down the motor , especially if the fault is in the commutator or brushgear.

dAn inertia type starter motor

An inertia-type starter motor is fitted with a Bendix gear on the shaft from the armature. When the started switch is turned on, the motor rotates rapidly, the shaft turns, and the Bendix gear screws the pinion along a thread until it engages with they flywheel ring gear on the engine.

A pre-engaged starter motor

A pre-engaged type of starter motor incorporates a solenoid. When the started switch is turned on, the solenoid activates a lever which first moves the pinion along the shaft to engage with the flywheel ring gear and only then operates the switch that starts the motor turning.

However, the Bendix gear of an inertia-type starter can be repaired  without stripping; removing the motor from the car gives sufficient access.

Some faults, such as a badly worn commutator, are beyond repair and you have to buy a new or exchange motor. However, you can renew worn bearings

Damage to the electrical windings is usually too difficult to deal with at home. An auto-electrician can cure a minor short circuit in the fieldcoils, but anything else calls for fitting a new or exchange motor.

A single fault in the motor is probably worth repairing, but if you find more than one fault check the cost of all the spares you need. You may find it cheaper – and quicker – to replace the motor with an exchange unit.

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Source:Howacarworks.com