The starter motor uses electrical power supplied by the battery to rotate the starter pinion, which is in mesh with the flywheel ring gear, at high speeds allowing the engine to crank over and start. The starter pinion is a small cog and the flywheel is massive in comparison and it is the ratio between two gears and high rotational speed of the starter gear that provide the necessary torque needed to turn the engine over. During cranking very high current loads are place on the battery so it is crucial the battery is in good condition for the starter to function correctly. Furthermore, an engine that is reluctant to start will place a massive load on the starter motor and lead to premature failure.
The pre-engaged starter motor is commonly used in modern vehicles. When cranking is required the starter circuit feeds the starter solenoid which causes the solenoid plunger to move back and push the engagement leaver forward allowing the starter pinion to mesh with the flywheel ring gear. Further movement of the plunger allows the main contacts within the solenoid to close and full battery power is transmitted to the starter motor armature and field windings. The current that flows through the windings cause a distortion in the magnetic flux and it this distortion that generates the torque that pushes the armature away from the field windings magnetic pole. The motor will continue to turn while the solenoid is energised.
Non start -It the starter fails to operate the most common cause is a flat battery, if the battery proves to be in good condition then the electrical feeds to the starter motor and solenoid must be checked for current feed and conductivity. If these connections and circuit are good then the fault will be with to starter motor itself.
Noisy Starter Motor - On occasion starter motors can be noisy in operation. If a starter is noisy, then the only solution is to remove the starter for inspection. While the starter is out it is essential the fly wheel ring gear teeth are checked for wear, damage and missing teeth. If there is serious wear and damage then the fly wheel ring gear will need to be replaced. On occasion a starter motor can be cleaned, lubricated and refitted which can resolve minor noises, but usually unacceptable noise will lead to the starter motor being replaced.