By harnessing the power of the battery, the starter motor powers the engine to "turn over" and start when the driver turns the ignition switch. The starter motor also enables the driver to restart the engine once it has been switched off.
When the starter motor causes the engine to revolve at about 350–450 rpm, the engine develops enough vacuum to enable combustion to occur, which is what causes the engine to start. Once the engine starts, the driver releases the ignition key from the start position, allowing the starter to disengage.
The starter motor is located where the engine and transmission join together.
When replacing the starter, the technician first disconnects the battery and then removes the electrical wiring and hardware securing the starter. The defective starter is removed and a new one is installed. Replacement can vary from easy and straightforward to time-consuming and difficult.
Mechanics' Corner: More Technical Detail
The starter is a powerful, electric DC motor that rotates the crankshaft 350–450 rpm to begin the starting process of the engine. The starter motor has a special gear that engages the flywheel, which is connected to the crankshaft (frequently located between the transmission and the engine). Usually the engine needs to rotate 720 degrees (two full rotations) in order for the computer sensors to register the correct crankshaft and camshaft positions. This enables the delivery of fuel and spark at the proper time for the engine to "fire" or begin to run.