It happens thousands of times over the life of a car, and seems like a pretty straightforward event--the ignition key is turned and the car starts. But what actually happens can involve up to four different circuits, and the process employs the principles of electromagnetism--all for the simple act of starting a car.
Ignition Switch and Starter Solenoid
The ignition switch, usually located on the steering column, has a "hot" wire (or a direct connection to the battery), which routes current to the starter solenoid. The starter solenoid can be located inside or adjacent to the starter motor. Sometimes there is a starter relay that opens this circuit and is activated by the ignition switch. A relay is a way that one circuit may be controlled by the current from a separate circuit. The starter solenoid is essentially a large relay itself. When the ignition switch is open, a small amount of current is delivered to the starter solenoid, which then opens the circuit that operates the starter motor. This circuit uses a tremendous amount of current. This function of the starter solenoid is accomplished by means of a coil and plunger within the solenoid. When current flows through the coil, it creates a magnetic field, which pulls the plunger down. A metal disc attached to the plunger then makes contact between two terminals which connect the battery directly to the starter. In addition, most starter solenoids will also cause the small gear on the starter motor to mesh with the teeth in the engine flywheel, thus allowing the starter to crank the engine.
Problems with the ignition switch can result in a no-start situation (starter motor fails to turn) or the starter motor may fail to stop when the key is released from the start position. This may be because the contacts in the switch have been worn or burnt. To test for this, use a test light in contact with the solenoid start terminal. A good ignition switch will cause the test light to light up when the key is turned to "Start." If the test light does not light up, there is either an open wire in the circuit or the switch is defective. Problems with the starter solenoid include failure of the starter to crank (sometimes this is accompanied with a clicking noise) and failure of the solenoid to disengage from the flywheel. Slow cranking may indicate that the disc that opens the circuit between the battery and the starter has become burnt and pitted, thus reducing the amount of current that reaches the starter.