he ignition switch generally has four positions: off, accessories, on, and start. Some cars have two off positions, off and lock; one turns off the car, and the other allows the key to be removed from the ignition. When the key is turned to the accessories position, certain accessories, such as the radio, are powered; however, accessories that use too much battery power, such as window motors, remain off in order to prevent the car's battery from being drained. The accessories position uses the least amount of battery power when the engine is not running, which is why drive-in movie theaters recommend that the car be left in theaccessories mode during the movie.
The on position turns on all of the car's systems, including systems such as the fuel pump, because this is the position the ignition switch remains in while the car's engine is running. The start position is spring loaded so that the ignition switch will not remain there when the key is released. When the key is inserted into the ignition switch lock cylinder and turned to the start position, the starter engages; when the key is released, it returns to the on position, cutting power to the starter. This is because the engine runs at speeds that the starter cannot match, meaning that the starter gear must be retracted once the engine is running on its own.
Either the ignition switch or the lock cylinder may fail in a car, but both circumstances have very different symptoms. When the ignition switch fails, generally the electrical wiring or the plastic housing develops problems. The car may not turn on and/or start when this happens. Also, the spring-loaded start position could malfunction, in which case the starter will not engage unless the key is manually turned back to the on position.
When the lock cylinder malfunctions, however, the operation of the key itself will become problematic. If the tumblers become stripped, the lock cylinder may be able to turn with any key, or you may be able to remove the key when the car is on. If the tumblers begin to shift, the lock cylinder may not turn. Sometimes the key can be wiggled until the lock cylinder turns, but it is important to remember that this is only a temporary fix.
Replacing an ignition switch can be tricky business, particularly in newer cars, because of the anti-theft devices used in cars. Once the ignition switch is separated from the back of the lock cylinder, the car can be started with a screwdriver, making it vital that this switch be difficult to get to. It is important to consult a shop manual before attempting this kind of repair, as the anti-theft devices may require special tools; attempting to remove an ignition switch without the proper tools can render the car inoperable.
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