The ignition switch has more functions than simply starting the vehicle. The common points on an ignition switch include:
The key can only be removed from the "Lock" position. When this occurs, all non-essential electrical circuits are disabled, and the steering column lock is enabled. If fitted, the engine immobilizer and theft deterrent systems are normally activated at this time.
Many modern vehicles also include an "Off" ignition point. Turning the key from "Lock" to the "Off" position unlocks the steering column, but it does not enable any electrical systems or disable the engine immobilizer or theft deterrent system.
The "Accessories" position allows power to be supplied to the vehicle entertainment system, blower fan and in some vehicles the electric windows and sunroof. This is mainly for passenger convenience but prolonged use of any of these without the motor running will drain the battery.
When the switch is turned to the "On" position all warning lamps on the instrument panel should illuminate. This is to test the operation of the lamps. On vehicles that are not fitted with engine immobilizers, this position also activates the charging and ignition system required for engine starting.
Vehicles that are fitted with engine immobilizers do not normally activate these systems until the key is turned to the "Start" position.
The "Start" position activates the starter motor solenoid, which enables the engine to crank and start the engine.
Vehicles without engine immobilizers will be able to start immediately, as the required electrical systems will have already been activated when the key was turned to the "On" position.
In vehicles fitted with engine immobilizers a number of the essential electrical systems are not normally enabled until the key is turned to the "Start" position. When this occurs communication between various control units determines whether to allow the engine to start or not. This is then achieved by deactivating the immobilizer, and activating the ignition, fuel, and charging systems. There may be a slight delay while this communication is occurring.
The Key itself can consist of two parts. The first is a mechanical tine, that works in the key barrel and turns to unlock the steering column and switch through the various stages of "Off", "Accessories", "On" and "Start". On later models, there can be a second electronic key that uses a randomly selected 'rolling code' that is communicated with the engine immobilizer and security system. If the correct code is sent, the vehicle will start, if not the vehicle will not respond.
In many automatic transmission vehicles, the ignition switch also has a transmission shift interlock device connected to it. In these vehicles the gear selector must moved into the PARK location before the key can be removed from the lock in the OFF position. In the same way the transmission shift lever cannot be moved into gear until the engine has started.