The typical car ignition switch is a complex rotary device with many brass terminals on the back. There are four main key positions and most switches also have an optional lock position. The majority of cars, trucks, boats, and small aircraft use this type of switch to start their engine and control the power to their electrical accessories. It is also called the universal ignition switch but not all cars use it. It has been replaced in some cars by a simple push button, along with a wireless fob that secures the ignition system as well as the doors and alarm.
The four main key positions on the car ignition switch are start, on, accessories, and off. The start position provides power to the starter motor solenoid, ignition coil, and accessories. It is spring-loaded so that the key returns to the on position when released. The on position provides power to the ignition coil and the accessories, and of course, the accessories position only provides power to the accessories. The off position disconnects power from all systems, but the clock and radio usually have direct connections to the battery to preserve their settings. Most switches also have a lock position, which is just another off position. The key can only be removed when it's in the lock position, but some switches don't have one so their keys are removed when they are in the off position.
The car ignition switch is usually secured to the plastic body of the steering wheel column by a nut and washer just behind the front face. To remove the switch, the column body usually has to be removed along with the steering wheel. The switch is comprised of a case, terminals, rotary contacts, and a key lock barrel. The barrel sits in front of the contacts and can be removed to gain access to them. This is the main problem with the universal ignition switch because it's fairly easy to start the car without the key. It's also one of the main reasons behind the push to replace them with push buttons and wireless fobs.
On the back of the car ignition switch are several terminals, each connected to a different color wire. There will be a terminal for the positive power wire, accessories wire, starter solenoid wire, and ignition coil wire. Each terminal usually has an abbreviation next to it showing which wire it should be connected to. Wire colors vary between different car manufacturers, so it is essential to refer to the service manual. If no manual is available, write down the color and the terminal connections before removing the wires.
The description of a car ignition switch often has lists the number of terminals and poles it has. This can be confusing since terminals and poles are not exactly the same thing. Terminals are the contacts on the back of the switch, while poles are the internal contact points. For example, the basic push button is single pole switch with two terminals, while the universal ignition switch can have many poles and terminals.
The common car ignition switch is fast disappearing from the motoring world, replaced by push buttons and wireless fobs. What was once an optional extra for luxury cars is now a standard feature in many new cars, as car buyers and insurance companies demand more protection from car theft. While a button may not seem like a more secure option than a key lock, it should be remembered that the wireless fob and immobilizer provide the security, and the ignition system will not work unless it detects the fob.