Ignition Switch:
In the image below, the rear side of an ignition switch is shown. The ignition switch cylinder is located at the front, and it moves the electrical part of the ignition switch with the control element at the back of the cylinder.
Because the electrical part (also called the contact block) moves relative to the housing, various circuits are possible. The position of the key then indicates which contacts in the electrical part are connected to each other (e.g., when the constant positive wire (terminal 30) is connected to the starter motor positive wire (terminal 50b), the engine starts. If the key is released, terminal 15 (contact) remains engaged. When the key is fully turned to the left again (in the off position), all contacts are disconnected, and the engine and contact are turned off).
When the key is removed from the ignition and the steering wheel is turned, the steering lock engages. A metal pin moves from the ignition switch into a notch in the steering column. To unlock the steering lock, the ignition key must be turned in the ignition switch while making a slight steering movement.
The operation of the lock cylinder is described on the page lock cylinder.
Nowadays, ignition switches are increasingly used without lock cylinders and operate fully electronically using a plastic chip or key.
Electrical Part of Ignition Switch:
Behind the lock cylinder is the electrical part, also known as the “contact block.” This essentially ensures that the movement made with the key is converted into the correct voltages on the appropriate wires; thus selecting the accessory position and starting.
The electrical part is often secured with a small screw, making it relatively easy to remove from the ignition switch housing if a repair is needed.
In the image on the right, the front of the contact block is shown. In the center is a yellow, round, rotatable part with a horizontal slot. When the key is inserted into the ignition switch, the end of the key will be slid into this slot. When the correct key is inserted into the cylinder lock, it can rotate thanks to the wafers in the ignition switch. What you are actually doing is rotating the yellow part in the contact block. Internally, the switch contacts (on which the plug is attached at the rear) are interconnected. This is further explained in the diagram in the next paragraph.
It can happen that the car can no longer be started. In that case, the contact block may be internally defective. To check that, the control voltage on the small wire to the starter motor can be checked.
In older cars, it was sometimes possible to remove the plug from the contact block and then connect certain contacts using your own wire. Or, with a screwdriver, turn the yellow part as you see in the image above. This was often how car thefts occurred.
To prevent this, every car now has an immobilizer built-in. The key number of the transponder in the key is compared with the stored values. If this does not match, the engine will immediately stall again after starting. It is therefore no longer possible to start and run the car by simply bridging wires or only operating the contact block.
Ignition Switch Diagram with Load Reduction Relay:
The diagram below is of the ignition switch. Here, the component codes such as CS of the ignition switch and RX of the load reduction relay can be seen. Also, the terminal codings 15, 30, 31, 50, and 75 are shown here.
At the moment the ignition switch is set to position 1, terminal 15 is connected to terminal 30. This powers the contact, and the consumers in, for example, the interior are supplied with power. During starting, terminal 50(b) is powered, causing the starter motor to turn. When the ignition switch is turned to position 0 but the key is not yet removed from the cylinder lock, power will remain on terminal 75. Consumers like the radio are connected to terminal 75. In the car of this scenario, the radio will, therefore, remain on while the contact is off, and the key has not yet been removed from the cylinder lock.
The function of the load reduction relay contact X (in English, called “load reduction relay”) is to switch off consumers that require a lot of voltage and current to operate, such as the blower (interior fan), radio, low beam, rear window defogger, seat heating, etc., at the moment the engine is started. During starting, switch X in the ignition switch opens (as the situation currently is in the diagram). The voltage to the electromagnet (between terminal 85 and 86 of the control current side of the relay) is interrupted. Consequently, no main current will flow from terminal 30 to 87, which temporarily turns off the consumers that are connected in another diagram with wire number 102. At that point, more current is available for the starter motor than when other consumers use a lot of current. Therefore, this component is called a load reduction relay.
When the ignition key is turned back after the engine has started, switch X in the ignition switch will close again. The relay RX is energized again, so the consumers are powered with voltage and current once more.