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HomeTips & GuidesIgnition Switch Wiring Harness: How does it benefit your ignition system?

Ignition Switch Wiring Harness: How does it benefit your ignition system?

About Ignition Switch Wiring Harness: As soon as you sit in your car, you turn your key over, you hear the engine rumble, and you are all set to go. You always expect your car to work like that when you take hold of the steering.

If any fault occurs in your ignition switch, the relay won’t be able to engage with the starter, and your car will stop after some time.

How does this ignition switch work?

You can get the answer if you know the wiring of this starter switch.

Table of Contents

What is an ignition switch?

You can know this switch by its other names, like start or starter switch, indicating its purpose.

You can find this electronically controlled switch on the dashboard or inside the steering column. The switch allows the electric signals to reach the vehicle’s computer so that you can control the ignition system by turning the key over.

The ignition switch works as a junction between your battery and the engine. As long as the ignition switch maintains this connection, it will keep the engine alive, allowing your car to move continuously. 

Similarly, the switch also acts as a mediator between the car’s gadgets and the battery. As soon as the ignition switch sets the connection between the two, all your car’s gadgets, be it power windows, AC, music system, etc., start working.

So, why do we need a junction? Why don’t we connect them both directly?

This is to maintain the charge of the car’s battery. If there was no ignition switch and the engine was not working, even the smallest electronic system in the vehicle would drain all the batteries.

Further, without an ignition switch, you won’t have any control of starting the engine and the electronics. 

Hand-holding car key

Hand-holding car key

Universal ignition switch wiring diagram:

The contacts inside the switch are arranged in a particular sequence. So, as you rotate the button, it leads to different connections mentioned below:

First Position: When you only insert the key in the key slot.

The first Position, or Position 0, is the position that lets you slot your car keys safely into place. You may also know it by the “LOCK” or “OFF” Position

When you insert your car key into position 0, there is no change in the electronics or the car’s ignition system. All the systems start when you move the key in the clockwise direction.

Further, this is the only position where you can remove the key after driving.

In an isolated switch, there are 2  M terminals, which act as its poles. These two M terminals connect only in the OFF/LOCK position. 

The terminals are called “M” because they connect the magneto kill or ignition kill circuits to the ground.

 Here, the lower M point connects to the ground (black) or the battery’s negative terminal, while the upper M connects to the magneto kill circuit through a black wire (with yellow stripes).

Now, as you know, the engine carries a high voltage of up to 300 volts; the circuit connects to the upper M terminal, isolating it from additional connections.

 You can also use the “M” terminal to connect the second switch, operating through a safety lanyard. On starting the second switch, it also connects both the M terminals. 

As a result, the engine kills despite the button being in OFF marking. 

Second Position: Turning on your car’s accessories.

You can call it Position I or ACC/accessory position. As the name clears, when you twist your car key to this position, you connect your car’s electrical accessories to the battery.

 In this situation, the engine is still in the OFF position, but you can enjoy the car’s gadgets like a music system, power windows, radio, heated seats, etc.

However, please don’t leave your car in this position, as it will gradually drain your car’s battery.

In this position, the battery’s B terminal (12 volts) connects to the I/A terminal to supply the needed voltage to the car’s accessory loads. In this state, your vehicle comes in a RUN or ON position

Third position: Preparing the car’s ignition system to start.

This is Position II or Ignition position. With the name, you can conclude you turn on all electric systems of your car. 

In this position, you turn on the fuel pump to circulate fuel into the engine. Your key becomes sealed at this Position, and you cannot remove it.

In any of the “ON” and “START” (fourth) Positions, the switch connects the B terminals of the battery to the switch’s C terminals to supply power to the engine circuit. 

Also, the battery’s B terminal connects to the S and A terminals.

Fourth Position: when you crank the engine,

This is Position III or the “START” position. You twist and hold the car key to crank the engine for this. 

In this position, the car’s starter plays a significant role, giving huge voltage to combust the fuel-air mixture inside the engine. 

Once the engine comes into running Position, the switch jumps back into the ON position and turns off the starter.

In this position, the B terminals connect to the A and the S terminal, giving voltage from the battery to the engine, engine starter solenoid, and accessory loads.

If you observe the outer portion of the ignition lock cylinder, where you insert your key, you can see some letters. These letters indicate different states of the ignition switch.

Letters on the ignition lock cylinder

Letters on the ignition lock cylinder

BBATTERYRed wire with Violet stripesBattery terminal positive through fuse
I or AI means Ignition or A means accessoryViolet wireAccessory loads; engine ignition
SMOTORYellow wire with Red stripesStarter solenoid
CCHOKEViolet wire with White stripesprimer solenoid 
M (raised)MAGNETO KILLBlack wire with Yellow stripesEngine ignition/magneto kill circuit
MMAGNETO KILLBlack wireEngine chassis or battery’s negative terminal

Caption: historical legend terminal identifiers on the switch vs. standardized color coding on conductors 

Symptoms to indicate a fault in the ignition switch:

Here are some signs that indicate that your ignition switch has some defect.

Your car doesn’t start:

When you twist your key to the IGNITION position, two things happen. First, the ignition turns on, and the switch sends the signal to turn on the motor.

All this happens due to the electrical connections in the circuit inside the switch.

 If there is any problem in this electrical circuit part, connections will not be set, and you will face problems starting the car.

Your car stops while driving:

If your car suddenly stops while you drive, the problem is again with the electrical connections within the ignition switch, shutting off the ignition.

If anything like this happens to you, observe the dashboard lights; if they all are also dying with the engine, it means there is some problem with the ignition switch. 

If you notice that your dashboard lights and other electronic parts blink or turn off, it indicates some issue with the connection plate of the switch.

In a situation like this, you can rectify the problem by only replacing the ignition switch; there is no need to replace the entire ignition lock.

Ignition is turned on continuously:

Sometimes, a bad ignition switch fails to turn your car off even if you want to do it. In such a situation, your car engine runs even if you remove the key.

Ignition does not turn on:

Sometimes, even after twisting, you fail to switch the ignition On even after twisting. 

When your vehicle’s battery is charged, and you face this situation, the problem is probably with the ignition switch.

You fail to turn your key:

Sometimes, you cannot turn the key when you want to start the car. This problem mainly occurs in keys having a cylindrical shape to their blade.

 The slot’s grooves and notches wear down and turn smooth with use. As a result, the key cannot move the pins in the barrel to the different positions.

If the pin does not move, the keys also won’t. In such a situation, you must replace your keyboard immediately.

Do you want to change the ignition switch wiring harness?

Facing internal delamination or some faults in the auto switches is common.

 However, it is easy to rectify the problem by replacing the old switch with a plug-and-play unit to avoid any issues in the future.

Factory switch wiring harness:

Most steering columns have a built-in switch. 

The only thing you have to do is to find the wiring diagram for the particular vehicle either in the book or on the internet. 

You will get a complete guide to wiring the switch there.

Aftermarket switches wiring harness:

You can get an aftermarket until the steering column does not have a factory switch. 

If you don’t like its looks, you can hide it by putting the unit under the seat.

Custom switch wiring harness at Cloom:

Cloom’s pigtail wiring harnesses have the right-sized connectors to fit into factory wiring seamlessly. 

With our customized harnesses, you need not cut or splice your factory wires and still have a high-quality connection.


We understand that all vehicles are different, and with technological advancements, modern vehicles come with more complex accessories. 

We design custom harnesses to suit all electronic operating components, including door locks, power windows, sunroofs, headlights, control panels, horns, and more.  

You can install our harnesses in all 12-volt marine, automotive, and industrial applications. 

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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