What does an ignition coil do?

An ignition coil is an essential part of your petrol engine electrical system, ensuring that the energy required to make the Suck, Squeeze, Bang, Blow happen at the correct voltage to ignite the fuel in the engine and in the correct order for the engine to run.

In essence, the coil takes the low voltage from the battery (6 or 12 volts, depending on the age of your vehicle), and jumps it up to the very high voltage needed to ignite the fuel. The distributor sits between the coil and the engine and distributes the power to the engine in the appropriate firing sequence.

Before you even consider playing with one, remember that electricity will always take the shortest and easiest route to ground  - literally the path of least resistance. Testing a coil can be dangerous if done incorrectly. If you don't already know how to do it safely, contact a specialist.

Where is the ignition coil located?

Now you're asking a tricky question. Most modern engines have multiple coils and they're usually located on top of the engine block. In older cars where there is less standardisation it can get more complicated. Also, you may need to be on the lookout for more than one ignition coil.

Classic cars and vans usually have one coil, but not always. Some two strokes in particular (such as some early Saabs) have one coil per cylinder.

The most common locations for ignition coils in older cars are in the engine bay on the bulkhead, on the inner wing or sitting on top of the dynamo.

Can a faulty ignition coil be repaired?

Yes, a faulty ignition coil can probably be rewound by a specialist. However, at the time of writing, a reputable new coil for a Morris Minor for example would set you back about £25. A rewind will probably cost rather more.  Whether to buy or to repair will come down to choice and the type (or scarcity) of coil your vehicle needs.

Does an ignition coil need to be earthed (grounded)?

Yes and no. Electrical circuits do need to be earthed. However, a standard ignition coil is connected to the distributor, which is earthed to the engine block, which in turn is earthed to the chassis. So you don't need to earth it separately.


As a result, what might appear to be a coil problem could actually be caused by damaged cabling or a poor engine earth - so check these first.

How does an ignition coil work?

A classic ignition coil usually looks a bit like an energy drink can with a plastic cap and spout sticking out of one end.

The outer casing contains a soft iron core surrounded by two tightly wound coils of copper, each separated by insulation.

The primary (outer) coil (or 'winding') is made of thick wire, one end of which is connected to the vehicle's battery and the other to earth (ground).

The secondary (inner) coil is made of much thinner wire, and there's a whole lot more of it.

There can be between 50 to 200 windings of the secondary for each one of the primary. The most common is probably a 100:1 ratio. This is called the 'transformation ratio'.

When the ignition is switched on, current from the battery passes through the primary winding, creating a strong magnetic field. The collapse of this field induces a current in the secondary winding, transforming it from low to high voltage.

The input from the battery can be either 6 or 12 volts, depending largely on the age of the vehicle. The voltage created in the secondary coil can be as high as 10,000 volts.

One end of the secondary winding is connected to earth, the other connects to the rotor arm in the distributor.

That arm rotates in the distributor cap, passing in turn each of the metal contacts to which the high tension (HT) leads are connected.

A high voltage spark jumps from the rotor arm tip to the distributor cap metal contacts (which is why having the correct gaps in the distributor cap is so important). The HT leads then carry  the high voltage  power to the spark plugs in the correct order for engine to fire. If the spark is too weak or distributed in the wrong order you end up with a very unhappy engine.

Can an ignition coil cause a misfire?

Yes, a failing ignition coil could cause an engine misfire, especially if it happens when the engine is idling or on sudden acceleration.


An irregular spark originating in the coil can result in the engine stalling. A faulty coil can also increase fuel consumption in more modern engines as it will fool the system into over-compensating on the fuel delivery.

What's the easiest way to test a coil?

The simplest (and safest) method is to swap the coil for one you know is working. If the problem persists, then you know you need to dig deeper.

Why do ignition coils fail?

Your ignition coil works in a pretty harsh environment, subject to extremes of heat and vibration. The coating the insulates the wiring is often the first element to begin to give up the fight. the reduction in insulation between the primary and secondary windings can lead to increased heat within the coil, further exacerbating the problem.

Vibrationfrom the engine and road over time can also lead to windings breaking.

Secondary problems on the engine side of the coil - damaged or worn out spark plugs can result in a greater power draw from the coil, shortening its effective life span.

Some ignition coil markings

CB : contact breaker (older style coils)

SW : switch (older style coils)

+ : positive

- : negative

COP : coil on plug

CNP : coil near plug

CPC : coil per cylinder

Related Reading
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Ignition Coil System.  Images courtesy of the New York Public Library
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