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A simple guide to your car’s ignition coil pack

A simple guide to your car's ignition coil assembly

If your car has trouble starting, is misfiring or if it’s doing fewer miles to the gallon, it may be there’s a problem with its ignition coil.

This is serious because not only does your car not run smoothly, if at all, but poor ignition can damage the catalytic converter in the exhaust system, which is expensive to replace.

The trouble is, the coil is an unknown quantity to many people. You occasionally hear of the coil giving trouble but while the spark plug is a familiar part of a car’s ignition system and one that is routinely replaced, typically as part of a car’s major service, the coil is this shadowing thing that does, well, what?

Below, we answer that question and explain, too, how it works, why your car needs one and what to do if it fails.

What is a coil?

It’s the part of a car’s ignition system that takes the battery’s 12-volt output (called low-tension current) and transforms it into as much as 45,000 volts (called high-tension current) before then supplying it to the engine’s spark plugs.

Why does an engine need one?

Because without it, the spark plug wouldn't receive voltage high enough to do its job of detonating the fuel/air mixture in the combustion chamber. The pressures are so high in there, and the need to get a good burn so great, that the voltage has to be extremely high for the spark to be effective.

What does the coil look like?

If your car is an older vehicle with traditional distributor ignition, it’ll look like a small metal cylinder (in fact, it’s often called a canister-type coil) with wires sprouting out of it, one connecting it to the battery, another to the distributor.

On a modern car, coils come in a variety of shapes and sizes including a single coil that looks like a long plastic tube – sometimes called a pencil or plug-shaft coil – one that incorporate an ignition module and another that looks like a domestic plug.

Some coils are constructed in series and called cassette or sequence coils, or coil rails.

Where is the coil?

The ignition coil sits between the battery and, if the car is an older model, the distributor that ‘distributes’ the HT voltage it produces to each spark plug, via thick rubbery HT leads (one per plug).

On modern cars with an electronic ignition system that uses a computer, rather than a distributor, to ‘fire’ the spark plugs at the right time, the coil is likely to be mounted directly to the spark plug or to a pair of spark plugs, without the need for HT leads. For this reason, a modern engine can have multiple coils.

A simple guide to your car's ignition coil assembly

How does the coil work?

A coil works on the ‘step-up’ transformer principle in that it transforms one voltage into another that’s higher. It does this using two separate wires coiled around each other with both coiled around a central iron core, all contained within an insulated body.

One wire, called the secondary, is made up of thousands more windings than the other one, called the primary.

This is important because it’s the number of windings (imagine they’re fibres) that determines the level of voltage the wire can handle. The magnetic core allows electrical energy to pass from the primary to the secondary wire. 

The primary wire receives the low voltage from the battery and generates a magnetic field around it.

However, the instant that flow is interrupted by the distributor, or in more modern ignition systems, the electronic control unit (ECU), the magnetic field collapses, creating or inducing a higher voltage in the secondary wire that travels to the spark plug.

Why does a coil fail?

A hot engine bay is a challenging place for any sensitive electrical equipment such as a coil. Being situated directly above the engine doesn't help.

Here the coil gets alternately hot and cold, and is subjected to strong vibrations from the engine. Over time these forces can break up the coil’s windings and insulation.

However, the primary cause of coil failure is voltage overload caused by worn spark plugs with electrode gaps that are outside specified limits or by damaged cabling and wires.

In time, the coil’s output voltage can rise to damaging levels, causing short circuits when it burns through the insulation.

How do you diagnose a faulty coil?

Engine misfires and backfires, poor starting, a lack of performance and poor fuel consumption are all possible indications of a faulty coil. If your car has a distributor-based ignition system, all the spark plugs will be affected but if it’s a modern car with electronic ignition, only one plug could be, or two if they share the same coil.

If your car was built after 1996 and has an OBD II (on-board diagnostic) port with misfire detection, interrogate it using a diagnostic tool, checking for the code P030X, X being the number of the cylinder that is faulty.

Of course, a faulty cylinder can be caused by all manner of ignition and fuel supply problems, not just a faulty coil. For this reason, you should remove and check the spark plug, and, if there is a distributor, the HT lead.

Check the security and integrity of the coil itself. Also, using a multimeter, check the coil’s primary and secondary resistance are to specification.

How do you repair a coil?

It’s easier to replace but be careful to turn off the ignition and disconnect the battery before starting work.