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A simple guide to your car's ignition coil

A simple guide to your car's ignition coil

Dan is an experienced motoring journalist who has more than 20 years of experience. He has been the editor of titles such as Fast Ford and Redline, and his latest project was converting an old Renault Trafic into a family campervan.

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 shadowy 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?

In order for electricity to jump between the spark plug electrodes, and ignite the pressurised fuel/air mixture in the combustion chamber, high voltage is required.

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 of the wires connects it to battery voltage, another to the contact points in the distributor, and the central high-tension lead going to the centre of the distributor cap.

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 that sits on the top of each spark plug

Some coils operate 2 spark plugs at the same time, with a system called ‘wasted spark’. Both of the spark plugs are fired at the same time, regardless of which cylinder is at the top of the compression stroke, and which one is at the top of the exhaust stroke. The ‘wasted’ spark has no effect on the running of the engine.

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. 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, it may have an engine management system that generates a fault code for any misfires detected. This code could help identify the component that caused the misfire. Connect a diagnostic tool to the OBD (On-Board Diagnostic) port, and retrieve any stored fault codes.

Of course, a misfire 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 not really possible to repair a coil. Replacement is the only realistic option. They’re easy to replace, but can be expensive.