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How ignition coils work & failures

By autotech-nic on June 18, 2016

An Overview

  • Linked to the car’s ECU, the ignition coil controls the sequence and current to the spark plugs. febi supplies almost 100 different  ignition coils for a variety of vehicle makes and models
  • There are various types of ignition coil fitted to different types of ignition systems, such as single coils providing ignition voltage through a distributor, ‘wasted-spark’ coil packs and individual coil-on-plug units.


  • Vital to the running of your car’s engine
  • Provides smooth engine starts


  • Improved combustion performance
  • Lower fuel consumption
  • High quality parts, like the components featured in febi’s electrical range, ensure more efficient engine performance and reliability

The technical answer

In petrol engines, the ignition coil is located in a vehicle’s ignition system in order to convert a car battery’s low voltage, of around 12 volts, to the thousands of volts needed to create an electric spark in the spark plugs and ignite the fuel.

An ignition coil supplies a high voltage that is made to jump from one electrode to the other at the spark plug, creating a spark. The voltage needed depends on a variety of factors.

These include:

  • width of the electrode gap
  • electrical resistance in the spark plug or ignition leads
  • air/fuel mixture
  • compression pressure / engine load
  • temperature of the spark plug

The level of voltage required can change and vary from 5,000 volts up to 25,000 or more. The voltage can reach as high as a whopping 30,000 volts.

How do they work?

The ignition coil actually consists of two coils; a primary winding which has relatively few turns of heavy wire, and a secondary winding which has thousands of turns of very thin wire. Both windings are wound around each other.

The primary winding is connected to battery voltage and its earth can be switched on and off by the engine control unit (ECU). The secondary winding is connected to the spark plug. When the primary winding is switched on, the current flow creates a strong magnetic field around both windings. When it is switched off the magnetic field collapses, moving over both the primary and secondary windings. This induces a high voltage in the secondary winding which causes a spark at the spark plug.

All modern variations of ignition coils, including single coils and coil rails, work in a similar way – with the exception of capacitive discharge coils.

Ignition Coil Failures

Ignition coils have a relatively long service life, but they can fail for a variety of reasons. Heat and vibration can damage the coil’s insulation, but the biggest cause of ignition coil damage is voltage overload, which is caused by poor quality or faulty spark plugs and/or HT leads.

If the ignition coil’s output voltage rises too high, it burns through the coil’s internal insulation, causing a short – the coil’s insulation can be damaged if the output exceeds 35,000 volts. If this happens, the coil’s output voltage may drop causing ignition misfire. In worst case scenarios, the coil may cease to put out any voltage at all, preventing the engine from starting or running. febi recommends fitting new spark plugs (and HT leads where applicable) at the same time as replacing a faulty ignition coil.

You can find the ignition range within the online catalogue on febi Live! Sign in or register now at



About Autotechnician
Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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