The ignition coil

The Ignition coil, popularly referred to as  “coil”, is the heart of the ignition system. Its job is to transform the battery voltage (12V) into the thousands of volts necessary to initiate the ignition on the spark plug.

In other words, the ignition coil is in charge of providing the energy required by spark plugs, some have an internal resistor, while others rely on a resistor wire or an external resistor to limit the current flowing into the coil from the car’s 12-volt supply.

The wire that goes from the ignition coil to the distributor and the high voltage wires that go from the distributor to each of the spark plugs are called spark plug wires.

There are three main types of ignition systems, hence three main types of ignition coils.

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Conventional

The conventional breaker point-type ignition system has been in use since the early 1900s. In this system, the primary circuit of the ignition coil receives power from the battery through a resistor. The power is grounded through closed ignition points in the distributor. Current flows through the windings of the primary coil, creating a magnetic field. When the points are opened by the rotation of the distributor cam, the current’s electrical circuit is broken, collapsing the magnetic field. The force from the collapse crosses the windings of the secondary coil and creates an electrical current within them. The current flows into the distributor cap and eventually into the spark plugs, all in a split second.

 

Electronic

Electronic ignition systems were popular in the mid-1970s and were developed to be more reliable and produce fewer emissions. This type of ignition is very similar to the conventional system, with the same configuration in the secondary circuit of the ignition coil. From the battery to the coil terminal, the primary circuit is also the same. But instead of a distributor cam and points, the electronic system uses a pickup coil to signal the control module, which then fires the ignition coil. On some electronic systems, the ignition coil is located inside the distributor cap.

 

Distributor-less

In a distributor-less ignition system, which came out in the 1980s, its design allowed more energy to be available from the coils. Instead of two coils, there are typically three or more mounted together in a coil pack, each responsible for firing either one spark plug or a pair. This system uses a magnetic triggering device to determine engine speed and crankshaft position. The triggering device sends a signal to the engine control module or the ignition control module which, in turn, sends energy to the coil.

Despite the mechanical and technological changes through the years, the basic function of the ignition system has remained basically the same; take the voltage from the battery, convert it to a higher voltage, then send this electric current to the combustion chamber in time to ignite the compressed fuel and air.

 

How does an ignition coil work?

Basically, an ignition coil is a transformer. As any transformer, it has a primary winding and a secondary winding. It receives battery voltage on the primary winding and when the current is disrupted the magnetic field collapses causing a huge voltage output on the secondary winding.

Most recent designs of ignition coils have output voltages in excess of 45,000 volts, able to ignite the ultra-lean mixture of fuel injection systems.

Several factors determine the amount of energy required to create the electric arc in the spark plug. Some of them are the engine compression, air-fuel ratio, combustion chamber temperature, spark plug materials, spark plug calibration, etc.

Over the years, vehicle manufacturers have focused on developing more efficient engines. That implies engines with higher levels of compression and leaner fuel delivery. These changes demand more and more energy from the ignition system. As a consequence, single-coil engines became obsolete opening the way to multi-coil systems.

Nowadays, the most common multi-coil system is known as Coil-On-Plug (COP). This particular design consists of one ignition coil per cylinder. Each coil is directly attached to the spark plug, which contributes to eliminating electrical loses from spark plug wires.

Unlike traditional single-coil ignition systems, on current COP systems, the Electronic Control Unit is in charge of activating each individual coil when it’s needed. Most COP electrical connectors consist of three wires: one for ground, one for battery voltage and one for ECU activating signal. It’s not uncommon that all three wires connect directly with the ECU because many designs incorporate current and/or ground monitoring for each coil.

 

Symptoms of a bad ignition coil

A weak coil will not be able to accomplish its mission, having as a possible consequence, a “no spark” condition.

As explained at the beginning of the article, the ignition coil is vital for appropriate combustion. The most common symptoms of a bad ignition coil are:

1. Check engine light

Due to its crucial role in the ignition system expect the Check Engine Light to illuminate after a few drive cycles with a damaged ignition coil.

 

2. Poor fuel economy

Without a proper spark, the combustion process will not be efficient and a noticeable decrease in fuel economy will be evident.

 

3. Rough idle

Depending on the vehicle year and maker the ECU may not compensate for this failure during idle. This might cause a rough idle or even an engine dying occasionally while idling.

 

4. Difficulty during engine start

Depending on the number of defective coils, you may experience difficulty during engine start. Difficulty to start the engine can also be caused by other problems with the electrical system such as a flat battery, so it’s worth checking these out too.

 

5. Engine misfires

Ignition energy degradation will cause engine misfires that will affect overall performance.

 

6. Engine hesitation during acceleration

Having one or more faulty COP will cause a severe lack of power in the engine, especially during acceleration. This can also be caused by bad injectors, so it’s worth looking at these too. For vehicles that use a single ignition coil as the source of spark for all of the cylinders, a faulty coil will affect the operation of the entire engine. If the coil fails completely, it will leave the engine without spark, which will result in a no spark, no-start condition.

 

7. Exhaust back-fire

Fuel particles won’t burn in the combustion chamber as they usually do and will enter the exhaust pipe with a high energy charge. That energy will be unleashed in the form of mini-explosions.

 

8. Fuel smell from exhaust pipe

The failure to burn the fuel in the combustion chamber will cause a noticeable petrol smell coming from your exhaust pipe.

It is worthy of note that the ignition coil (irrespective of the design) is a maintenance item and requires replacement at certain intervals. Please consult the vehicle’s service /workshop manual for the recommendations.