SPECIAL NOTES: Note that the information provided in this guide is of a generic nature, and is intended for informational purposes only. However, since the basic operating principles of any given engine sensor is largely similar across all makes and models, it is possible to apply the information provided here to a large range of applications. Nonetheless, be aware that neither similarities in operation, appearance, or location, nor effects on engine operation when any given sensor fails is guaranteed, and it is therefore recommended that the relevant technical manual be consulted for details on the location, manufacturer specific diagnostic information, replacement procedures, and other technical information pertaining to the affected application. END OF SPECIAL NOTES.
What does the ignition coil do?
All spark ignition engines need a mechanism to create a high voltage current with which to create a spark to ignite the air/fuel mixture in the cylinders. Essentially, an ignition coil is a high-voltage step-up transformer that converts a low voltage, which is the ignition system’s primary circuit, into a secondary high voltage that can be as high as 40 000 Volts on some applications.
In practice, the ignition coil creates a high voltage current that is strong enough to “jump” across the gap between the spark plug’s electrodes, which spark serves as the heat source that ignites the compressed air/fuel mixture.
Why is an ignition coil needed?
Put simply, on spark ignition engines, ignition of the air/fuel mixture is not possible without the high-energy sparks that ignition coils deliver through the spark plugs.
How does an ignition coil work?
Regardless of their design or appearance, all ignition coils contain an iron core that connects to the spark plugs’ central electrode, and two sets of copper wire windings. One set of windings, called the “primary” windings, is connected to the vehicle’s low-voltage primary ignition circuit through the ignition coils’ positive and negative terminals. The positive terminal (+) connects to battery positive voltage, while the negative terminal (-) connects to the ignition module, which provides a ground.
NOTE: It should be noted that while the iron core and two sets of windings typically produce a spark in only one spark plug, there is an exception to this rule. Some applications have “waste spark” ignition systems, in which one ignition coil produces a spark for two different cylinders at the same time. One spark goes to the cylinder on the compression stroke, while the second spark simultaneously goes to a cylinder that is opposite the firing cylinder, where the spark is expended on the opposite cylinders’ exhaust stroke, hence the term, “waste spark”.
The second set of windings, known as the “secondary” windings, is connected at one end to the primary positive terminal, while the other end is connected to the iron core in the coil that connects to the spark plug. Note however that the number of windings in each set of windings is different, or otherwise the ignition coil would not be able to create the high current voltage required to create the ignition spark
Typically, the primary winding is wrapped around the core only about 100 or so times, while the secondary winding (which insulated from the primary winding) can have several thousand turns around the core, although this ratio of windings between the primary and secondary rarely exceed about 80:1 on original, OEM ignition coils. Nonetheless, coils with higher ratios are available for performance applications, since the bigger the ratio between the windings, the higher the ignition coil’s output becomes.
In practice though, when the ignition module completes the primary circuit by connecting the ground, low voltage current flows through the primary windings, which charges up the ignition coil in less than about 15 milliseconds. This charge is in the form of an intense magnetic field in the iron core, which is transferred into the secondary windings when the ignition module breaks the ground connection, which in turn, causes the magnetic field in the primary windings to collapse.
Since energy cannot be destroyed on the one hand, and therefore has to go somewhere, on the other hand, the collapsing magnetic field induces a magnetic field in the secondary windings that then amplifies the field by one hundred times or more, depending on the number of windings in the secondary coil. When the induced magnetic field reaches the intensity required to jump the gap across the spark plugs’ electrodes, the resulting spark discharges the windings.
NOTE: An ignition coil’s ability to create an electrical current depends on the relationship between magnetism and electricity. We need not delve into the complexities of this relationship here, but suffice to say that neither magnetism, nor electricity can exist, or be created without the other being present in the same system.
Where is the ignition coil located on the engine?
The image above shows the typical location (arrowed) of CoP (Coil over Plugs) ignition coils, also known as “pencil coils”. In this example, each individual ignition coil serves one cylinder, but note that on many applications, the ignition coils many be incorporated into one unit, although each ignition coil in the unit still serves only one cylinder. While individual ignition coils can be replaced individually, the whole unit, known as a “coil pack”, has to be replaced when an ignition coil within the pack fails.
Note also that since the appearance and location of coil packs can vary greatly between applications, it is important to refer to the manual for the affected application to locate and identify the ignition coils correctly.
What does the ignition coil look like?
The image above shows an example of an igniion coil that is used in a “waste spark” ignition system, while the image below shows a typical example of a CoP ignition coil that serves only one cylinder. Note however that while many CoP ingnition coils are similar in appearance, and may even be identical in all outward aspects, ignition coils are designed to be used only on the application they were designed for, meaning that even though some ignition coils may look the same, their technical specifactions can vary greatly, and they should therefore NOT be used interchangebly with applications they were not designed for.
Possible symptoms of a failed/failing ignition coil
NOTE: It is a common mistake to blame failed or failing ignition coils for misfires, poor performance, poor fuel economy, or hard/no-start conditions. Note that since all of these issues have multiple other possible causes that do not involve failed or failing ignition coils, a “shotgun” approach to diagnosing these kinds of issues should be avoided, since the symptoms themselves have no, or very limited diagnostic value.
However, if a suitable scan tool is not available, or the application is not OBD II compliant, a logical approach to the kinds of issues listed above can go a long way towards isolating the root cause(s) of many issues. Below are some tips on diagnosing ignition coil issues on engines with distributor-based ignition systems if-
All the cylinders are affected
Note that all the cylinders can be affected in different ways: a spark can be absent on all cylinders, or, a spark may be absent from different cylinders at different engine loads. In the latter case, the misfire may appear to “jump” between cylinders. If the primary, low voltage ignition circuit checks out OK, the spark plugs/spark plug leads are known to be good, and there are no mechanical issues such as los of compression are present, replacing the ignition coil will almost always resolve the issue.
Diagnosing distributor-less ignition systems
Fortunately, diagnosing the same kinds of issues on distributor-less ignition systems is much easier since any fault will set a trouble code, which removes (almost) all of the guesswork from diagnosing ignition related issues. Below are some of the most common symptoms of failed or failing ignition coils on distributor-less ignition systems if-
Only one cylinder is affected
If only one cylinder is affected, the most likely causes include a failed ignition coil, defects in the primary ignition circuit, or a failed ignition driver in the PCM (Powertrain Control Module)
Two cylinders are affected
If the engine has a waste spark ignition system, two cylinders will be affected if an ignition coil fails. Note however that the two affected cylinders will be opposite each other in the firing order, in the sense that one affected cylinder will be on the compression stroke, while the other affected cylinder will be on the exhaust stroke.
All cylinders are affected
In these cases, the cause is extremely unlikely to involve all the coils. The most likely causes to consider in these instances include-
- Failed or defective CKP (Crankshaft Position Sensor)
- Failed or failing ignition module (where fitted)
- Failed or failing ignition coil driver circuit(s) in the PCM
- Total PCM failure
NOTE: Be aware that the symptoms listed above assume that-
- the spark plugs are known to be good, and of the correct type and heat grade
- all fuel/air metering sensors are known to be fully functional
- fuel pressure is within specified values
- all fuel injectors and their control systems are known to be good
- engine temperature is within acceptable limits,
- that there are no engine vacuum leaks, and,
- that there are no mechanical issues such as loss of compression for any reason
NOTE: One or more of the following generic codes may be present, in addition to one or more manufacturer specific codes-
- P0320 – Ignition/Distributor Engine Speed Input Circuit Malfunction
- P0321 – Ignition/Distributor Engine Speed Input Circuit Range/Performance
- P0322 – Ignition/Distributor Engine Speed Input Circuit No Signal
- P0323 – Ignition/Distributor Engine Speed Input Circuit Intermittent
- P0350 – Ignition Coil Primary/Secondary Circuit Malfunction
- P0351 – Ignition Coil A Primary/Secondary Circuit Malfunction
- P0352 – Ignition Coil B Primary/Secondary Circuit Malfunction
- P0353 – Ignition Coil C Primary/Secondary Circuit Malfunction
- P0354 – Ignition Coil D Primary/Secondary Circuit Malfunction
- P0355 – Ignition Coil E Primary/Secondary Circuit Malfunction
- P0356 – Ignition Coil F Primary/Secondary Circuit Malfunction
- P0357 – Ignition Coil G Primary/Secondary Circuit Malfunction
- P0358 – Ignition Coil H Primary/Secondary Circuit Malfunction
- P0359 – Ignition Coil I Primary/Secondary Circuit Malfunction
- P0360 – Ignition Coil J Primary/Secondary Circuit Malfunction
- P0361 – Ignition Coil K Primary/Secondary Circuit Malfunction
- P0362 – Ignition Coil L Primary/Secondary Circuit Malfunction
- P0363 – Misfire Detected – Fuelling Disabled
- P0364 – Ignition Coil L Primary/Secondary Circuit
How to test the ignition coil
WARNING: Do NOT EVER disconnect any ignition coil’s output lead from the output while the engine is running in an attempt to isolate a defective coil. Doing so could cause serious damage to the vehicles’ ignition system, serious personal injury, or worse, instantaneous death due to electrocution by as much as 40 000+ Volts. Therefore, if you are in any doubt about your ability to diagnose ignition coil related issues, refer the affected vehicle to the dealer or other competent repair facility for professional diagnosis and repair.
Note that while professional repair shops have ignition simulators with which to test the operation of ignition coils, the average DIY mechanic is unlikely to have access to equipment of this type. Therefore, the average DIY mechanic is limited to only one safe method to test an ignition coil, which is-
Testing the ignition coils’ internal resistance
Doing this requires the use of a digital impedance ohmmeter that can make measurements in the 10 mega-ohm range. To test the primary windings, simply connect the red lead of the ohmmeter to the ignition coils’ positive (+) terminal and the black lead to the negative (-) terminal.
Most ignition coils will have a resistance of between 0.4 and about 2 ohm in the primary windings, but compare the obtained reading with the manufacturer’s specified resistance value before condemning the ignition coil out of hand. However, zero resistance in the primary windings indicate an internal short circuit in the ignition coil, while an abnormally high resistance indicates an open circuit in the ignition coil. In both cases, the ignition coil is defective and must be replaced.
To test the secondary resistance, connect the red lead of the ohmmeter to the ignition coils’ positive (+) terminal, and the black lead to the high voltage output point. Most coils will show a resistance of between about 6 000 ohm and about 15 000 ohm, depending on the application. Compare the obtained reading with the manufacturer’s specified value before condemning the ignition coil.
Notes on “spark testers”
Some auto parts stores sell devices that connect between the spark plug and the ignition coil that in theory, should produce a flash of light if the ignition coil being tested works. Note however, that while most of these devices do produce flash of light when a pulse from the coil passes through them, there is no way to relate the brightness of the flash with the ignition coil’s actual output with any degree of accuracy.
Thus, in practice, and assuming that the “spark tester” actually works, the best it can do is to differentiate between an ignition coil that may or may not produce a current of sufficient strength to create a spark, and a coil that does not work at all.
How to replace the ignition coil
If an ignition coil is known to be defective, replacing it is usually a relatively easy procedure that should be well within the capabilities of any DIY mechanic. In most cases, the procedure will follow the general pattern described below-
- Make sure the engine is cold to avoid burns and scalds
- Disconnect the defective coils’ low-voltage electrical connector
- Remove any retaining bolts/screws, and place aside
- Extract the defective coil, and insert the replacement
- Insert and tighten the retaining bolt(s)/screw(s), but do NOT over tighten to avoid damaging either the replacement coil or the threads in the engine
- Reconnect the electrical connector
- Clear any trouble codes present
- Test drive the vehicle, or rescan the system to verify that the issue had been resolved
Notes on ignition coil replacements
- Always double check that the replacement coil is suitable for use on the affected application; unsuitable, or substandard aftermarket replacement ignition coils may fail again in very short order
- Always check the electrical connector for signs of damaged, or corroded terminals; poor contact between terminals can make even OEM replacement coils fail in short order
- If ignition coils fail repeatedly, ensure that the correct spark plugs are fitted, and make sure that all spark plugs are gapped correctly. Note however that the gaps on spark plugs with platinum or iridium electrodes, or spark plugs with multiple side electrodes should not be adjusted. Replacing these types of spark plugs after about 60 000 to 80 000 miles can go a long towards preventing ignition coil issues.