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How To Test a Vehicle’s Ignition Coil

By Joe Ballard
Published on June 12th, 2024

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Ignition coils convert the low voltage from the 12-volt battery into the high voltage the spark plugs need to ignite the fuel-air mixture in the combustion chamber. In this article, I will explain the different types of ignition coils, the warning signs of a bad ignition coil, and how to test them.

Ignition coil types

Vehicles use different types of ignition coils. Depending on the vehicle manufacturer and age, your vehicle may use any one of the following:

Coil-On-Plug (COP)

These coils are easy to identify as they sit right on top of the spark plug and are usually bolted to the valve cover or cam cover. Having the coil directly on each spark plug simplifies the diagnosis of an issue because there are no secondary ignition wires or other electrical components to deal with.

Additionally, it makes it easier to test a coil by swapping it with the neighboring coil to see if the misfire moves to the neighbor. The best way to monitor the misfire is with an OBD2 scanner like the model 5610 from Innova.

Distributorless Ignition System (DIS) “waste spark” ignition coil packs

These coil packs have been used in vehicles since 1984. This system consists of a single or a series of coils with terminals that connect to spark plug wires.

They usually have a pair of wires, each connected to the spark plug on two different cylinders, that fire in unison. One spark fires on a cylinder’s compression stroke, and the other spark fires on the other cylinder’s exhaust stroke. The spark that fires on the exhaust stroke is the "waste spark." Since an ignition module usually powers the coil packs, this is another point of failure.

The best way to determine which coil pack is failing is by moving the coil pack to a neighboring cylinder to see if the misfire follows the move. If you are a DIYer, you can also use an OBD2 scanner to look for misfires.

Traditional single coil

This is one ignition coil that serves all the engine’s cylinders. These systems have been used in vehicles in one form or another since the 1920s. This type of system is no longer used in modern vehicles, but millions of older "classic" vehicles still use distributors. Doing an inline spark test will determine if the coil is firing or not.

For a DIYer, this is a simple test where the coil wire is removed from the distributor and put near the engine block to see if a spark arcs across the gap while cranking. If you still suspect that the coil is bad, you can use a multimeter to test the resistance of the coil windings. Since these systems can also use electronic ignition or mechanical points, checking those components is also a good idea.

One thing to keep in mind is that the coil is susceptible to heat and vibration, which may cause it to work intermittently. If this is the case, replacing it with a new coil should address the issue.

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Warning signs of a bad ignition coil

Most ignition coils fail in two ways: complete failure with total lack of power or fails under engine load. The failing under engine load situation is obviously much more challenging to diagnose. In this case, looking at some common bad coil symptoms as a preliminary diagnosis is a good idea. Here are a few symptoms of a bad ignition coil:

Engine misfires

This is usually the first and most common symptom and will also almost always trigger a Check Engine Light (CEL) to come ON. The misfire will either be on a specific cylinder or randomly on multiple cylinders.

A misfire on a specific cylinder is easier to diagnose. If the engine is running rough, stumbling, or lacking acceleration and the CEL is on, then there is a misfire on one or more cylinders. If you are a DIYer, you can usually identify the misfiring cylinder using an OBD2 scanner like the model 5610 from Innova.

Engine idling rough or hesitation under load

Most misfires are caused by ignition issues, which result in rough idling. If your vehicle is a "Classic" and does not have a CEL, then the most likely culprit is the secondary ignition system which includes the spark plug wires, coil wire, distributor cap, rotor and/or spark plugs.

Engine will not start with no spark

In this case, most people would immediately assume that the coil is bad, especially on older vehicles that use a distributor. Although a good guess, several ignition system components could still be the culprit. It is a good idea to check the wiring and distributor points (on older vehicles) before you replace the coil.

How to test an ignition coil

It is important to recognize that many ignition systems (DIS or COP) use an ignition module to send signals to the ignition coils. So, an issue with an ignition coil may actually be an issue with the ignition module. However, here are a few ways to test the ignition coil:

Swapping the coil – this is by far the easiest method where, on your Classic car, you swap the suspected bad coil for a new one; or, on COP or DIS systems, you simply swap the ignition coil to another cylinder.

General inspection – on any coil type, visually inspect it for cracks, burns, or melting; or, on Classic cars, inspect for oil leakage out of the coil. This will sometimes reveal the issue straightaway so check carefully. It is also a good idea to check the distributor cap for cracks on older single coil ignition systems. Just a small amount of moisture seeping into the cap will cause intermittent sparking on the cylinders.

Testing the coil windings—if you are a DIYer with a multimeter, you can quickly test the primary and secondary resistance of the coil windings. This method has been used for many years with great success. The only downside is that the coil is not being tested under load, so although it passes the winding test, it still could be bad.

Spark active analysis – often, the coil case will start to degrade and become weak but still produces a spark that is adequate when the engine is running in the driveway but under load produces a misfire. As mentioned previously, the easiest way to do this is to swap the ignition coil with another cylinder or a known good ignition coil. However, as a DIYer, you can also use other inline testing tools to help identify the intensity of the spark while the engine is running. There are many such tools, and the most appropriate tool for testing the coils will depend on the type of ignition system used in your vehicle.

Conclusion

Diagnosing issues with the ignition system and coils is often quite challenging. Swapping out parts can sometimes be time-consuming, expensive, and not very effective. If you are a DIYer, you stand a better chance of identifying the issue using an OBD2 scanner, multimeter, or other testing tools, as well as a good dose of common sense and experience.

If you decide to take your vehicle to a service center, they will have sophisticated tools at their disposal to identify the issue with the ignition system. Using a service center may be the most cost-effective and less time-consuming route, but there is still no guarantee that they will resolve the issue, especially if it is intermittent. It may take a few visits before the issue is completely resolved.

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