Using a multimeter to test an ignition coil involves connecting the multimeter across the terminals of the coil that feed the primary and secondary windings of wire inside the coil. You will also need the appropriate service manual or coil specifications for your vehicle to perform this test. Set the multimeter to read resistance, or Ohms (Ω), and connect the probes of the multimeter to the terminals indicated by your service manual. Once you have a resistance reading, you can simply compare it to the specifications listed in the service manual for your coil, and if it is out of specification or if the reading indicates zero resistance, or excessively high resistance, the coil needs to be replaced.
This method has one major shortcoming – it is not possible to test the coil under engine load with a multimeter. Often when coils fail and engine misfire occurs, it is at higher RPMs when the engine is working its hardest. A coil that is producing appropriate results in a bench test may still be weak enough to fail to generate spark when a load is introduced.
When your car won’t start, won’t stay running, seems to be running rough, or lacks performance, keeping calm is the last thing you might think to do. But if you can keep your head clear, you may be able to diagnose why your vehicle won’t start more easily than you might expect. If your engine turns over but doesn’t fire, the two most important areas to check are the fuel delivery and the ignition systems. In this article, we’ll discuss ignition coil failures, which are one of the most common ignition system failures, and also one of the best places to begin your diagnosis to determine the source of your starting problems.
What is an Ignition Coil or a Coil Pack?
An ignition coil generates the power needed to make the spark that ignites the fuel mixture in your cylinders. Ignition coils receive 12-volt power from the battery or alternator and transform it internally into thousands of volts that are required for ignition. Depending on the age, make, and model of your vehicle, it may have one coil that relies on a distributor to time the delivery of voltage to the spark plugs using the spark plug wires, or it may have individual coil packs. A coil pack or coil-on-plug system utilizes individual coils and coil insulators for each spark plug on the engine, and these types of systems are usually timed by the engine computer. With the help of the camshaft position sensor or other engine sensors, the computer sends a signal to the coil to tell it when to begin building voltage and when to fire the spark plug. If your vehicle has a failing ignition coil, it could cause the spark plug to not fire, which leaves unburnt fuel in the engine’s cylinder and can result in a strong gasoline odor coming from the engine bay, usually accompanied by a rough, stumbling idle.
Misfires
With time or due to environmental factors, the ignition coil or one or more of the coil packs on your vehicle may begin to wear out and eventually fail. If you have a single ignition coil and distributor on your vehicle, a bad ignition coil could mean that your engine fails to start entirely, or if the coil is producing too little voltage, you may notice that your engine occasionally or consistently misfires. Engine misfires can make your engine run noticeably rough, cause a flashing Check Engine light, and can cause damage over time if not addressed promptly. One of the most common engine misfire codes is P0300, which indicates a random or multiple misfires in the engine, but you may also get more specific codes that indicate only one cylinder is misfiring. This is often a sign that one specific coil is not working as it should, and can help you narrow your search and allow you to test only the ignition coil for that cylinder. It’s typically advised that you drive as little as possible (or not at all) if your engine is misfiring until you can diagnose the problem and have it fixed.
Active Check Engine Light
In modern engines with one coil pack per spark plug, known as a coil-on-plug ignition system, coil failure may not prevent the engine from starting. Often you will experience a misfire in one or more cylinders that will trigger a Check Engine light, which can easily be diagnosed using a code reader or by visiting your local O’Reilly Auto Parts store to have a parts professional perform a free Check Engine Light diagnostic test.
Hard Starting, Bad Gas Mileage, Lack of Power, or Backfiring
A failing ignition coil can also display several symptoms before it fails entirely. These can include hard starting, poor gas mileage, a lack of power, or backfiring. If you notice any of these symptoms, it may be worth taking the time to fully test your ignition coil or coils and try to determine whether or not the ignition system is causing any performance issues.
How to Test an Ignition Coil - The Easy Way
Once you’ve had your codes read and determined that the issue you’re experiencing is a misfire, and figured out which cylinder or cylinders are misfiring, there are a few ways you can proceed. Since misfires can be caused by a few different factors, including a failed ignition coil or coil boot, a fouled spark plug, or a lack of compression, it’s a good idea to test only one thing at a time. In most older vehicles with a single coil and distributor, the simplest test is often to replace the coil with a known-good or new ignition coil, as well as inspecting the coil for signs of damage or oil buildup, which could indicate that the coil case has been damaged. Having a spare ignition coil for your older vehicle is often a good idea, as they are simple to change out even on the side of the road and could be the difference between getting back on the road or calling a tow truck.
In vehicles with multiple coil packs, the easiest way to test your ignition coil is often to switch the coil on the misfiring cylinder for another coil on the engine. If the misfire continues but is now appearing on the cylinder with the suspect coil, then you can be nearly certain that a failing ignition coil is the source of your problem. If the problem persists in the original cylinder, it may be a good idea to pull the spark plug and inspect or clean it, or it may be necessary to test compression or incoming signal to the ignition coil, as a corroded connector or wiring damage could also prevent it from firing.
If swapping the known good and suspected bad coils resulted in the misfire jumping from one cylinder to another, the next consideration should be the condition of the coil and the coil boot. Inspecting the ignition coil can tell you a lot about how it may have failed. In some cases, you may notice “hot spots” or discoloration due to the coil burning out, or you may notice deterioration or cracks in the coil boot. Coil boots are made of rubber or silicone and are designed to insulate the voltage to ensure it reaches the spark plug. A crack or other damage to the coil boot could cause voltage from a working coil to arc to the cylinder head, which will prevent the spark plug from receiving the power it needs to fire. If you notice damage to the coil boot, it may only be necessary to replace the boot if it can be separated rather than replacing the coil itself.
How to Test an Ignition Coil with a Multimeter - The Less-Easy Way
There are a few ways to test an ignition coil more thoroughly than by process of elimination as described above. In order to perform these tests, you may need a multimeter and spark tester to isolate any potential issues.
Ignition Coil Ohm Test
How to Use a Spark Tester
Using a spark tester, also known as an ignition spark tester or an engine spark tester, allows you to see what the coil is doing when the engine is placed under a load more easily than the multimeter test, and may help confirm that a coil is only failing under specific circumstances. Spark testers are very simple to use, but should be used with caution. If they are connected incorrectly, it may cause spark to short to the engine or generate spark outside of the tester that could be dangerous. Confirm there are no fuel leaks before performing this test to ensure that any spark that may be generated outside the engine isn’t potentially hazardous.
How to Use an In-Line Spark Tester
There are two types of spark testers, including in-line spark testers and spark testers that actually take the place of the spark plug and ground to the engine block to allow you to see the spark. In-line spark testers are often the safest spark tester to use because they allow you to continue to run the engine and see the spark jump across electrodes inside a containted space. These can simply be connected to the coil output on one end and the spark plug on the other end. When the engine is cranked, you should see spark inside the spark tester, and this should continue once the engine is running. Once the engine is running, having a helper increase the RPMs slowly and keeping an eye on the spark tester can help you determine if the spark stops at any point throughout the power and load curve. If at any point the ignition coil stops firing, you can confirm that the coil is failing under load and should be replaced.
How to Use an In-Line Spark Tester
There are two types of spark testers, including in-line spark testers and spark testers that actually take the place of the spark plug and ground to the engine block to allow you to see the spark. In-line spark testers are often the safest spark tester to use because they allow you to continue to run the engine and see the spark jump across electrodes inside a containted space. These can simply be connected to the coil output on one end and the spark plug on the other end. When the engine is cranked, you should see spark inside the spark tester, and this should continue once the engine is running. Once the engine is running, having a helper increase the RPMs slowly and keeping an eye on the spark tester can help you determine if the spark stops at any point throughout the power and load curve. If at any point the ignition coil stops firing, you can confirm that the coil is failing under load and should be replaced.
If you find that your ignition coil has failed, they are often relatively easy to replace. In most cases, replacing your ignition coil only involves the use of basic hand tools, but on some vehicles, they may be more difficult to access due to the presence of other parts and might need more advanced service.
Older canister-style ignition coils are often secured to the engine block with a bracket and connected to the electrical system via one or two wiring connectors and a coil wire that goes to the distributor. Simply removing the wiring connectors and coil wire and loosening or removing the bracket will allow the coil to slide out and the new one to slide back in. Be certain that the coil cannot move once the bracket is tight, reconnect the wiring harness and coil wire, and you should be done. On older coils with two separate wiring connectors, be sure not to confuse which terminal each wire connects to. This will not only prevent the coil from working, but can also damage your new coil. The wires are nearly always color-coded, and terminals of the coil will be labeled “BATT”, or positive, and “DIST”, or negative.
How to Replace An Ignition Coil
Replacing a coil pack once you have determined which one has failed is also relatively straightforward, and typically only involves removing a single bolt and a wiring harness connector. It’s typically better to remove the wiring connector from the coil while it is still attached to the engine, but be certain not to pull on the wires as this could damage the harness or terminals. Once the wiring connector is removed from the coil, the retaining bolt or nut can be loosened and removed, and the coil simply pulls up and off of the spark plug. Installing the coil is simply a matter of reversing these steps; installing the coil and boot over the spark plug and pushing down, reinstalling the retaining bolt or nut, and securing the wiring connector.
Ignition Coil Testing and Professional Service
Hopefully this instructional guide on diagnosing and replacing an ignition coil has provided the confidence you need to inspect and test ignition coils yourself. However, if these tests are inconclusive, or if you’re not comfortable diagnosing or repairing your own vehicle, visit one of our stores and a parts professional will be happy to recommend a local shop to help get your vehicle running at its best. You can also find recommended local shops on our Find a Repair Shop page. If you’re ready to replace a failing ignition coil, you can also shop our selection of ignition coils for your vehicle, as well as the tools you need to complete the job.
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