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A battery in a traditional car cannot directly create engine spark. It’s only rated at 12 volts, after all, so it needs a little help boosting the signal to the spark plugs. To make that happen, a car’s ignition system uses devices known as ignition coils.
Like spark plugs, ignition coils are essential to a fully functioning engine. And like spark plugs, ignition coils can slowly degrade and fail throughout extended periods of time. With symptoms similar to that of bad spark plugs, faulty ignition coils are a fairly easy issue to diagnose and solve.
Throughout our years of garage tinkering, the editors at The Drive have worked our way through all sorts of spark issues, and we’ve gleaned some info along the way. Now we’re here to impart that info onto you through our guide to testing ignition coils. Let’s get started.
What Is an Ignition Coil?
An ignition coil is an electromagnetic induction-type transformer that turns low electrical voltage from the battery into high voltage provided to the spark plug, which then sparks and ignites the air and fuel mixture inside the engine’s combustion chamber.
Types of Ignition Coils
Not all coils are the same. Learn what sets them apart below.
Distributor Ignition Coils
Distributor ignition coils will most likely be found on older vehicles, as it is old technology. In these setups, the ignition coil sends the current to the distributor, which has spark plug wires that link to the spark plugs.
Block Ignition Coils (Ignition Coil Pack)
Some cars have all of their ignition coils assembled together in a single pack or box with outputs for each cylinder. The ignition cables connect from the pack to the spark plugs.
Pencil Coils/Stick Coils
Due to the downsizing of engines and the continuing mission to make everything more compact, the pencil coil was invented. In these applications, the coil is built into the plug and into a singular tube to make a space-friendly plug. They require no ignition cables.
Plug Top Coils/Coil-On-Plug
These types of coils are individually located directly on top of the boot that connects to the spark plug.
Ignition Coil Rails
An ignition coil rail is exactly what it sounds like, a collection of ignition coils connected by a single line rail.
Common Symptoms of a Failing Ignition Coil
You’ll know if you have a failing ignition coil. Here’s how.
Check Engine Codes
Spark and ignition issues will likely throw a code that signals the check engine light to illuminate.
If a spark is weak, is timed incorrectly, or doesn’t work at all, it could result in misfiring.
One of the tell-tale signs of a bad ignition coil or bad spark plug is a fluttery or inconsistent idle.
Car Won’t Start or Rough Start
A car needs a spark to run. If it can’t get a consistently strong spark, it’s not going to be easy to start.
The Basics of Testing an Ignition Coil
Estimated Time Needed: 30 minutes to 1 hour
Skill Level: Beginner
Vehicle System: Engine
Working on your car can be dangerous and messy, so here’s exactly what you’ll need to ensure you don’t die, poke your eye out, or lose a finger in the process.
Everything You’ll Need To Test An Ignition Coil
Prepare for the job by making sure you have these tools.
Organizing your tools and gear so everything is easily reachable will save precious minutes waiting for your handy-dandy child or four-legged helper to bring you the sandpaper or blowtorch. (You won't need a blowtorch for this job. Please don’t have your kid hand you a blowtorch—Ed.)
You’ll also need a flat workspace, such as a garage floor, driveway, or street parking. Check your local laws to make sure you’re not violating any codes when using the street because we aren’t getting your ride out of the clink.
Here’s How To Diagnose and Test An Ignition Coil
Let’s do this!
- Park your car on an even surface, turn your car off, and allow it to cool down.
- Use your OBD II scanner tool to determine the cylinder with the issue.
- Remove the ignition coil.
- While you’re here, it’s a good idea to inspect the spark plugs for damage, and if it’s been long enough, replace them, as well.
- Inspect the housing for cracks, holes, or any other damage. They could be the source of your issues.
- Use a spark tester to check the ignition coil.
- Plug the tester into the coil.
- Attach the ground wire.
- Plug in the coil connector.
- Adjust the spark gap to the correct measurement.
- Start the engine.
- If there’s spark, great, it works! If there’s no spark, it’s a bad coil.
Testing With a Multimeter
If you don’t have the required tools above, you can also test your ignition coils with a multimeter. Check the resistance on both the primary and secondary circuits.
Get Help On How To Test An Ignition Coil From a Certified Mechanic
As much as The Drive loves to put the "you" in do-it-yourself, we know that not everyone has the proper tools, a safe workspace, the spare time, or the confidence to tackle major automotive repairs. Sometimes, you just need quality repair work performed by professionals you can trust like our partners, the certified mechanics at Goodyear Tire & Service.
FAQs About Ignition Coils
You have the questions, The Drive has the answers.
Q. So Is a Spark Plug Part of an Ignition Coil?
A. No, the spark plug and ignition coil are two separate parts. The ignition coil feeds the spark plug high voltage.
Q. Yeah, But How Much Does It Cost To Replace an Ignition Coil?
A. Multipacks of 4-8 ignition coils can range anywhere from $100-$600, on average.
Q. Ok, So How Many Volts Are In an Ignition Coil?
A. An ignition coil takes in current from the 12-volt battery and produces 30,000-40,000 volts, on average.
For a more in-depth look at the science behind ignition coils, check out this video from the National High Magnetic Field Laboratory (MagLab), the “largest and highest-powered magnet laboratory in the world,” in Tallahassee, Florida.
Got a question? Got a pro tip? Send us a note: firstname.lastname@example.org