Having difficulty starting your car? Is your check engine light popping on?
The culprit could be a faulty ignition coil. A bad coil can cause engine stalling, poor performance, and higher fuel consumption. However, ignition coil failure is often sporadic, making diagnosis difficult.
So, how to test an ignition coil?
We’ve listed several methods for you. Let’s get testing.
This Article Contains:
- How Does an Ignition Coil Work?
- 3 Ignition Coil Types: Know Which One You’re Testing
- What Tools Do You Need for an Ignition Coil Test?
- How to Test an Ignition Coil
- Other Ways to Test the Ignition Coil
- 7 Failing Ignition Coil Symptoms
- How Long Does an Ignition Coil Last?
How Does an Ignition Coil Work?
Car batteries deliver 12V to the ignition coil. Each coil consists of a primary winding (primary circuit), a secondary winding (secondary circuit), and an iron core.
Electricity flows from the ignition switch through the windings, amplifying the secondary voltage to 50,000V or more. This high voltage then goes to the spark plug in the combustion chamber, starting the engine.
Now, before you get into testing your coil, you need to know what you’re dealing with:
3 Ignition Coil Types: Know Which One You’re Testing
Ignition coils come in different shapes and sizes depending on the car:
A. Single Coil
With a single ignition coil, the amplified voltage goes through the coil wire (a high-voltage wire) into a distributor. From there, it reaches each spark plug through another high-voltage wire, known as the ignition cable or spark plug wire.
The single ignition coil (also known as a canister coil) is often found in older vehicles.
B. Ignition Block Coil Pack
The high voltage from an ignition coil pack passes through a spark plug wire to the spark plug.
This pack contains multiple ignition coils and is typically used in “waste spark” systems, where one coil fires two cylinders.
They can typically be found in older models.
C. Coil-on-Plug (COP)
The coil-on-plug sits directly on top of the spark plug, allowing each cylinder its own ignition coil. The secondary voltage is delivered directly to the spark plug, eliminating any wires for a strong spark in the combustion chamber.
If you have a modern vehicle, you’re more likely to have a COP system.
This type of ignition coil is sometimes called a pencil coil because of its shape.
NOTE: In some vehicles with coil pack(s) or COPs, an ignition module triggers the coils. Sometimes, the problem is a bad ignition module instead of a faulty coil.
Now that you know your type of coil, let’s see what equipment you require to diagnose an ignition coil issue.
What Tools Do You Need for an Ignition Coil Test?
Before you or a mechanic begins testing ignition coils, make sure these tools are present:
- Common hand tools such as socket wrenches and screwdrivers.
- The owner’s manual to locate ignition coils and check testing specs.
- A multimeter to measure resistance.
For detailed diagnostics, you may need the following:
- An oscilloscope to analyze the coil’s voltage output in detail.
- An OBD-II scanner to help identify trouble codes related to ignition system failures.
- A spark tester to check for a strong spark.
Once you’ve gathered the necessary tools, let’s review the testing process.
How to Test an Ignition Coil
There are several ways to test for ignition coil issues. Let’s go over four common methods:
1. Visual Inspection
Here’s what to do:
- Use the tools to remove the cover from the engine block to access the ignition coils.
- Inspect the coil, ignition cable, and coil wire (if applicable) for visible damage, such as cracks, burns, or melting.
- If you notice visible damage, the ignition coil likely needs to be replaced.
- If the coils and wiring are fine, move on to other methods.
2. Coil Swap
The easiest way to test a faulty coil is to swap it with a known, good coil. If your check engine light is on, a code reader can help identify which cylinder (and, as such, which ignition coil) may have issues.
Here’s what to do:
- If you have a single coil or use a block ignition coil pack, you can swap out the faulty ignition coil with a spare to see if the issue reoccurs.
- If you have coil-on-plugs or multiple coil packs (like a V6 engine with 3 coil packs), you can swap the suspect coil with one from a different cylinder if you know which one is misfiring. For example, if cylinder 2 is misfiring, you can swap the coil with a good one from cylinder 4. Then, see if the issue reoccurs on cylinder 2 or moves to cylinder 4.
If the issue persists after swapping coils, the problem may not be ignition coil failure. A faulty spark plug, a widened spark plug gap, or a damaged electrical connector or spark plug wire can display similar symptoms. It could even be something not directly related to the ignition system, such as a clogged fuel injector, a damaged fuel pump, or a faulty fuel pump relay.
3. Power Balance Check (For COP)
The power balance check is a fairly standard ignition coil test on older engines using COPs with 2 pins. However, it’s not advisable to do this on an engine block using newer COPs with 3 or more pins, as this test can damage these ignition coils.
IMPORTANT: Please take safety precautions to prevent an electric shock during this test.
Here’s what to do:
- Turn on the engine.
- Unplug the suspect ignition coil.
- If the RPM drops and becomes rough, that ignition coil is OK
- If the RPM doesn’t change, then you have a bad ignition coil.
4. Using a Multimeter
You only need a multimeter for this ignition coil test. Weak coils may still pass because they’re not tested under a load, but this method can help detect an ignition problem.
Here’s what to do:
- Identify the touchpoints for the coil you’re resistance testing. The primary circuit and secondary circuit will have different test points.
- Look up the resistance measurement values that you need. Your car manual should state it, but if it doesn’t, here are some typical values:
- For a 4-stroke ignition coil (cars, bikes, lawnmowers):
- Primary winding (primary coil) = 0.5 – 2.5 ohms
- Secondary winding (secondary coil) = 2500 – 5000 ohms
- For a 2-stroke ignition coil (smaller applications like chainsaws or boat motors):
- Primary winding = 0.5 – 2.5 ohms
- Secondary winding = 5000 – 10,000 ohms
- For a 4-stroke ignition coil (cars, bikes, lawnmowers):
Pro TIP: If you don’t know the specifications, you can test the resistance on similar coils from a different cylinder for comparison.
- Prep the coil for resistance testing. Remove all rust, oil, and debris. If possible, detach all connections (high-tension lead, kill wire)—we want to remove all likely elements other than the coil itself that could contribute to high electrical resistance.
- Set the multimeter to ‘Resistance’.
- To test the primary winding, dial down to the lowest resistance setting available (e.g., 200 ohms).
- For secondary windings, set the resistance setting to 20 kiloohms (20,000 ohms)
- If you have an auto-ranging multimeter, you don’t have to worry about the resistance setting.
- Touch the two leads together and ensure there’s no audible beep as this confirms continuity. The resistance value on your multimeter should be zero.
- If there’s some resistance (e.g., 0.2 ohms), remember to deduct this value from the tested coil resistance later.
- Touch the relevant touchpoints and check the reading. Resistance isn’t directional, so it doesn’t matter which lead you apply to which touchpoint.
If the resulting readings are a little out of range, it’s still fine and likely won’t be the cause of spark issues. For example, you may get a primary winding value of 7 ohms when its required range is 0.5-2.5 ohms. This is still acceptable. However, a resistance reading at 100 ohms could imply a coil problem.
Ultimately, extremely high or low resistance readings compared to the required range, or an open circuit/open loop on the multimeter, indicate an ignition coil issue.
NOTE: On some ignition coils, you may get inconsistent (or very high) readings when testing the primary winding. This could be due to electronic components within the coil, like diodes, transistors, and resistors, contributing to the reading. These are made of semiconductor material that drastically increases the resistance on the primary coil.
Some causes of bad multimeter readings:
- Damaged primary or secondary windings.
- The multimeter is set incorrectly to continuity instead of resistance mode.
- Semiconductor material in electronic circuitry increases the electrical resistance.
- Resistor-type spark plug boots.
Now that we’ve covered the simpler methods for ignition coil testing, let’s go over a few more.
Other Ways to Test the Ignition Coil
Failing coils can still fire a weak spark and may only produce misfires under load. Testing ignition coils under load is a more accurate way of verifying issues, such as using a spark tester or inductive COP testers that don’t even require removing the COP.
Here’s how you or your mechanic can use a spark tester to diagnose an ignition problem:
- Connect the spark tester to the coil and a grounded part of the engine.
- Start the engine or crank it. Watch the spark tester’s gap for a strong, blue spark.
- If no spark or a weak spark is visible, the ignition coil is likely faulty and needs replacement.
NOTE: When using a tool like a spark tester, you’re testing the coil while your car is running. Take preventive measures to avoid getting an electric shock.
There you have it! All the ways you can test your ignition coil.
Now that that’s out of the way, let’s go over some failing coil symptoms
7 Failing Ignition Coil Symptoms
Here are some things that happen if an ignition coil is failing:
- Engine dies when it’s hot
- Difficult to restart the engine when it’s hot
- Engine stutters at high RPMs
- Engine can’t reach full RPMs
- Starting the engine is inconsistent
- Poor fuel economy, as the engine struggles to burn fuel efficiently.
- Loud banging or popping noises from the exhaust.
Remember that other problematic ignition components (spark plugs, electrical connector wires, etc.) can also cause these symptoms.
This brings us to our final question:
How Long Does an Ignition Coil Last?
Ignition coils typically last between 80,000 and 150,000 miles, but their lifespan can vary depending on driving conditions, the make and model of your vehicle, and the quality of the ignition coil itself.
It’s best to get your ignition system checked by a mechanic after 80,000 miles.
Keep Your Ignition Coils Firing with AutoNation Mobile Service
Your ignition coils are vital to keeping the engine running, but when a coil problem arises, diagnosing it can be challenging, as ignition system issues can stem from various sources.
Fortunately, our professional mechanics at AutoNation Mobile Service have the necessary equipment to conduct a thorough inspection.
We’re a mobile car repair and maintenance solution, available 7 days a week. We offer an easy online booking system, and a 12-month | 12,000-mile warranty covers all repairs and maintenance.
Contact us, and we’ll send our best mechanics to help you, right in your driveway!