Having difficulty starting your car? Your check engine light pops on?
The culprit could be a faulty ignition coil. A bad ignition coil can cause engine stalling, poor performance, and increased gasoline bills. However, 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:
- Ignition Coil Types: Know Which One You’re Testing
- How to Test an Ignition Coil
- Other Ways to Test the Ignition Coil
- Failing Ignition Coil Symptoms
Ignition Coil Types: Know Which One You’re Testing
Ignition coils come in different shapes and sizes, but all work the same way. Car batteries deliver 12V to the ignition coil, which converts that to a high voltage that fires the spark plug in the combustion chamber.
Each coil consists of a primary winding (primary circuit), secondary winding (secondary circuit), and iron core — working together to amplify the secondary voltage to 50,000V or more. This high voltage then goes to the spark plug:
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 called the ignition cable or spark plug wire. The single ignition coil is also called a canister coil.
B. Ignition Block Coil Pack
The high voltage from an ignition coil pack passes through a spark plug wire to the spark plug. These ignition blocks contain multiple coils and are typically used in “waste spark” systems, where one coil fires two cylinders.
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. 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 how you can test for ignition coil issues.
How to Test an Ignition Coil
There are several ways to test for ignition coil issues. Let’s go over three common methods:
1. 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.
Note: You may need to remove an engine cover 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 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 coil failure. A faulty spark plug, a widened spark plug gap, or a damaged connector or spark plug wire can display similar symptoms. It could even be something not directly part of the ignition system, like a clogged fuel injector, damaged fuel pump, or faulty fuel pump relay.
2. 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 engines 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 the ignition coil is faulty
3. 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 issues.
Here’s what to do:
- Identify the touchpoints for the coil you’re testing. The primary circuit and secondary circuit will have different test points.
- Look up the resistance 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
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 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 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 make sure there’s no audible beep—this ensures 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 problem.
Ultimately, extremely high or low resistance readings versus 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 resistance
- Resistor-type spark plug boots
We’ve covered the simpler methods for ignition coil testing. Here are 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 the coil under load is a more accurate way of verifying issues — such as using a spark tester (which lets you see the generated spark) or inductive COP testers that don’t even require removing the COP.
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. Also, make a point to check the spark gap while you have the spark plug out.
Let’s look at some signs pointing to a bad ignition coil.
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
Remember that other problematic ignition components (spark plugs, wires, etc.) can also cause these symptoms.
Keep Your Ignition Coils Firing with AutoNation Mobile Service
Your ignition coils are vital to keeping the engine running, but when problems crop up, diagnosing them can be tricky, as ignition system issues can stem from different directions.
Auto professionals have the equipment to do a thorough inspection.
If you need help, why not get in touch with AutoNation Mobile Service?
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.
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