How To Check Ignition Coils With A Multimeter

Checking your ignition coils with a multimeter is a straightforward process that can help you diagnose engine problems like misfires, rough idling, and poor fuel economy. A faulty ignition coil can prevent the spark plug from firing correctly, leading to these performance issues. By using a multimeter, you can test the coil's resistance and identify if it's functioning within the manufacturer's specifications.

Why Checking Ignition Coils Matters

The ignition coil is a critical component in your car's ignition system. Its job is to transform the low voltage from the battery into the high voltage needed to create a spark at the spark plug. This spark ignites the air-fuel mixture in the cylinder, powering the engine. When an ignition coil fails, the cylinder doesn't fire, resulting in a misfire.

Misfires can lead to several problems:

• Reduced Engine Power: A misfiring cylinder means less overall power output from the engine.
• Poor Fuel Economy: The unburned fuel from the misfiring cylinder gets wasted, lowering your gas mileage.
• Rough Idling: The engine will shake and vibrate noticeably when idling due to the uneven firing.
• Check Engine Light: The car's computer (ECU) will likely detect the misfire and trigger the check engine light.
• Catalytic Converter Damage: Prolonged misfires can dump raw fuel into the exhaust system, potentially damaging the catalytic converter, a costly repair.

Identifying a faulty ignition coil early on can prevent these issues. While a mechanic can diagnose the problem, learning to test the coils yourself with a multimeter can save you time and money. It's a relatively simple diagnostic procedure you can perform at home.

How to Choose the Right Multimeter

For testing ignition coils, you don't need a high-end, expensive multimeter. A basic digital multimeter (DMM) with the following features will suffice:

• Resistance Measurement (Ohms): This is the most important feature. You'll use it to measure the primary and secondary resistance of the coil.
• Auto-Ranging (Recommended): An auto-ranging multimeter automatically selects the appropriate range for the measurement, making it easier to use.
• Continuity Test (Optional but Helpful): This feature can help you quickly check for broken circuits or shorts within the coil.
• Voltage Measurement (Volts): While not directly used for coil testing, a voltage measurement function is always useful for general automotive diagnostics.
• Accuracy: Look for a multimeter with reasonable accuracy (typically +/- 1% or better) for reliable readings.

Safety First: Choose a multimeter that is CAT III rated or higher. This indicates that the meter is designed to withstand transient voltage spikes that can occur in automotive electrical systems. Always follow the multimeter manufacturer's safety guidelines.

Budget: Decent multimeters suitable for automotive use can be found for under $50. Brands like Fluke, Klein Tools, and Innova offer reliable options, but many affordable alternatives exist.

How to Test Ignition Coils with a Multimeter

Here's a step-by-step guide on how to test your ignition coils:

1. Safety Precautions:
   • Disconnect the negative battery cable to prevent electrical shock or damage to the vehicle's electrical system.
   • Work in a well-ventilated area.
   • Allow the engine to cool down completely before working on it.
2. Identify the Ignition Coils: Locate the ignition coils. These are usually mounted directly on top of the spark plugs (coil-on-plug) or located near the engine with wires running to the spark plugs. Consult your vehicle's repair manual for specific locations.
3. Remove the Ignition Coils: Carefully disconnect the electrical connector from the ignition coil. Then, remove the coil from the engine. Some coils may be held in place by a bolt or screw.
4. Set the Multimeter: Set your multimeter to the Ohms (Ω) setting. If your multimeter is not auto-ranging, start with the lowest resistance range and increase it if necessary.
5. Test Primary Resistance: The primary winding is the low-voltage side of the coil.
   • Identify the primary terminals on the coil (usually two terminals).
   • Place one multimeter probe on each primary terminal.
   • Record the resistance reading.
   • Compare the reading to the manufacturer's specifications. These specifications can be found in your vehicle's repair manual or online. Typically, primary resistance is a very low value, often less than 1 ohm. A reading of 0 ohms (or very close) indicates a short circuit, while an infinite reading (or very high) indicates an open circuit.
6. Test Secondary Resistance: The secondary winding is the high-voltage side of the coil.
   • Place one multimeter probe on one of the primary terminals and the other probe inside the spark plug boot (where the spark plug connects).
   • Record the resistance reading.
   • Compare the reading to the manufacturer's specifications. Secondary resistance is typically much higher than primary resistance, often in the range of several thousand ohms (kΩ). Again, a reading of 0 ohms indicates a short circuit, and an infinite reading indicates an open circuit.
7. Interpret the Results:
   • If the resistance readings are within the manufacturer's specifications, the ignition coil is likely good.
   • If the resistance readings are outside the manufacturer's specifications (too high, too low, or infinite), the ignition coil is likely faulty and needs to be replaced.
8. Repeat for All Coils: Repeat steps 3-7 for each ignition coil in your engine.
9. Reinstall the Ignition Coils: If the coils tested good, reinstall them in the engine, ensuring the electrical connectors are securely attached.
10. Reconnect the Battery: Reconnect the negative battery cable.
11. Start the Engine: Start the engine and check for any improvement in performance. If the misfire persists, further diagnosis may be needed.

Real-World Owner Experiences

Many car owners have successfully used multimeters to diagnose ignition coil problems. Here are some common experiences:

• Identifying Intermittent Misfires: Some owners have reported using a multimeter to pinpoint ignition coils that were failing intermittently, causing occasional misfires. The coil might test okay when cold but fail when hot.
• Saving Money on Repairs: By diagnosing the problem themselves, owners were able to avoid expensive diagnostic fees at a repair shop and replace the faulty coil themselves, saving a significant amount of money.
• Preventing Further Damage: Early detection of a failing ignition coil prevented further damage to the catalytic converter and other engine components.
• Learning More About Their Cars: The process of testing ignition coils with a multimeter helped owners gain a better understanding of their car's engine and ignition system.

However, it's important to note that a multimeter can only test the resistance of the coil. It cannot detect all types of coil failures. Some coils may have internal shorts or other problems that are not detectable with a simple resistance test. In these cases, a more advanced diagnostic tool, such as an oscilloscope, may be needed.

FAQs About Ignition Coil Testing

Q: What are the symptoms of a bad ignition coil?

A: Common symptoms include engine misfires, rough idling, poor fuel economy, reduced engine power, and a check engine light.

Q: Can I drive with a bad ignition coil?

A: While you *can* drive with a bad ignition coil, it's not recommended. It can lead to further engine damage, reduced fuel economy, and potential damage to the catalytic converter.

Q: How often should I replace my ignition coils?

A: There's no set replacement interval for ignition coils. They should be replaced when they fail or show signs of wear. Many manufacturers recommend replacing them as a set when one fails, especially if they are all the same age and mileage.

Q: What if the coil tests good, but I still have a misfire?

A: If the coil tests good, the misfire could be caused by other issues, such as faulty spark plugs, a clogged fuel injector, a vacuum leak, or a problem with the engine's computer. Further diagnosis is needed.

Q: Can I swap ignition coils to diagnose a misfire?

A: Yes, swapping ignition coils is a common diagnostic technique. If you suspect a specific coil is bad, you can swap it with a coil from a different cylinder. If the misfire moves to the cylinder where the swapped coil is now located, it confirms that the coil is faulty.

Q: Where can I find the manufacturer's specifications for ignition coil resistance?

A: The manufacturer's specifications can be found in your vehicle's repair manual, online repair databases (like Alldata or Mitchell OnDemand), or by contacting a local auto parts store or dealership.

By following these steps and understanding the process, you can confidently test your ignition coils with a multimeter and diagnose engine problems at home, potentially saving time and money.