What is the best way to test an ignition coil?

An ignition coil is a crucial component in any gasoline-powered engine. It transforms the vehicle’s 12-volt electrical system into the thousands of volts needed to create an electrical spark in the spark plugs to ignite the fuel-air mixture in the engine’s cylinders. Testing ignition coils is an important part of routine engine maintenance and diagnosing engine performance issues. There are several methods for testing ignition coils depending on the available tools and the specific coil design.

Purpose of an Ignition Coil

Inside an ignition coil are two coils of wire, a primary coil and a secondary coil, wrapped around an iron core. When power from the battery flows through the primary coil, it creates a magnetic field in the core. As this magnetic field collapses, it induces a high voltage in the thousands of turns of wire in the secondary coil. This high voltage travels out of the ignition coil to the distributor or coil pack, then to the spark plugs. It jumps the gap at the spark plug, creating the spark for ignition.

The ignition coil serves as an electrical transformer, stepping up the lower battery voltage to the high voltage needed for the spark plugs. It allows the lower 12-volt current in the vehicle’s electrical system to generate a spark across the spark plug’s gap, which can be 20,000 volts or more.

Symptoms of a Bad Ignition Coil

Some common symptoms that indicate a faulty ignition coil include:

• Misfiring – This occurs when the coil fails to provide adequate voltage to the spark plug, preventing it from sparking properly.
• Rough idle – The engine may run unevenly or erratically due to misfiring.
• Loss of power – Performance will suffer as misfiring worsens.
• Poor fuel economy – Misfires waste fuel, lowering gas mileage.
• Difficulty starting – Weak coils struggle to generate sufficient voltage for starting the engine.

These symptoms could also be caused by other ignition system problems, so testing the coil is necessary for diagnosis.

Methods for Testing Ignition Coils

There are several DIY methods that can be used at home to test an ignition coil’s operation:

Spark Test

This basic test determines if the coil is capable of producing a spark:

1. Disconnect the ignition coil from the spark plug.
2. Use an insulated jumper wire to connect the coil’s output terminal to a good ground source on the engine.
3. Have an assistant crank the engine while you watch for a spark at the coil output terminal.

If a strong, blue spark is visible, the coil is functioning properly. No spark indicates a bad coil.

Multimeter Resistance Test

Testing the coil’s primary and secondary resistance values with a multimeter can detect internal faults:

1. Remove the coil from the vehicle.
2. Set the multimeter to ohms and connect the leads to the coil’s primary terminals.
3. Check that the reading is within manufacturer specifications, often 0.5-2 ohms for primary resistance.
4. Connect the meter leads to the secondary terminals and note this reading, typically 6,000-30,000 ohms.

If either reading is noticeably outside the specification range, the coil has an internal fault and should be replaced.

Output Test

A specialized ignition coil tester can measure the coil’s voltage output:

1. Connect the tester following manufacturer directions and activate it per instructions.
2. Crank the engine and observe the voltage reading on the tester.
3. A good coil should generate at least 20,000-30,000 volts or more.
4. Lower voltage readings indicate a weak coil requiring replacement.

This requires using a professional coil tester but provides quantitative voltage measurements to definitively evaluate the coil.

Professional Diagnostic Testing

For more thorough ignition coil testing, the vehicle can be taken to a professional mechanic shop. Trained technicians have additional diagnostic tools to thoroughly test the coil and ignition system.

Coil-on-Plug Testing

Many modern engines use coil-on-plug (COP) ignition coils mounted individually on each spark plug. Testing them involves:

• Visually inspecting coils for damage.
• Checking electrical connectors for corrosion.
• Measuring coil primary and secondary resistance values with a multimeter.
• Using an oscilloscope to check the coil secondary waveform and voltage output.
• Performing a cylinder power balance test to identify weak coils.

Distributor Coil Testing

Older vehicles often use a single distributor ignition coil. Tests for it include:

• Inspecting distributor cap and rotor for wear or damage.
• Checking coil primary and secondary resistances.
• Testing coil output voltage with specialized tools.
• Evaluating pickup coil and magnetic sensor performance.
• Measuring trigger signals with an oscilloscope.

The technician can also conduct crank and cam sensor tests, engine compression tests, and other system checks to gain a complete picture of the ignition and engine condition.

When to Replace an Ignition Coil

Here are some general guidelines for when to replace an ignition coil:

• The coil fails any diagnostic tests described above.
• You observe visible damage like cracked casing or burnt wiring.
• The engine has persistent spark-related issues like misfires, rough idle, or poor performance.
• The coil reaches the age or mileage limit recommended by the manufacturer, often around 100,000 miles.

For preventative maintenance, many technicians advise replacing the coils at major service intervals to avoid being stranded by unexpected coil failures. Bad coils often affect multiple cylinders, so consider replacing all coils rather than just one.

Ignition Coil Replacement

Use the following general process when replacing an ignition coil:

1. Disconnect the negative battery terminal.
2. Remove any components blocking access to the coils.
3. Label and disconnect any electrical connectors.
4. Remove any mounting fasteners to detach the coil from the engine.
5. Install the new coil and reconnect all connectors.
6. Reinstall any components that were removed.
7. Reconnect the battery.

Consult a vehicle repair manual for the proper coil replacement procedure for your specific vehicle. Use care when disconnecting components and note the position of any small parts. Compare old and new coils to transfer any necessary parts over to the new coil.

In some cases, bad coils may result from other underlying problems. Be sure to address any other issues found during testing to prevent new coils from soon failing again.

Conclusion

Regularly testing and replacing worn ignition coils is essential for maintaining engine performance and preventing breakdowns. Use a spark tester or multimeter for DIY checks, or take advantage of professional diagnostic services for more in-depth evaluations. Replace any coil that fails testing or shows outward signs of damage or significant mileage. Following the manufacturer’s recommendations and specified replacement intervals can help avoid being left with a no-start situation when a coil fails unexpectedly.

Test Method	What It Tests	Required Tools
Spark test	Coil’s spark output	Jumper wire, helper
Multimeter resistance	Primary and secondary resistance	Multimeter
Coil output test	Coil voltage output	Ignition coil tester
Professional diagnostics	Comprehensive testing	Shop tools like oscilloscope

Having the right ignition coils in good working order is crucial for an engine to start and run properly. Following scheduled testing and replacement procedures will help ensure your vehicle’s ignition system remains in peak condition.