How do you test a spark plug coil?

Testing a spark plug coil is an important part of diagnosing issues with your vehicle’s ignition system. A bad coil can cause the engine to misfire, resulting in poor performance, reduced fuel economy and increased emissions. Here is a step-by-step guide on how to properly test your spark plug coils.

What is a Spark Plug Coil?

The ignition coil, also known as a spark plug coil, is an induction coil that converts the vehicle’s 12 volts into the several thousand volts required to create an electric spark in the spark plugs to ignite the fuel. Inside the coil are two coils of wire wrapped around an iron core.

When power flows into the primary coil, a magnetic field is created. When the current is interrupted, the collapsing magnetic field induces a high voltage in the secondary coil. This voltage jumps the gap in the spark plug, igniting the compressed air-fuel mixture in the combustion chamber.

Tools and Supplies Needed

Testing a coil requires a few basic tools and supplies:

• Multimeter
• Spark plug socket set
• Ratchet and extension
• Replacement coil (in case faulty one is found)
• Small piece of insulated wire (for ground)
• Shop rag

How to Test Coil Primary Resistance

The primary resistance test checks the electrical resistance of the primary windings inside the coil. Perform these steps:

1. Locate the coils on top of the spark plugs.
2. Remove all the coils by gently pulling up on them. Removing them will make it easier to take measurements.
3. Set the multimeter to read ohms (Ω).
4. Touch the meter probes to the two terminals on the coil.
5. Compare the reading to the factory specification, usually around 0.5-2 ohms.

If the reading is significantly higher than specification, the coil is likely faulty and should be replaced. Very low or no resistance indicates a shorted coil.

How to Test Coil Secondary Resistance

The secondary resistance test indicates if the secondary windings inside the coil have worn or broken wires. Follow these steps:

1. Check the service manual for the correct secondary resistance value, usually between 6,000-30,000 ohms.
2. Connect the meter probes between the large high tension terminal and one of the two primary terminals.
3. The reading should be within 10% of the factory spec. If not, the coil is defective.

How to Test Coil Output

This test checks the coil’s ability to provide adequate high voltage to the spark plug. You’ll need an insulated jumper wire for this.

1. Clip one end of the jumper wire to the negative (-) battery terminal.
2. Hold the other end about 1/2″ from the coil’s high tension terminal.
3. Crank the engine and check for a blue snappy spark across the gap.
4. Repeat test for all coils. A weak or lazy orange spark indicates a bad coil.

How to Do an On-Vehicle Coil Test

This test checks the coil’s functionality while still installed on the engine. It can identify coils not firing properly during operation.

1. Start the engine and let it reach operating temperature.
2. Use an insulated jumper wire connected to the negative battery terminal.
3. With the engine running, hold the grounded jumper wire about 1/2″ from each coil’s high tension terminal. A snappy blue spark should jump the gap each time that cylinder’s spark plug fires. Be extremely cautious not to touch the exposed wire.
4. A coil not producing a spark is defective and should be replaced.

How to Perform a Spark Plug Wire Leakage Test

Excessive voltage leakage in the spark plug wires can prevent sufficient voltage from reaching the spark plugs. Here’s how to check for leaking plug wires:

1. Turn off the engine and remove all spark plug wires from the plugs.
2. Clean the silicone boots at the end of the wires to ensure you have a good connection.
3. Attach one end of the jumper wire to the negative (-) battery terminal.
4. Turn the ignition on but don’t start the engine.
5. Hold the other end of the jumper about 1/2″ from the silicone boot at the end of each wire, one at a time.
6. A snappy blue spark should jump the gap. Little or no spark indicates the plug wire is leaking voltage. Replace any wires that fail the test.

Using a Spark Plug Tester

A spark plug tester is a convenient tool for testing coil output on most modern vehicles. It substitutes for the spark plug and features a light that flashes when it detects a spark. To use:

1. Remove a spark plug and thread the tester into the cylinder in its place.
2. Crank the engine while observing the tester light.
3. The tester light should flash brightly and consistently while cranking. No light or an intermittent/dim light points to a bad coil.
4. Repeat test for each cylinder.

Testing Coils with an Oscilloscope

A lab scope gives precise measurements of the coil’s voltage output and firing patterns. While not required, an oscilloscope can detect subtle issues not found by other tests. Hook up the scope by:

• Attaching the ground clip to the negative battery terminal.
• Connecting the scope lead to the coil’s high tension terminal.
• Cranking the engine and observing the coil’s waveform patterns.
• Checking that the peak voltage, rise time, and waveform shape match specifications.

Conclusion

Testing your ignition coils takes just a few tools and simple procedures. Start with basic resistance and output tests, followed by confirming operation while installed on the running engine. Check for leaking spark plug wires as well. If issues are found, replace the faulty coil and any deteriorated wires. Your engine’s performance and fuel efficiency rely on a robust ignition system.