The ignition coil is a key component of your vehicle’s ignition system. It transforms the 12V battery power into the high voltage needed to ignite the fuel–air mixture in your cylinders. Knowing how to test an ignition coil ensures proper spark delivery, smooth engine operation, and prevents misfires or starting issues.
What Is an Ignition Coil?
An ignition coil is essentially a high-voltage transformer. It converts low voltage from the car’s battery (12V) into thousands of volts required by the spark plugs to ignite fuel.
Modern vehicles may use single coil systems for older cars, or coil-on-plug (COP) systems in newer engines like BMW, Toyota, or Ford, where each cylinder has its own ignition coil.
The ignition coil works with these components:
- Spark plugs – deliver the spark to ignite fuel.
- Distributor or ECU – controls timing and firing sequence.
- Battery and wiring – supplies low voltage.
A faulty coil can cause engine misfires, rough idling, and poor fuel economy.
How Does an Ignition Coil Work?
The ignition coil has two main windings:
- Primary winding – connected to the battery and ignition switch, carries low voltage.
- Secondary winding – generates high voltage (up to 40,000V) to fire the spark plug.
When the ignition system opens and closes the primary circuit rapidly, a magnetic field collapses inside the coil, inducing high voltage in the secondary winding. This voltage jumps across the spark plug gap, igniting the fuel–air mixture.
Proper coil function ensures consistent spark timing, smooth combustion, and reliable performance.
Common Symptoms or Signs of Ignition Coil Failure
A failing ignition coil can lead to noticeable performance issues, including:
- Engine Misfires or Rough Idle: One or more cylinders are not firing properly.
- Hard Starting: Weak or no spark prevents ignition.
- Reduced Fuel Efficiency: Poor combustion wastes fuel.
- Check Engine Light: Error codes like P0300–P0306 indicate misfires.
- Backfiring or Poor Acceleration: Erratic spark timing.
- Burnt Smell or Coil Overheating: Electrical faults generate heat.
If ignored, a bad coil can damage spark plugs, catalytic converters, and other engine components.
How to Test an Ignition Coil
Testing your ignition coil can be done with a multimeter or specialized coil tester. Here’s a step-by-step guide:
Tools Needed
- Digital multimeter
- Screwdrivers or socket set
- Spark tester (optional for visual testing)
1. Visual Inspection
- Remove the ignition coil.
- Inspect for cracks, corrosion, or carbon tracking.
- Check the wiring harness and connectors for damage.
2. Test Primary Resistance
- Set the multimeter to ohms (Ω).
- Connect probes to the coil’s primary terminals.
- Compare the reading with manufacturer specifications (usually 0.4–2 ohms).
- Infinite resistance or zero indicates a faulty coil.
3. Test Secondary Resistance
- Connect the multimeter between the coil output terminal and ground/negative terminal.
- Check the reading against specs (commonly 6,000–15,000 ohms).
- Outside this range = defective coil.
4. Spark Test (Optional)
- Use a spark tester connected to the coil output.
- Crank the engine — a strong, blue spark indicates the coil is functioning.
- Weak or no spark = replacement required.
5. Check Coil-on-Plug Systems
- Modern COP systems require a scan tool to monitor live data.
- Test for misfire patterns and cylinder-specific voltage drops.
Ignition Coil Replacement Cost in South Africa
Replacing a faulty ignition coil in South Africa typically costs:
- Standard coil (single coil): R400 – R900
- Coil-on-plug (COP) per cylinder: R800 – R1,800
.
Related Car Parts and Maintenance Tips
When testing or replacing an ignition coil, consider inspecting:
- Spark plugs: Old or fouled plugs reduce coil lifespan.
- Battery and alternator: Ensure proper voltage supply.
- Camshaft and crank sensors: Accurate timing is essential for proper spark.
- Throttle body: Keeps proper air–fuel mixture for combustion.
- Fuel injectors: Dirty injectors increase load on ignition system.
Other helpful guides include:
Regular inspection of these components ensures engine reliability and optimal performance.
Conclusion: Why Ignition Coil Maintenance Matters
A functional ignition coil is critical for engine starting, smooth idling, and efficient combustion. Knowing how to test an ignition coil can save you time and prevent misfires, costly repairs, or premature wear of spark plugs and the catalytic converter.
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