Ignition System Basics
Coils, plugs and the primary/secondary circuit.
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What you will learn
- Describe the ignition circuit
- Explain dwell and firing
- Recognise misfire causes
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The ignition system creates the high-voltage spark that ignites the petrol-air mixture. Current flows through the coil primary to build a magnetic field (dwell); when the ECU interrupts that current, the field collapses and the secondary winding produces the high voltage that jumps the plug gap.
Modern engines use coil-on-plug, waste-spark or COP systems controlled entirely by the ECU, timed from the crank and cam sensors. Ignition faults show up as misfires, rough running, poor economy or a no-start, and are best confirmed with a scope on the primary/secondary or by swapping components.
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Key points
- Dwell builds the field; interruption fires the spark.
- The ECU times ignition from crank and cam signals.
- Confirm ignition faults by measurement, not by swapping parts blindly.
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Key points
- The coil steps 12 V up to high voltage
- Dwell builds the field, collapse fires the plug
- Ignition faults cause misfires and no-starts
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
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