Ignition
Ignition - what the exam tests
Coil operation, dwell, available spark voltage, distributor versus distributorless versus coil-on-plug systems, HT leads, spark plug specification and reading, timing and knock control.
Principles
- Coil: primary winding about 0.5–1.0 Ω, secondary about 5–15 kΩ, turns ratio near 1:100, producing 20–40 kV.
- Collapsing primary current induces the HT pulse. Dwell is the time primary current flows: too short gives a weak spark, too long overheats the coil.
- Firing order and timing come from the distributor or the ECU map; advance rises with speed and light load, and retards when knock is detected.
- Plug gap is typically 0.8–1.1 mm; the heat range must suit the engine; torque to specification to protect alloy heads.
Testing
Use a spark tester for a consistent blue spark, measure coil and lead resistance against specification, inspect cap and rotor for carbon tracking, and read plug colour: light brown is normal, black and sooty is rich, white or blistered is lean or overheating.
Worked example
Misfire under load on one cylinder. Coil primary 0.7 Ω (spec 0.5–1.0 Ω) is fine; secondary reads 22 kΩ against a spec of 8–12 kΩ. The high secondary resistance limits available voltage, so the plug cannot fire under cylinder pressure - replace the coil. Reasoning: pressure raises the voltage needed to jump the gap, so a weak coil shows up first under load.
Traps that cost marks
- Measuring coil resistance with the coil hot, or with fingers across the meter probes.
- Blaming the plug for every misfire when low compression or a faulty injector gives the same symptom.
- Refitting HT leads in the wrong firing order.
- Setting plug gaps by eye instead of with a gauge.
- Touching HT circuits while the engine is cranking.