Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairHard
A technician is diagnosing a vehicle with an intermittent misfire under heavy load. They suspect a weak spark due to a failing ignition coil. Using an oscilloscope, they observe that the coil's inductive kick (firing voltage) is consistently high (e.g., 25kV-30kV) while the spark line duration is relatively short (e.g., 0.5ms). All spark plugs are new. What does this combination of high firing voltage and short spark duration most likely indicate?
- AA rich air-fuel mixture causing high resistance at the spark plug.
- BAn open primary circuit in the ignition coil.
- CExcessive resistance in the secondary circuit, possibly due to a faulty spark plug boot or carbon track.
- DA shorted secondary winding in the ignition coil.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive resistance in the secondary circuit, possibly due to a faulty spark plug boot or carbon track.
High firing voltage combined with a short spark duration, especially with new spark plugs, typically signifies excessive resistance in the secondary circuit. The coil must build higher voltage to jump the increased resistance, but this resistance also limits the current flow, causing the spark to extinguish quickly.
Why the other options are wrong
- A. A rich air-fuel mixture would typically cause a lower firing voltage and a short spark duration, not a high firing voltage.
- B. An open primary circuit would result in no spark at all, as the coil wouldn't be able to build any energy.
- D. A shorted secondary winding would result in very low or no firing voltage and a very short or absent spark.
Secondary Waveform High kV Short Duration
An oscilloscope pattern in secondary ignition that shows a high firing voltage (kV) but a short spark duration, often indicating excessive resistance in the secondary circuit.
- Coil works harder to overcome resistance.
- Resistance limits current, shortening spark.
- Commonly caused by faulty wires, boots, or plugs.
Memory trick: High voltage, short spark: The coil's 'struggle' to 'jump' a 'gap'.