Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairHard
A technician is using an oscilloscope to diagnose an intermittent misfire on a vehicle with a Coil-On-Plug (COP) ignition system. They observe that the secondary ignition waveform for cylinder #3 occasionally displays a very short spark line duration, often followed by an immediate 'coil oscillation' without a proper spark event, while other cylinders show normal waveforms. What is the most likely cause of this specific waveform pattern?
- AAn open primary winding in the ignition coil.
- BHigh resistance or an open circuit in the secondary winding of the ignition coil or spark plug.
- CExcessive resistance in the spark plug wire of cylinder #3.
- DA short to ground in the primary circuit of the ignition coil.
Show answer & explanationAnswer & explanation
Correct answer: B. High resistance or an open circuit in the secondary winding of the ignition coil or spark plug.
A very short spark line duration followed by immediate coil oscillation, without a proper spark event, is a classic indication of an open or very high resistance in the secondary circuit. The coil builds voltage, but cannot find a path to discharge through the spark plug, leading to internal arcing or immediate collapse of the magnetic field.
Why the other options are wrong
- A. An open primary winding would prevent the coil from building any voltage, resulting in no spark at all.
- C. Excessive resistance in a spark plug wire (if present in a COP system) would typically show a higher firing voltage and potentially a shorter spark duration, but not necessarily an immediate collapse and oscillation without a proper event.
- D. A short to ground in the primary circuit would cause excessive current flow and likely damage to the coil or ECM, resulting in no spark, not a short spark followed by oscillation.
Secondary Ignition Waveform Interpretation (Open Secondary)
Analyzing oscilloscope patterns to identify faults in the high-voltage secondary ignition circuit, where a short spark duration followed by coil oscillation often indicates an open circuit.
- Short spark line indicates poor discharge path.
- Immediate oscillation means energy cannot dissipate.
- Points to issues in coil secondary or spark plug.
Memory trick: The spark's 'story' is told by its 'shape' and 'duration'.