A technician is performing a diagnostic on a vehicle experiencing an intermittent misfire under heavy acceleration. Using an oscilloscope, they observe the secondary ignition waveform. On the misfiring cylinder, the spark line (firing line) is significantly shorter and lower in voltage than normal, and the spark duration is reduced. What does this waveform pattern MOST likely indicate?
- AAn open circuit in the spark plug wire or secondary coil winding.
- BA shorted ignition coil primary winding.
- CExcessive resistance in the primary ignition circuit.
- DFouled spark plug or excessive resistance in the secondary circuit.
Show answer & explanationAnswer & explanation
Correct answer: D. Fouled spark plug or excessive resistance in the secondary circuit.
A short spark line (low firing voltage) and reduced spark duration typically indicate that the ignition system is having difficulty finding a path for the high voltage or that the path has high resistance. This is characteristic of a fouled spark plug, a wide spark plug gap, or excessive resistance in the secondary circuit (e.g., a high-resistance spark plug wire), which requires more voltage to jump the gap but then quickly extinguishes the spark.
Why the other options are wrong
- A. An open circuit in the secondary would result in an extremely high firing voltage spike (often off-screen) with little to no spark line/duration, as the voltage has no path to ground.
- B. A shorted primary winding would prevent the coil from building sufficient magnetic field, resulting in very low or no kV output across the board, not just a short spark line.
- C. Excessive primary resistance would typically reduce the energy stored in the coil, leading to a lower overall kV output, but might not specifically shorten the spark line and duration in this manner.
Secondary Waveform: Short Spark Line/Duration
A short spark line and reduced spark duration on a secondary ignition waveform typically indicate excessive resistance in the secondary circuit, a fouled spark plug, or a wide spark plug gap.
- The ignition system struggles to deliver consistent energy to the spark gap.
- Often associated with misfires under load when higher voltage is needed.
- Can be caused by carbon fouling, worn electrodes, or high-resistance wires.
Memory trick: Waveform tells the spark's life story.