A technician is using an oscilloscope to analyze the secondary ignition waveform of a cylinder. The waveform shows a very short spark line duration (arc duration) and a high firing voltage (kV). All other cylinders show normal spark line duration and firing voltage. What does this specific waveform characteristic MOST likely indicate?
- AAn open circuit in the spark plug wire or a severely fouled spark plug.
- BA rich air-fuel mixture in that cylinder.
- CA loose connection at the ignition coil primary terminal.
- DA shorted ignition coil secondary winding.
Show answer & explanationAnswer & explanation
Correct answer: A. An open circuit in the spark plug wire or a severely fouled spark plug.
A high firing voltage indicates that the coil had to build up a large amount of voltage to overcome a high resistance or open circuit to initiate the spark. A very short spark line duration means the spark quickly extinguished, often because the path was too difficult to sustain or there was no fuel to burn (in the case of a severely fouled plug or open circuit). This combination points to a high resistance or open circuit in the secondary circuit, common with a faulty wire or extremely fouled plug.
Why the other options are wrong
- B. A rich air-fuel mixture typically leads to a lower firing voltage and a longer, more stable spark line duration due to easier ionization in the rich environment.
- C. A loose primary connection would cause an erratic or absent spark, but the specific pattern of high firing voltage and short duration points to secondary circuit issues.
- D. A shorted secondary winding would result in a very low or non-existent firing voltage and no spark, not a high firing voltage.
High kV Short Duration Spark
An oscilloscope waveform showing high firing voltage (kV) and a very short spark line duration, indicative of high resistance or an open circuit in the secondary ignition.
- High kV to jump large/difficult gap.
- Short duration due to inability to sustain spark.
- Common causes: bad wire, severely fouled plug, wide plug gap.
Memory trick: High voltage, short spark: open circuit's stark mark.