A technician is using an oscilloscope to diagnose an intermittent misfire on a vehicle. The technician connects the oscilloscope to the primary ignition circuit of a cylinder and observes the waveform. During the misfire event, the primary voltage trace shows a significantly lower peak voltage during the 'coil turn-on' phase and a much shorter spark line duration compared to a good cylinder. What does this waveform pattern MOST likely indicate?
- AAn open circuit in the primary ignition coil winding.
- BA shorted secondary ignition wire or spark plug.
- CA faulty oxygen sensor causing an incorrect air-fuel mixture.
- DLow primary circuit voltage supply to the ignition coil.
Show answer & explanationAnswer & explanation
Correct answer: B. A shorted secondary ignition wire or spark plug.
A lower peak primary voltage during the 'coil turn-on' phase and a shorter spark line duration indicate that the coil is not building or sustaining enough energy to produce a strong, long-duration spark. This pattern is characteristic of a 'short to ground' or 'shorted' condition in the secondary ignition circuit (high voltage side), such as a shorted spark plug or secondary wire, which allows the high voltage to dissipate too quickly, preventing proper spark formation and duration.
Why the other options are wrong
- A. An open primary winding would typically result in no primary current flow and thus no spark at all, not a reduced spark.
- C. While an incorrect air-fuel mixture can cause a misfire, it would not directly alter the primary ignition waveform in this specific manner (lower peak, shorter spark line). It would affect combustion, not the coil's electrical characteristics.
- D. Low primary circuit voltage supply would affect the overall voltage levels but might not specifically cause the 'shorter spark line' characteristic as prominently as a secondary short.
Ignition Primary Waveform (Shorted Secondary)
An ignition primary waveform exhibiting a lower peak voltage during coil turn-on and a shorter spark line duration suggests a short circuit in the secondary ignition system (spark plug, wire), preventing proper high voltage buildup and discharge.
- Lower primary peak voltage indicates less energy stored/released.
- Shorter spark line shows rapid discharge of secondary voltage.
- Often caused by carbon tracking, fouled spark plugs, or damaged plug wires.
Memory trick: Low peak, short spark, secondary short leaves its mark.