ASE A1 Engine RepairFuel, Emissions, and Ignition Systems (General)Hard
A vehicle with distributorless ignition system (DIS) experiences a misfire on cylinder #2. The technician determines the spark plug is good. Using a scope, the technician observes that the primary ignition coil waveform for cylinder #2 is flat. What is the most likely cause?
- AA grounded spark plug wire for cylinder #2.
- BA faulty engine control module (ECM) providing an incorrect trigger signal.
- CAn open in the primary circuit to the ignition coil for cylinder #2.
- DA shorted secondary (high voltage) winding in the coil pack.
Show answer & explanationAnswer & explanation
Correct answer: C. An open in the primary circuit to the ignition coil for cylinder #2.
A flat primary ignition coil waveform indicates that the coil is not being energized and de-energized, meaning no current is flowing through the primary winding. This is most likely caused by an open circuit in the primary side, preventing the ECM from switching the coil's ground or power. A shorted secondary would still show primary activity, and a faulty ECM would likely affect multiple cylinders or show a different waveform.
Why the other options are wrong
- A. A grounded spark plug wire affects the secondary circuit and would likely cause misfire but would still show a primary waveform as the coil attempts to fire.
- B. A faulty ECM might not send a trigger, but the waveform might show the power side active if the ground side is controlled by the ECM, or it could show no activity if power isn't supplied. A flat line typically points to a direct circuit interruption.
- D. A shorted secondary winding would likely still show primary activity, but with a different 'spark line' or duration, not a flat line.
Primary Ignition Waveform
A graphical representation of the voltage and current in the primary winding of an ignition coil, used to diagnose ignition system faults.
- Shows coil saturation, inductive kick, and dwell time.
- Flat line indicates no primary circuit activity.
- Helps pinpoint issues in power, ground, or switching circuits.
Memory trick: Waveform flat? Primary's dead, check the wires, or lose your head!