Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a P0351 trouble code (Ignition Coil 'A' Primary/Secondary Circuit Malfunction). Using an oscilloscope, they observe a normal primary ignition waveform but no corresponding secondary ignition waveform. What is the most likely cause of this issue?
- AA short circuit in the primary winding of the ignition coil.
- BAn open circuit in the secondary winding of the ignition coil.
- CHigh resistance in the spark plug wire to cylinder #1.
- DA faulty ignition coil driver in the Powertrain Control Module (PCM).
Show answer & explanationAnswer & explanation
Correct answer: B. An open circuit in the secondary winding of the ignition coil.
A normal primary waveform indicates that the PCM is commanding the coil to fire and the primary circuit is functioning. The absence of a secondary waveform, despite a good primary, points to an issue within the coil's secondary circuit, most commonly an open winding.
Why the other options are wrong
- A. A short in the primary winding would typically result in an abnormal primary waveform, such as a missing or very low inductive kick.
- C. High resistance in a spark plug wire would cause a misfire or a degraded secondary waveform, but not a complete absence of the secondary waveform if the coil is producing voltage.
- D. A faulty coil driver would prevent the primary circuit from being energized, leading to an abnormal or absent primary waveform.
Ignition Coil Secondary Open
An open circuit in the secondary winding of an ignition coil prevents the high voltage necessary for spark plug firing, even if the primary circuit functions correctly.
- Primary waveform appears normal.
- No secondary voltage or spark is produced.
- Common cause of misfires or no-start conditions.
Memory trick: Primary's fine, but secondary's silent, no spark for the engine's plight.