Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A technician is investigating a vehicle that stalls intermittently and has difficulty restarting when hot. The primary ignition coil resistance measures within specifications when cold, but after the engine stalls, the resistance is found to be significantly higher than specified. What is the most likely cause of this issue?
- AAn internal thermal breakdown or increased resistance in the primary winding of the ignition coil.
- BA short to ground in the primary wiring harness.
- CA faulty ignition control module (ICM) that fails when hot.
- DAn open circuit in the secondary winding of the ignition coil.
Show answer & explanationAnswer & explanation
Correct answer: A. An internal thermal breakdown or increased resistance in the primary winding of the ignition coil.
If the primary coil resistance increases significantly when hot, it indicates an internal fault within the coil's primary winding. This 'thermal breakdown' causes the coil to lose its ability to build a strong magnetic field, leading to a weak or absent spark when hot, and prevents restarting until it cools down.
Why the other options are wrong
- B. A short to ground would typically result in very low resistance or a blown fuse, not significantly higher resistance when hot.
- C. While an ICM can fail when hot, the question specifies a change in the ignition coil's primary resistance, pointing directly to the coil itself.
- D. An open secondary winding would prevent spark, but the question specifically refers to the primary resistance changing with heat.
Ignition Coil Thermal Breakdown
A condition in an ignition coil where its internal electrical resistance changes dramatically (typically increases) when subjected to heat, causing intermittent misfires or no-start conditions when hot.
- Resistance changes with temperature.
- Often causes hot-start or hot-stall issues.
- Requires testing when hot to diagnose.
Memory trick: The 'hotter' the coil, the 'higher' its 'resistance', the 'weaker' the spark.