Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A technician is testing a Coil-On-Plug (COP) ignition system on a vehicle experiencing an intermittent misfire. They use a spark tester to check for spark at the affected cylinder. The spark tester shows a weak, yellowish spark. What is the most likely reason for a weak, yellowish spark instead of a strong, blue-white spark?
- AA short circuit in the secondary winding of the coil.
- BInsufficient voltage or current in the secondary circuit.
- CExcessive primary current to the coil.
- DAn overly rich air/fuel mixture in the cylinder.
Show answer & explanationAnswer & explanation
Correct answer: B. Insufficient voltage or current in the secondary circuit.
A weak, yellowish spark indicates insufficient energy in the secondary ignition circuit. This is typically caused by inadequate voltage or current delivery from the coil, which can be due to high resistance, internal coil weakness, or low primary voltage/current.
Why the other options are wrong
- A. A short circuit in the secondary winding would lead to very low or no output, not typically a weak yellowish spark.
- C. Excessive primary current would likely lead to a stronger spark or coil damage, not a weak, yellowish one.
- D. An overly rich air/fuel mixture would primarily cause a fouled spark plug, leading to a misfire, but the spark itself might still be strong if the ignition system is healthy. The spark's color is more indicative of its energy level.
Spark Color Diagnosis
The color of the spark indicates its energy and heat: a strong, blue-white spark is healthy, while a weak, yellowish spark suggests insufficient voltage or current in the secondary circuit.
- Blue-white: strong, hot spark.
- Yellowish/Orange: weak, low energy spark.
- Weak spark causes misfires.
Memory trick: Blue is good, yellow is sad, the spark's energy is truly bad.