Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a misfire on cylinder #4. They use a spark tester to check for spark at the spark plug wire (or COP unit). The spark tester shows a very weak, orange spark that struggles to jump the gap. The technician has already confirmed good power and ground to the coil, and the spark plug is new. What is the most likely cause of the weak spark?
- AHigh compression in cylinder #4.
- BA lean air-fuel mixture in cylinder #4.
- CAn open circuit in the primary winding of the ignition coil.
- DExcessive resistance in the secondary circuit of the ignition coil or spark plug boot.
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive resistance in the secondary circuit of the ignition coil or spark plug boot.
A weak, orange spark that struggles to jump a gap, despite a new spark plug and good power/ground to the coil, strongly suggests excessive resistance in the secondary ignition circuit. This resistance impedes the high voltage from reaching the spark plug with sufficient intensity.
Why the other options are wrong
- A. High compression would actually require a higher voltage to jump the spark plug gap, but the coil should still produce a strong, blue spark if healthy, not a weak, orange one.
- B. A lean mixture can cause misfires but wouldn't directly result in an observably weak, orange spark at the tester, which is a high-voltage issue.
- C. An open primary circuit would result in no spark at all, not a weak, orange one.
Weak Orange Spark Diagnosis
A visual indication during spark testing that suggests insufficient high voltage or excessive resistance in the secondary ignition circuit, leading to poor ignition.
- Healthy spark is strong, blue/white.
- Weak/orange indicates low energy.
- Often points to secondary circuit issues.
Memory trick: The 'color' and 'strength' of the spark 'reveal' its 'health'.