Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairEasy
A technician is diagnosing a vehicle with an intermittent misfire that seems to worsen during acceleration. Using a digital multimeter, they measure the resistance of a spark plug wire at 25,000 ohms. The manufacturer's specification for spark plug wire resistance is typically 5,000 to 12,000 ohms per foot. Assuming the wire is 1.5 feet long, what is the most likely conclusion regarding this spark plug wire?
- AThe spark plug wire has excessive resistance and should be replaced.
- BThe spark plug wire resistance is within specifications and is not contributing to the misfire.
- CThe spark plug wire has insufficient resistance and should be replaced.
- DThe measurement indicates a short circuit within the spark plug wire.
Show answer & explanationAnswer & explanation
Correct answer: A. The spark plug wire has excessive resistance and should be replaced.
A spark plug wire with 25,000 ohms of resistance significantly exceeds the typical maximum specification for its length, indicating excessive resistance. This excessive resistance impedes the flow of high voltage, leading to a weak spark and potential misfires, especially under load.
Why the other options are wrong
- B. The measured resistance is well above the typical maximum specification, indicating an issue.
- C. Insufficient resistance would generally be a very low reading, not 25,000 ohms.
- D. A short circuit would result in extremely low or no resistance, not a high resistance reading.
Spark Plug Wire Resistance
The electrical opposition within a spark plug wire, crucial for delivering high voltage to the spark plug. Excessive resistance can cause misfires.
- Typically measured in ohms per foot.
- High resistance impedes voltage delivery.
- Can vary with wire length and material.
Memory trick: Resistance reveals if the spark plug wire is ready to deliver the spark.