How to Identify a Faulty Ignition Coil?
3 Answers
Methods to identify a faulty ignition coil are: 1. Replace the suspected faulty ignition coil with a known good one—if the issue disappears, it indicates the original coil is faulty; 2. Use a multimeter to measure the resistance of the primary and secondary windings. If the reading shows infinite resistance, it means there is an open circuit in the winding. If the resistance is abnormally high or low, it indicates poor contact or inter-turn short circuits in the winding. The ignition coil is located in the engine compartment, directly connected to the spark plug at its upper part. Symptoms of an aging engine ignition coil include: adhesive overflow, cracking, connection tube issues, and erosion of the high-voltage terminal.
With decades of experience in auto repair, I've got a few practical methods to check ignition coils. First, start the engine and unplug each ignition coil connector one by one. If the engine shakes noticeably worse when you unplug a specific cylinder's coil, that coil is working fine. But if there's hardly any change in engine performance after unplugging, that cylinder's ignition coil is most likely faulty. Another way is to measure resistance with a multimeter: primary coil resistance typically ranges from 0.8 to 1.5 ohms, while secondary coils should read between 5 to 15 kilohms. If measurements show significant deviation - like a secondary coil reading over 30 kilohms - it's almost certainly defective. Some coils work fine initially but fail after driving a few kilometers when hot; if the coil casing feels unusually hot to touch, that's a warning sign. Finally, always inspect the coil casing for cracks or burn marks - the high-voltage wire connector area is prone to aging and current leakage, causing cylinder misfires, engine shaking, and jerky acceleration.
Last time my car had weak acceleration and was shaking, the mechanic taught me how to check the ignition coil. Remove the ignition coil, connect it to the spark plug in the same cylinder, and ground the metal part of the spark plug against the engine's iron casing. Then have someone help start the engine to see if the spark plug can produce a strong blue-white spark. If the spark is very weak or completely absent, the ignition coil is probably dead. Another method is to swap the suspected faulty ignition coil to a normally functioning cylinder. If that cylinder also fails to ignite, it confirms the ignition coil is to blame. You can also pay attention to the engine check light and use an OBD scanner to read fault codes. Codes like P0301 for a specific cylinder misfire are clues. During regular maintenance, have them check the ignition coil connectors for oil contamination or corrosion, as poor contact can also cause similar issues.
