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Why a multimeter is not the best way to test an ignition coil.

 

Ignition coil test with multimeter

 

When it comes to testing ignition coils, the first thing we all want to do is grab our multimeters to check the resistance. That method will only tell if there is something seriously wrong with the coil (aka if it's dead). If there is an ignition coil that is occasionally misfiring, testing the resistance with a multimeter does no good. Modern ignition coils typically have an epoxy resin that surrounds the actual coil windings. As the coils age, this resin may crack. If you're lucky, you might see the crack from the exterior of the coil, making it easy to determine that it needs to be replaced. Other times, they may develop a hairline crack that is internal only, and not visible from the exterior. The crack usually expands when warm and cause a very occasional misfire that may not be enough for the main vehicle computer (ECU, PCM, ECM, etc) to recognize it as a consistent problem and illuminate a check engine light. This means the if the coil is cold, it may test out, and operate perfectly fine, but still misfire occasionally when the vehicle warms up, and the crack expands.  

 

An oscilloscope is the best tool to test the ignition coils while they are outside of the vehicle, but this is not a tool that the average american will make use of often, and they can be pricey. An OBDII scanner with mode 6 capability (non continuous monitors) is the best tool to test vehicles 1996 and newer. These mode 6 scanners usually start at around $100, and they can be used on all of the vehicles you own. Mode 6 uses hexadecimal computer language, so when accessing mode 6, you will notice things like the $ sign used instead of the # sign. This is a great method to test out your own personal vehicle, or even a vehicle you are looking to purchase. You can even find problems that even the dealer may not know about.