Forget testing ignition coils. First of all, we charge for our time, and customers don't like being charged to play around with testing stuff. Second, what test results would you expect? Service manuals provide resistance values that apply to testing with an ohm meter, but those are for reference only. A typical value might be 200 to 500 ohms, which is a real broad range. If a coil is good at 499 ohms, is it bad if it reads 501 ohms? It can't change by only two ohms without there being some other major problem, and that is going to result in a huge change in resistance, typically an open circuit, meaning infinite resistance.
More commonly an arc can occur inside the coil. Any place arcing occurs, a carbon track is left behind, and that carbon conducts electricity, so further arcing continues to occur. You'll never read that carbon with an ohm meter, so what's the point of testing it? The coil will test fine, and you'll overlook the cause of a misfire.
If you have the luxury of owning a diagnostic scope, you can watch the wave forms from the ignition coils to determine if they're developing sufficient voltage. The faster test is to swap the coils between the misfiring cylinders and some that aren't misfiring, erase the fault codes, then drive the vehicle and see if misfire codes set for the cylinders you moved the suspect coils to.
Tuesday, March 14th, 2017 AT 3:57 PM