I'll let someone else handle your other question. For the ignition coils, I don't know of any mechanic who actually measures the resistance other than to verify one has an open coil. First of all, they go by symptoms. Simply due to the way the coils are wound, it is very unlikely the two ends will short together. The only other thing that can happen is a wire corrodes or breaks off causing an open circuit. That would read "infinite" on your ohm meter. You don't have that condition either, so electrically the coil is good.
In electronics, unless something is specified otherwise, the industry standard is plus or minus ten percent. Assuming your other coils are within specs, the one in question just needs to be within plus or minus ten percent of the good ones. Even brand new coils will all read a little different.
Most of the time when a coil fails it still reads the correct resistances. It's the insulation that breaks down and lets the high voltage cause a spark internally. Once it occurs inside the coil, it won't occur at the spark plug. Carbon tracks are left behind where arcing occurs, and that carbon is a conductor, so once it occurs, it will tend to continue and be real easy to find.
If the insulation breaks down just around some of the loops of wire, that part of the coil can short leaving just the remainder to generate the high voltage. Once the maximum voltage that can be achieved is lower than what is needed for the spark to jump the spark plug's gap, a misfire occurs. That seems to be the cause of all the trouble Ford has been having with their coil-on-plug ignition coils. Here again, the secondary coil is going to measure the correct resistance. Ohm meters only have 6 or 9 volt batteries so they won't cause arcing in the coil. During operation the secondary will develop up to 20,000 volts or more, as needed. That's when the arcing will occur.
I suspect you're using an auto-ranging ohm meter. I have over a dozen digital meters that I used in tv / vcr repair, and I never used those. When your entire day is spent taking voltage and resistance readings, you don't have time to waste waiting for the meter to pick the right scale. I know what to expect for a reading so I set my meters to the proper scale, but more importantly, the few times I've used other people's auto-ranging meters, I made too many mistakes by not paying attention to which scale it selected. It's hard enough to concentrate on the circuit and my test results. It looks like that happened to you too. The secondaries will have thousands of ohms of resistance. I'm pretty sure you overlooked the "k" or that the meter selected the "x 1000" scale. That is not the issue in looking for your misfire, and that happens to everyone. Auto-ranging meters work perfectly fine. Just remember to double-check your results when you come up with a reading that appears to show a defect. This is especially important when reading a sensor voltage where 3.0 volts and 0.3 volts are both acceptable values but mean wildly different things.
Monday, February 17th, 2014 AT 3:56 PM