If you replaced the battery, you erased any fault codes when you disconnected it. Code 12 just means the battery was disconnected. That code will self-erase after 50 key starts. Also, the Engine Computer needs to relearn "minimum throttle" before it will know when it has to be in control of idle speed. Until then, you may need to hold the accelerator pedal down 1/4" to get it to start and stay running. You won't get the normal idle flare-up to 1500 rpm for the first few seconds, and it will usually stall at stop signs. Once the engine is running, minimum throttle will be learned when you drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals.
As for the ignition coil, I've never actually tested one. I did have one fail intermittently and I chased it for over a year on my '88 Grand Caravan. It finally failed completely two years ago, THEN I could diagnose it. That was the first time since the van was new that it didn't get me back home.
The problem is the resistance checks you're having done are rarely going to not pass. The wires don't break that often. What usually happens is the spark arcs to the metal case instead of going to the spark plugs. Current takes the path of least resistance, and once that occurs inside the coil, it leaves a carbon track behind, just like can happen inside a distributor cap, and that carbon track is a conductor. Resistors used in electronics are made from the same carbon.
It is also possible for some of the loops of wire in the secondary part of the coil to short together or the varnish insulating those wires can burn off. The range of specs for resistance is so broad that a partially-shorted secondary can read lower resistance than it did when it was new but will still fall within the acceptable range. The voltage it can develop will be reduced, and if it can't develop enough to jump the gap between the rotor and distributor cap terminal AND the spark plug, AND the high-resistance spark plug wire, you'll have a no-start, hard-start, or misfire condition.
The symptoms on my van mimicked a bad accelerator pump in a carburetor, which at first didn't make sense. That's why I never suspected the ignition coil. By the time I got under the hood whenever the engine stalled, I never caught it having no spark until the day it failed completely. That's when it finally presented the same symptom you have now.
Thursday, September 26th, 2013 AT 10:59 PM