Hey Mick, The coil wire is the spark plug wire, black wire with the boot over the spark plug. Like RG said, if you take 1 wire off while the engine is running well tell you if that coil is bad or weak. If it starts sputtering after you remove it, then it is still working. If the engine dies that means the other coil is bad, or weak because it won't provide enough spark to run the engine on 1 cylinder. If it sputters but keeps running put it back on the plug, and pull the other plug wire off and see what happens. I like to do this when it's acting up , running rough, because I've noticed coils that are going bad go bad slowly maybe over a few weeks. Change the coils runs great.