Let's play with some ignition systems and see how the spark teaches us.
A 1989 Jeep ComancheThe distributor ignition system has been around for as long as I can remember. Originally, a mechanical set of electrical contacts called points was used on the ground side of the coil primary to control current flow. They were opened and closed by small ramps built into the distributor shaft, which the camshaft drove.
A 2006 Kia SportageThis four banger uses a distributorless, or DIS, ignition system. Instead of one ignition coil for use by all the engine's cylinders, a DIS system uses one coil for each pair of cylinders. Also called an EI, or electronic ignition, system by some manufacturers, this design puts out a significant increase in spark energy of 30,000-plus volts, which is needed to ignite the leaner mixture in later model engines.
The secondary coil winding is the source, and the two spark plugs are the loads in the circuit. You know the battery has a positive and negative side as does the secondary. Current flows from the negative side of the coil, through the first spark plug and the cylinder head to the second plug and back to the coil. The compression plug is going to need the most energy, but if a problem occurs on the first plug's side of the coil (excessive gap, weak coil output, etc.), there may not be enough left for the second plug. That is why you can have a single misfiring cylinder even when the coil is shared.Testing the spark energy can be done with the spark tester, just like with the Jeep. A scope and amp clamp can be used to test these systems, and often spot weaknesses you may miss with a mechanical tester.
A 2007 Toyota CorollaThe Toyota uses a version of the newest ignition system design, the coil on plug or COP ignition. In these systems, one coil is assigned to each cylinder. Some of these systems can produce as much as 50,000 volts, so I wouldn't hold one in my hand while it was working if I were you!
And the igniter COP coil is not unique to Toyota. If you see more than two wires headed into a COP coil, spend some time studying exactly what makes that system tick before beginning any fault diagnosis. Troubleshooting ignition systems is still based on some old principles, but it's how they are applied on a modern car that can create the challenge.