Originally posted by stuvx:
EEC-IV, I believe, had the capability of doing a power balance test during the KOER test. Not sure if OBD-II (EEC-V) has this capability.

In any event, a cylinder power balance test involves shorting the secondary ignition to ground, not primary. This does not involve providing an open in the secondary circuit (e.g., pulling off a spark plug wire to disable a cylinder). You are in essence allowing spark to go directly from the coil tower to ground instead of allowing the spark to jump across the spark plug gap to ground. This is no way damages the coil, ignition drivers (transisters), computer, etc.

[snip]

As far as the test not being applicable to waste-spark ignition systems, that is simply not true. The secondary circuit is never opened during the test. The spark only bypasses the spark plug gap on the cylinder being tested, thus not allowing power to be produced on that particular cylinder only. The companion cylinder spark plug fires normally on both the exhaust and power strokes.

Sorry about the long post, but this is an extremely useful and powerful diagnostic tool that has many misconceptions.



Please don't take this as a flame or an argument, just a desire to understand better why shorting a shared coil is not at risk to cause damage or possibly mask a bad injector on the shared cylinder.

Spark plug wires are resistance wires, and the spark plugs have internal resistance (except on some race cars). This is to reduce the effect of electrical noise in the electrical system. Early on, the electrical noise would severly effect AM radio reception, but now days the noise could also effect the electronic engine control. Electrolytic capacitors used in the PCM and other electronics to filter noise are rated for certain peak voltage spikes. Everytime this limit is exceeded, the capacitor degrades. As the capacitor degrades, its effectiveness at filtering noise also degrades. The passing noise can cause transistors in the PCM and other electronics to attempt to switch too often, or break down the internal P-N junctions.

The resistance will also limit the surge current provided by the inductance of the coil secondary. If you provide a short circuit from the coil tower to ground, that is an extremely low resistance path. Electrical current will take the path of least resistance, so if you provide a 2 ohm path from one tower on the coil and have 10kohm of resistance on the other path, the majority of the current is going to flow from the secondary of the coil through the short to ground, very little current (probably not enough to fire a spark plug) will travel through the other plug wire and plug.

The only control the PCM has over the coils is on the primary side. If the spark is turned off for the KOER cylinder balance test, it is turned off on the primary side. I'm not sure if the PCM controls the spark or the fuel for the KOER test.

-Rod