The test to perform will depend on the type of the coil (integrated amplifier or not). In all cases you can lift the coil pack and use the plug extensions, but this may not be the quickest way. Either primary current (both types of coil) or primary voltage (non integrated amplifier) can show up issues with the ignition system, and the influences on the ignition of other issues. The key performance indicator is sparkline voltage and spark duration. A comparison of both (using voltage or current) will point you to the cylinder(s) in trouble.
You can do a primary current parade (using a cylinder reference trigger such as an injector) or superimposed (where all primary current ramps will appear on after the other at the same point on the screen)is using a simple edge trigger as long as you clamp the common supply. Primary and secondary issues will show up in the current waveform if you choose this method, but you may need to zoom in after the vertical drop at the end of the ramp, as this is where the secondary issues appear (and where you can look at spark duration). The spark line voltage is less easy to analyse when you're new to scopes using the current clamp when compared to seeing the ignition primary voltage.
We have used this current ramp technique successfully, on Toyota coils for instance, where there is no raw primary voltage (due to the integrated amp) and the coils are shielded fairly well rendering the COP method redundant. In my view primary V and A are great if possible, but A is good (and it may be the only nonintrusive option).
The misfire/hesitation sympton which can lead you to need to perform this test can be cause by primary issues (supply, control or physical primary winding issues), secondary issues (plugs, connecting 'wire' or secondary windings) as well as a host of other things (injection/fuel system, engine physical, event timing etc.).
James Dillon - Technical Trainerwww.techtopics.co.uk
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