Waveform of COP

Dears,

Does a bad, broken, or wornout spark plug changes the waveform of coil over plug?

Sometimes, when I use power probe with verus for diagnosing COP, the waveform is good but when I do snap throttle, the waveform doesn change but the size of the waveform become smaller.

Is this bad coil?

Thank you

Yes. The wave form is a function of voltage, resistance, and current so any change in one or more will cause a change in the wave form no doubt.

At idle, the cylinder conditions are very different than at a snap throttle. So the wave form will be different. Scanner Danner has numerous awesome videos demonstrating this. For example, there will be more A/F and load will be slightly higher with a snap throttle. Thus the spark plug spark has to overcome more resistance on a snap, causing the spark like to climb.

Let's pick a bad spark plug wire, as voltage increases, the voltage drop would increase causing the spark like to not be as high. Clearly, one could get a PhD in spark wave form analysis. I am far from a PhD. But, after much practice, I have been able to find issues with this wave form.

Generally, a shorted coil will have a vertical climb in voltage. This is caused by one or more wires in the windings 'touching' each other.

Maybe attaching some idle and snap throttle wave forms will help. The spark line will appear shorter since there is less time for the spark to occur.

I just want to point out that ignition waveforms at idle or slightly elevated rpms may not tell you the story your trying to find. You really need to do a snap throttle test when looking at the waveforms. This will stress the ignition system and possibly point you in a direction. I think this is different though with amperage waveforms as those waveforms are present just before the coil fires.

[quote="GeekDIYMechanic" post=16647
]Generally, a shorted coil will have a vertical climb in voltage. This is caused by one or more wires in the windings 'touching' each other.

Maybe attaching some idle and snap throttle wave forms will help. The spark line will appear shorter since there is less time for the spark to occur.[/quote]

A shorted coil will show a sudden rise in current flow during coil on time, not in voltage. This will cause a weak magnetic field and hence little energy to be transferred into the secondary once the coil turns off....

The firing line - if present - always is vertical.

A completely shorted primary may show no self inductive kick at coil turn off but probably will blow the fuse before.

Yes. My bad. You're absolutely right Mr Scholl. I should have said a quick climb in current and not voltage.

That was my fault. Thank you for correcting that.

Don't worry, you're well.
We all learn, each one from every one.

(-:

When you snap the throttle 6 main things change inside the cylinder.
1 The cylinder suddenly gets filled at a higher pressure near atmospheric resulting a much higher pressure and temperature at the end of compression
2 Fuel air ratio changes suddenly and can't be tightly controlled by the ECU
3 The velocity of the air rushing into the cylinder suddenly increases increasing the inlet tumble swirl turbulence.
4 The "squish" turbulence inside the cylinder cause by the piston squeezing the gas against the cylinder head increases.
5 Dwell time may be reduced.
6 The ignition timing advances from say 13 btdc at idle to say 32 btdc at 2,500 rpm that's a big change.

The mixture going lean and the cylinder pressure increasing push the KV requirements and the increased turbulence tend to make the burn line less neat and tidy looking. Also remember so many different ignition systems these days and no two look quite the same on the scope.

Some manufacturers current limit the primary by reducing dwell time a low rpm others saturate the primary and some Ford do multi spark at low RPM even good COP coils from different manufacturers on the same car can have primaries and secondaries that look very different on the scope.