Coil current ramp and coil voltage

I have watched a number of scannerdanner's videos. The one that has interested me recently has been about misfires, and using the current ramp of a ignition coil to determine what cylinder is the issue.

I had done some work to my car recently, which is a 99 chevy metro. It has a 3 cylinder engine in it. Afterwards, I had time to get my scope out and try some stuff out. I hooked up the amp clamp to the coil primary wiring. This is the waveform i got with the engine runing correctly.

www.flickr.com/gp/152203134@N05/M965Y4

I then put a spare used spark plug in the # 1 cylinder, but the electrode had no gap. I smashed it flat to similate a shorted plug wire.
I had expected the currecnt ramp to change and have a straight up line at the begining of the ramp, but it looked exactly the same as the normal one.

I thought about it for a little while then I realized my engine has a distributor, cap and rotor. So it still has to jump a air gap inside the distributor even if the plug no longer has a gap.

With my engine configuration, I will never see a straight up line at the beginning of current ramp will I, unless the coil secondary is shorted?

Good question! A straight line in the current ramp would indicate a shorted primary winding, not a secondary problem. That's why your current ramp remained unchanged.

I will have to go back and check some of the videos then, I thought for sure he said it was a secondary ignition issue.

Thanks for the response.

Here is one of the videos were he saya shorted secondary. Near the 5:30 mark.

This video explains why I didn't see any difference in my spark waveform (i didn't post a picutre of it), when I had the spark plug gapsmashed flat. I have a distributor, with a cap and rotor. I should of snapped the throttle to see if there was a increase in KV voltage.

You wouldn't see a plug with a gap set to 0 on the primary side with an amp clamp since current is not flowing in the secondary circuit in that waveform you showed in the first post. As far as I know, which may be incorrect, a short in the secondary coil windings will have that straight line jump instead of coil oscillations, but I have forgotten whether that's just the behavior of the magnetic field with a short or due to the secondary and primary being connected.

You need to watch the voltage to see what the spark is doing, which means a capacitive clamp around the spark plug wire or coil wire going into the distributor. In which case you would see a high Kv with a sharp downward ramp where the firing line would be since all that voltage is going straight to ground

Thank you for the response.

I believe your right, I will need to check the voltage going to the spark plug with a inductive clamp. There should be close to no burn line after the high kv line on the waveform.

I get a chance I will have to try that again.