Why am I seeing a secondary ignition wave form while checking the primary signal

Background: Just playing with my new scope after watching primary ignition lecture.1. Car 2011 Dodge Journey 3.6L engine, 2 wire ignition coil setup.2. Equipment: Foxwell OS100, 4 channel lab scope, Dell laptop, AES wave amp probes, AES Wave fuse block current loops3. Testing: A. signal from PCM (Pin 1) to

(sorry I lost all my text when I tried to add a picture).

Bottom line ... I'm seeing a secondary wave form on the lab scope channel I was using to see the square wave signal from the PCM to the coil. Can anyone explain this? It's a COP system.

And, how in the world does a guy attach a picture here. Site keeps telling me that a 93KB .JPG image is not allowed.

Not sure how to attach pictures here.

On this setup, a two wire coil, there is NO trigger signal the pcm sends out to the coil and you could observe. In this application the pcm itself controls the coil(s) directly, the ignition module lives inside the pcm so to speak.

The mentioned trigger signal is detectable only when the ignition module is integrated into the coil itself. This coil type is identified by 3 or 4 wire connected to the coil instead of only two. On these coils you'll not be able to see the coil's primary voltage though.

To attach a picture it has to comply with a couple of conditions, such as format, file size....Make sure to obey these criterias.

Do you happen to know wether this foxwell scope is a rebadged Hantek? At least this is what the soft- and hardware looks like...

SK wrote:

Bottom line ... I'm seeing a secondary wave form on the lab scope channel I was using to see the square wave signal from the PCM to the coil. Can anyone explain this? It's a COP system.

That is not the SECONDARY ignition that you are seeing. That is the PRIMARY ignition waveform. A two wire coil has a constant power, and a ground that is controlled by the PCM. If you backprobe the "control" side of the coil, when the coil is NOT energized (coil is NOT turned on), you will see the constant BAT(+) power, that goes into the coil, THROUGH the coil, and OUT the control wire. This is the 'high" side of the "square wave". When the PCM turns the coil "on", it supplies a GROUND. On the scope, you will see the the BAT(+), that is coming OUT of the coil on the ground side, drop to near zero volts, as the PCM grounds it. (This is the downward transition/low side of the "Square Wave".) Now, the coil Primary winding has a POWER and a GROUND. The energized primary winding will create a magnetic field, (which, also, surrounds the SECONDARY winding). When the PCM releases the Ground (the "square wave" transitions back to "high"), the magnetic field will collapse. The collapse of the magnetic field, around the primary and secondary windings, will create a voltage spike. This is the Primary Voltage waveform. (<---This is what you are seeing. ) On the Primary winding, the spike will be close to 400 volts, on a good coil. The Secondary winding contains about 100x more windings than the Primary winding. As a result, the Secondary voltage spike will be about 100x higher than the primary. (polarization will be reversed.)



As Juergen pointed out, 2-wire coils are energized, turned on/off, directly by the PCM. 3 & 4-wire coils are energized, turned on/off, by a transistor, internal to the coil, which is triggered by the PCM on the 3rd wire. This will be a square wave WITHOUT a voltage spike, as this wire does NOT, directly, create a magnetic field. It supplies a small current to a transistor, inside the coil. The transistor, then, supplies a path to ground that will energize the coil.

Thanks guys for the replies!

I figured it out, and I understand why the wave form looks identical to a secondary ignition wave form. When the primary field collapses, it induces voltage into the secondary winding. The counter EMF from the secondary is what I'm seeing on my scope.

SK wrote:

The counter EMF from the secondary is what I'm seeing on my scope.

Not really!

What you´re seeing in the primary voltage pattern - as far as the "spark section" is concerned - is the effect of (mutual) induction. The effect of CEMF would be observable in the current waveform while the CEMF is fighting the incoming current through the primary windings. This is seen as ramping up/shark fin shape of the current trace instead of an instantaneous, vertical rise. In this case the CEMF originates in the primary circuit, not in the secondary windings.