[FIXED]94 Blazer 5.7 TBI coil current ramp anomaly

This Blazer has what seems to be timing issues. It pops, sputters and backfires during acceleration. No communication with PCM. Spark was VERY weak. A 1/4" arc was all I could get to a testlight connected to engine ground. The customer declined to spend any time/money. Before it left, I pulled a current ramp of the coil during a backfire. After looking at this wave, I swapped the coil for a known good. No change. Can anyone explain, or have any ideas what is going on in this waveform? At idle, and not backfiring, it looks normal.

Sounds like an ignition timing issue, I don't know how to justify that with the current ramp you have there.

It almost looks like the turn off isn't a clean cut decision so it wouldn't be completely collapsing the windings and creating a good spark. I'm looking at the oscillation on the down slope. Curious to hear others inputs.

I remember sitting in a John Thornton class in 04 where he talked about some control or de-spiking circuits a module has during this turn off event. He showed us a faulty module in this area but I haven't seen one since. This turn off area doesn't look right at all. Some type of module issue. I want to see the primary or secondary voltage waveform. I bet you see it there too.
Seems to me the magnetic field is not getting a good collapse, causing your weak spark. Put a module in it.

matt.white
Nice post! I didn't even see it when I wrote mine. Sorry about that. Thank you!

ScannerDanner wrote: matt.white
Nice post! I didn't even see it when I wrote mine. Sorry about that. Thank you!

No, thank you. That one post proves some of what you've been teaching all of us has been sinking into this thick skull of mine.

I'm with Paul and Matt about a module problem. It seems like the driver isn't collapsing the field quickly enough. What you're seeing in that current ramp is taking around 1ms to fully collapse, but even that is too slow.

Also, customers are strange sometimes... They brought it in, but don't want to spend any time/money :blink:

So you guys are thinking that a transistor internal to the module is kind of hanging open in a sense? Or maybe leaking voltage through the emitter under higher demand is a better way to describe it?
It makes sense that a slowly collapsing field would cause a weaker than normal spark.
Too bad you don't have this one to play around with. I also would like to see the primary voltage compared to the current during the event.

Yeah well my thought would be it collapses the windings, creates a spark but instead of having any sort of burn time, it starts building the magnetic field again which would surely stop the spark.

My first reaction was reminded me of a points ignition with a faulty condensor.
Searching on faulty condensor wave form I found this article on the Auto Service Profesional site --- coil driver issue similar wave form.
Link to article

Andy.MacFadyen wrote: My first reaction was reminded me of a points ignition with a faulty condensor.
Searching on faulty condensor wave form I found this article on the Auto Service Profesional site --- coil driver issue similar wave form.
Link to article

Nice find, Andy! I remember reading about this phenomenon in a trade magazine a few years ago, might have been this article...

Thanks, everyone. A lot of nice input. The good news is that a new module fixed the problem. The bad news is that I cannot provide any voltage waveforms. With Thanksgiving and a surgery the following Monday, I have been a rare sight around the shop. But, curiosity got the best of me and I sent a helper over to change the module, pro bono. I told him to save the old module so, maybe, I'll play with it when I return, next week.

Hey sir, thanks for the update! I think we all learned from this one, honestly. I mean, we usually see drivers either working perfectly, or not at all, but this was something in between :blink:

Hoping your surgery recovery goes well

This is actually a common problem for these trucks. My 95 k1500 tbi was running very similar and I went out on a whim and changed the ignition module. Luckily it fixed the issue. These trucks are notorious for ignition module failure. Probably due to no heat sink on the module itself and it being located under the distributor cap.