308 No spark at plugs for cylinders 4 and 8

Johnk - Yes it started and idled well with the new sensors, (the rest of the story is listed in a post above where it drove for a short bit, performance started failing, CAT got red hot and I luckily coasted into my driveway…).

I did recheck to make sure I had connected the RPM sensors to the correct harness leads.

Thanks for sending that diagram. What is that device under the digiplex’s, coming from terminal 6 ( with the pink/black wire going into it )? I can’t read the name it’s too blurry. In any case I don’t have that on my car. My wiring/digi inputs look like the diagram attached.

Back to the RPM sensors though – Do you or anyone know if the results of my tests could be caused by any of the indicators that are mounted on they flywheel accidentally coming off on reassembly? Seems like that is not likely since when switching Digis the issue did not follow to the other bank. Would you agree?

Mike996 – I hope I can let you know soon !!!
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Yes, I had done this and do have good connections from cap to plug wires but still problem exists.

 

This is what I thought too: that if it were the RMP sensors, or the digiplex boxes, then the issue would follow to the other bank when swapping digiplex boxes at the large connectors on the boxes. But it does not.
The problem remains only on the rear bank. And effected only when swapping the wires at the coils - on the Rear Bank cylinders 2 and 3 fire as wired originally, and 1 and 4 fire when coil wires are swapped.
I'm stumped.

 

I purchased a $36 oscilloscope..
I’d like to confirm where the scope’s leads connect to– I’m not clear on this yet… Do you disconnect both the black and white connectors that connect the RPM sensors to the harness, and then check between the two contacts within each connector on the sensor side connectors? And your comment about having to have the sensors plugged in to the digiplex, is that only for when using a voltmeter? Meaning you don’t need power when using the oscilloscope?
Thank you for assisting and this is all super helpful as I want to get the testing right…

 

Thank you both so much. Makes total sense to me now in terms of how it's all wired and where to test. I'll report back with results.
Johnk - I see you are located in CT. Me too. Do you happen to be anywhere near Westport, in Fairfield County?

 

Good Afternoon.

Steve and John, I hope you are still following along, I very much appreciate it... and everyone else's advice too.

I finally had time to do some testing. I used the diagnostic port and tested each bank's TDC and coil trigger.

I've linked below a number of videos of the scope readings but I really don't know what I am looking at exactly. I've never used a scope before and I tried to adjust the settings in order to get some meaningful visual readings. I don't see any steady patterns that I would have expected though.

I also linked to a video of each bank sparks at the plugs (I purchased those adaptors that light up when the plug fires.). Notice that cylinder 1 and 4 don't fire at all, and 8 just barely. And the firings seem random.

Can you make any suggestions after seeing these videos? And/or, are there any adjustments I should make on the scope to get cleaner readings?

I had read some very old threads (like 20 years old) where new out of the box sensors from Magneti Marelli were problematic. That's the brand I had installed. Based on these readings, should I put my old sensors back in?

I thank everyone for taking the time to look at the videos.

John I had really hoped you were in the next town over. lol.

Scope Videos:

Front Bank Sparking Indicators: https://www.youtube.com/shorts/ahwexMFlFzA
Rear Bank Sparking Indicators: https://www.youtube.com/shorts/e1CA9_UhwnA

Bank 1-4 TDC Sensor Test:
Bank 5-8 TDC Sensor Test:

Bank 1-4 Coil Trigger:
Bank 5-8 Coil Trigger:

Thanks again.

 

Thanks John, I tried 1-4/TDC again on with a faster time and got this result. (link below). I'm curious to see what your settings and readings look like of course. Once I see those, I'll match your settings and test all again. Greatly appreciated...

 

OK got it, my next garage session, (likely end of the week) will include the Shadetree test and getting additional scope readings including on the coil triggers. Of course this assumes the engine will run. I have not tried to start it since the red hot CAT episode which led to all this.

I'm still a novice to say the least at using and reading the scope but I'll continue to do my best at that. This may be a stupid question, but does it matter how long the scope testing leads are? Can I put extensions on the leads so I can be in the driver's seat with the scope in my hands and turning the engine over? (I rarely have a helper)

One thing I thought to restate since it's been such a long thread is the issue with swapping the negative (digi) feeds at the coils themselves: With the green wires (from digi1) connected to the rear bank's coil, only 2 and 3 fire on the rear bank, and then switching and connecting the yellow/black wires (from digi 2) to the rear bank's coil, only 1 and 4 fire. It's the opposite. This seems to point to something I just don't know what... That chart I posted a while back shows these various scenarios. But one thing that popped up since then is that 8 is not firing consistently now with the stock wiring set up.

Also @tuttebenne, If i get the engine running I'll take a new alternator output reading, but about 2 months ago, the battery was not holding a charge, and I checked the alt output at the battery (with the engine running) and it was reading about 13.5. Took the battery to NAPA and it tested bad, so there is a new battery installed now.

I'll keep you all posted. Can't thank you guys enough. Have a good week.

 

I'm back with some scope videos per John's specs. (Btw I'm pretty jealous of how good your engine sounds, John.)

I tested bank 1-4 coil trigger at 5ms, 10ms, and 20ms, and then bank 5-8 at 10ms and 20 ms.

I would certainly appreciate interpretations of the scope readings. To me they look fairly similar to John's readings but I really don't know. Of course too if there are any other settings to try or do overs that would help, please let me know.

I also did a dark night start up to see if I saw any sparking in the bay, but did not.

Here are the scope videos and hope you guys have some good news about the readings... meaning the TDC and flywheel pins seem to be doing what they are supposed to do.

Look forward to hearing your comments.

1-4 Coil Trig 5ms:
https://www.youtube.com/shorts/FuKqrqVcdZw

1-4 Coil Trig 10ms:
https://www.youtube.com/shorts/EA9BexkM9cs

1-4 Coil Trig 20ms:
https://www.youtube.com/shorts/B87nvT3Hrnk

5-8 Coil Trig 10ms:


5-8 Coil Trig 20ms: