308 No spark at plugs for cylinders 4 and 8

I always thought I was pretty good at diagnosing such issues but none of this makes any sense to me. The only thing that comes to mind that can explain it is bad plug wire or a carbon tracked (or defective) distributer cap. But it sounds like you have eliminated wires and caps as the source of the problem. I'll be really interested to hear what it was when you figure it out because it sounds impossible!

 

Did this start after installing the new RPM sensors? Any possibility they are connected wrong?

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I also had this experience, and had to drill the holes deeper for proper wire connection.

 

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.

 

Yes, not used on 4 valve cars.

 

Very doubtful IMO that the RPM Sensor is part of this problem as its signal is used by both Digiplex ECUs, but you can measure its signal either with an oscilloscope or even just an AC voltmeter (but the sensor has to be connected to the Digiplex ECUs to be valid). This is the idealized RPM Sensor signal (ignore the TR pin numbers):
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And this is a shot of someone using a (now obsolete ) oscilloscope to measure the RPM Sensor signal on a 308i-2V (you can buy a digital oscilloscope on Amazon for about $30):

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You can make the same measurement on the TDC Sensor signals to see if they are OKish, or not, and whether they are similar, or not. There are far fewer pulses in the TDC Sensor waveform so the AC voltage measures a lot lower (and would be lower on a 308 as it only gets 2 pulses each flywheel revolution whereas a TR gets 3 pulses each flywheel revolution):
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This is what the TDC sensor signals actually look like at idle on an '85 QV.

https://www.youtube.com/shorts/ooT-ZvNZ6ao

 

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…

 

For the TDC sensors use the diagnostic port. You can also look at the coil trigger signal there.
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It's always best to have everything connected as it normally is when making voltage measurements (whether using a voltmeter or an oscilloscope). I think you can "back probe" the connector at the Digiplex ECU themselves:
Measuring between pins 1 and pin 5 will be the TDC sensor for that bank
Measuring between pins 2 and pin 3 will be the RPM sensor
or you can do it at the Diagnostic Connector as John has shown.
On the older style (bullet) Flywheel Sensor connectors you can just partially unplug them to expose the male metal terminals (while they are still electrically connected) to have a place to connect the voltmeter/oscilloscope leads, but later on they went to a different style connector (so don't know which you have).

 

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?

 

1/2 way between Hartford and Middletown.

 

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.

 

The problem with those videos is that the time sweep is too slow. I have the same scope and I'll try to get some good scans on my car and see what the time base is.

 

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...

 

What is the alternator output? 14V?

 

The scope is showing you what you already know - some cylinders aren't firing. Let's try some good old shade tree stuff:

After dark, raise the hood and start the engine look for any sparking from the wiring/distributer cap/coil, etc. If none, sprits some water in a fine spray on the coils/wires/distributers. Any sparking in either case will indicate an insulation/carbon tracking deficiency which could be the cause of the problem. Shadetree? Yep, but I've seen a bunch of ignition "issues" identified this way. Costs nothing and sometimes it can result in quite an entertaining light show! This, of course, can only show secondary ignition issues, not primary component/wiring problems.

 

While I basically agree with this I think it's not a bad idea to once and for all eliminate the primary side of the ignition. Plus, looking at the trigger signal can tell you about what's happening on the secondary side. It should be done with everything as it would be when installed in the motor. In the case below, the bank 1 distributor rotor had a carbon track that shorted the coil out put to ground. In Sam's case we may see something like this for one or more cylinders.

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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.

 

Ok, Sam, I happen to have one of those pocket scopes like yours. The settings are 10V, X1, 10ms, DC.

This is for the coil trigger. With ignition off the signal is at ground. With the ignition turned on it jumps to a steady 12V before cranking. Then cranking and starting you see the repeated signal. It drops from 12V to ground then slowly rises over about 5ms to maybe a couple of volts. Then it jumps positive and quickly settles back to 12v. The signals repeat for each cylinder and should be equally spaced in time, and should all look basically the same. Don't worry about the differences in the peak positive voltages. That's an artifact of the scope sampling rate and display resolution. What you don't want to see is the longer, exponential return to 12V like in the left hand picture in my post 45. That indicates some kind of resistive short to ground that prevents the coil discharge voltage from rising enough to create a spark while at the same time causing the coil to discharge more slowly.

If you want to look more closely at the signal you can reduce the time to 2ms, but initially use 5ms so you can see several pulse displayed on the screen at the same time and observe if there are differences between them.

This won;t tell you which cylinder has a problem, just if a problem exists. Fortunately you already know which cylinders are problematic.

Hope this helps.


https://youtube.com/shorts/Iifh-2boNcc

 

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:

 

They all look good. They all indicate that you are getting proper coil discharge, i.e. spark. Doesn't seem to be any premature discharge or resistive discharge. Are you sure you have an ignition problem?

 

VERIFY my understanding (or not) that you pulled the plugs, connected them to the wires/grounded the plug to some point on the engine, cranked the starter and there was NO spark at the plug on the "bad cylinders" but good spark when the plug was connected to other cylinder wires or directly to the coil itself.