Ignition problem. What am I missing?

Morning Phil,

Never a dull moment with this problem...although I'm glad to hear that she's running awhile (hours versus minutes) before she quits. If I recall, before she'd lose a bank...but continue to run. Is that correct?

Now, I'm presuming that she just dies altogether? If so, we'll need to determine if she's lost fuel pressure, voltage to the fuel pump or voltage from the ignition switch/fuel box to the coils/ignition system. Are we monitoring voltages/pressures when she dies?

Does she re-start (immediately) after the engine dies?

I'm speculating, but I believe that we've got the initial problem fixed and have revealed another...

Regards,
David

 

Are you saying that when you unplug BOTH crank sensors AT THE SAME TIME the engine still runs? Also, are these observations made when all is OK or when the 5/8 bank is dead? I.e., if the 5/8 bank is dead, and you unplug the 1/4 crank sensor, the engine keeps running on the 1/4 bank only?

How about swapping the Digiplex ECUs to see if the problem moves to the 1/4 side or stays on the 5/8 side? Maybe you've already tried that, but you didn't list the Digiplex ECUs specifically in your list of swapped components.

Does the problem stay present long enough that you could make an AC voltage measurement of each crank sensor while the engine is running on just the one bank?

 

broken wire making intermittent contact? strange how it it comes and goes.

 

Wow! -- you might as well just remove them completely because they just must be dead weight

Seriously, I don't see how this is even possible on a stock 308QV ignition system unless the Digiplex ecus are much, much smarter than I thought they were (and it is somehow using only the RPM flywheel sensor signal once it has "learned" the relative relationship between the RPM flywheel sensor signal and the TDC sensor signals -- and can then synthesize the "missing" TDC signals). If you unplug both TDC sensors when the engine is stopped, will it start-up and run with the TDC sensors unplugged? If it does, I would have to conclude that you have a modified, non-stock ignition system.

 

Steve,

I've had Phil's spark boxes up here (at my shop) for testing:

-They're 100% stock
-They pass every test I've put them through, both on and off car, as well as an extreme heat soak test.
-Advance curves for both boxes have been mapped and are well within all factory specs and match one another
-Over 200 miles of "road test" miles have been put on them, without so much as a hiccup.

Phil and I have had a few conversations about this problem. My feelings are that we're dealing with several problems, with a cross-talk EMI-type issue likely. As we know, these boxes, by themselves, are not "smart enough" to run without the TDC sensor signal present. However, if the coaxial shields have been damaged, or if there is a hidden wiring problem to the spark boxes, who really knows what signal they're really receiving, as a "substitute" input.

At this point, I'm pretty confident that the problem is not a simple "let's swap out this or that part" and she'll all "be fixed". I'd like to get the car up here on a multi-trace scope and really watch what actually is occuring, or not when she drops a bank.

Regards,
David

 

David -- No argument from me that you're now beyond the easy solution of just replacing a component (and I'm still amazed that you can disconnect the TDC sensors once it's running and it keeps running ). Seems like you've done a lot of further independent testing of the Digiplex units themselves, but, once swapped, and the trouble stays on the same bank, I'd say that alone rules them out as the cause. And, since everything else has already been swapped, it gets down to probably a problem in the wiring harness -- so you're left with just what you said "...really watch what actually is occuring, or not when she drops a bank" at the Digiplex connectors and the coil connectors.

Phil -- Do you have a voltmeter? If so, you might at least be able to confirm/deny +12V power yourself at the Digiplexes and coils before resorting to the more advanced scope testing David suggested -- just a thought...

 

Steve,

PM me your phone number. I'd like to run a wild theory by you..

Thanks,
David