Since my manual was for a 96 (which is pretty different from prior years in terms of the codes), I found the pages from a 95 LT1 Camaro FSM and read it. So I understand this situation a lot better now: it's focused on the connection between the PCM to ICM, not the connection from opti to PCM. It's interesting how it determines the problem: applying 5v to the ICM's load and wanting to see it drop to 4.5v (if it stays at 5v then there's a fault between the PCM and the ICM load). But the key here is that if the PCM diagnoses this fault during cranking, it will shut off the fuel injectors to prevent flooding because it assumes there will be no spark. So in your case, the noid test probably doesn't indicate a bad opti. You could still test the opti's output (it looks like an oscilloscope is only ) just to be sure.

If I understand things correctly, that code can't set unless you're cranking the engine. If you get it to run and it starts failing, the PCM can't diagnose Code 41 until after it shuts off and you crank it. So you've gone through the whole test procedure and I see where it concludes that it must be either the connection of that signal wire (at PCM terminal A12) or the PCM itself. Did you inspect the pin for A12 to make sure it looks okay and has good contact? I wonder if it's possible that an ICM can fail in such a way as to have a fault between the B terminal connector and the load inside the ICM? It would be cheap/easy enough to swap that out to see. Also, any chance you tried to start the car after unplugging the A harness to the PCM and plugging it back in after these tests? If the issue is just a dirty or even loose pin, then maybe reconnecting it would be enough to get a good connection again. Same for the 4-wire plug into the ICM.

Failing that, I'm going to suggest that your PCM may be the culprit. I hate saying that, because much like the opti, these things don't fail as often as people think they do. But that's what the FSM is pointing to also, and there was that tuning that suspiciously coincides with the onset of all this. Tuning a 94-96 PCM shouldn't involve any physical disassembly or anything: it's just a reflash of the chip inside (and it can be done with the PCM still in the car via the OBD2 port). Therefore, it's hard to imagine how they could have screwed anything up...except that they or you might have messed up the pin in the process of plugging/unplugging the harnesses to it. That's I can come up with. REally check those pins. And if you know anyone else local with another 94 C4, see if they'll swap PCMs with you to determine if the problem follows the PCM or stays with your car.

If I think of anything else, I'll add it.

 

Very interesting. Since ive sent the computer out for a tune ive genuinely never touched it or the connectors. I removed connector B only to check for continuity between B5 and icm connector B and it was there. Never touched A but looking at my fsm terminal A12 is blank. Whats A12 for the camaro used for maybe its pinned somewhere else on the c4 and i can find it. Ill look over my pins and see, but again if its a faulty pcm why does 1. Changing the distributor fix the issue immediately, yet this same issue keeps happening and 2. Why would the car run so well for hours then in the span of 10 seconds die and never start again until i swap optis? When the car died for the 2nd time all i changed was the opti and coil, car started up just fine. Didnt touch the icm pcm or wiring inbetween besides unplugging and plugging the 4 pin connector back in (which ive done now many times to check the harness)
I can look around for someone with a 94 vette but cant say i know anyone locally so that may take time. When im back on monday ill be sure to check everything and run the test from that link you sent me

 

Okay, so PCM pins are one area where the 96 (the FSM for which I have) and the 94/95 pretty much match (except the 96 has some extras). So you're right, A12 on the Red connection (which the FSM labels as C1, as in connection 1) should be blank, and the ICM signal wire is on Black Connector 2 at the #5 position.

And something just hit me like a ton of bricks: in my 1996 FSM, it refers to the optical sensor inside the distributor as the "Ignition Control Module," and this is what sends the low-res and high-res crank position info to the PCM! What we typically call the "ICM" (the module up by the coil with the thermal paste) they call the "Ignition Coil Module!" That 96 FSM is a real hodgepodge: it mixes OBD1.5 test with OBD2 text in certain places!

It's hard to tell, but I think we're still looking at a short in the signal from the opti to the PCM.

 

So im back and was able to run the test. No voltage at terminal A followed all the instructions. I do have 12v at terminal d. checked all fuses and none were blown. What should i check next? unsure where the pink wire leads to where should i be looking?

 

Remove the MSD 6AL from the system. These control boxes tend to burn out factory modules, coils, etc.

 

box has been removed and failure still persists. ran msds tests for the box and it passed all so the box isnt the culprit.

 

i force fired the coil using the box and the coil fired so msd says the coil is most likely okay. ive ran tests without the box using the msd blaster coil and my stock gm coil and failure still persists. i believe the issue is occurring before the coil

 

Going back to the diagnostic procedure I linked before:

That's where to go next.

 

Checked fuses none are blown. i don't have a piercing probe so i checked terminal a while the harness was unplugged. does the harness need to be plugged in for 12v to flow through terminal A? if yes ill try again with it plugged in, if no what relay should i be inspecting?

 

so i swapped to my stock coil and got 12v on the pink wire terminal A. seems like my coil has failed again but swapping coils didnt fix my no start. coil exploded last time and this time it seems internally broken. unsure what would be causing that in addition to whatever else may be broken. ran test 2 and it passed. ran test 3 and got no blinking light just stayed off. 4th test same result no light. the c4 corvette doesnt have a crankshaft position sensor so thats out. only things left are pcm and opti if im not mistaken. unsure where to proceed next

 

Code 41 is almost always a bad Ignition Control Module. It is located on a heat sink on the same bracket as the coil. Code 41 can be set any time the engine is turning. The PCM knows if the engine is turning by reference pulses from the Opti. If the ICM fails while the engine is running, the engine quits, tach goes to zero, and a H62 "RPM signal malfunction" also sets in Module 9, ABS/TCS.

Codes 16 and 36 can only be set when the engine is turning. The PCM watches both lo-res (Code 16) and high res (Code 36) signals and if one is present without the other. the signal that is missing sets it's code. 16 and 36 will always be H-codes because they can only be "current" with the engine turning, which it isn't when you're pulling codes, and if 16 sets it doesn't run. If I find an H16 on a cranks but won't start, I clear it, crank it for a few seconds and recheck the codes. If H16 is back it is the opti.

Are you using the onboard diagnostics by jumping pins 4 and 12 in the diagnostic connector, then watching the speedo for the codes? If not, you should be.

 

Okay, so at this point you are passing Test 1 and 2, but failing Test 3 and 4, right? Make 100% sure you connected the test light per the instructions. I don't think it should stay off if there's not signal: I think it should stay on all the time. If you are very, very sure that the ICM is not getting a triggering signal from the PCM, then you need to make sure (again) that there's good continuity from the PCM to the ICM on that signal wire. There is a crank position sensor on a 96 C4, but not on your car. The PCM gets its position data (to generate triggers for the ignition and injectors) from one or both of the opti signals. However, I would expect to see code 16 or 32 if either opti signal were missing, as IHBD said. I would recheck your codes in the manner IHBD suggested. If the opti signals are good but the PCM is not sending any triggering signals to the ICM or injectors, then I would say the PCM or a ground to it is faulty.

 

I can buy a new icm worst case i waste 80 bucks and call it preventative maintenance. I have never gotten either of the opti codes before just the 41 and 62 codes. ive jumped pins 4 and 12 to pull my codes i believe i posted them earlier. per the 94 fsm to diagnose h41 you clear all codes then crank for 10 seconds. h41 came back the only thing that makes no sense is why does replacing my opti fix the no start issue each time? the icm i believe is stock to the car. if the icm is faulty could it cause the ignition coil and opti to fail by sending improper signals? one coil blew up now my current brand new coil isnt sending the 12v signal to the icm. unsure if a bad icm could cause all this or if it may just be a small piece in a long list of broken parts

 

You are correct. i have no bad opti codes and i repeated the tests several times and the results were the same. ill check continuity again but it did pass the first time i checked. again what makes no sense is why does replacing the opti fix the no start condition? can a faulty pcm somehow destroy optis and blow up ignition coils?

 

The statements I highlighted in red caught my attention.

Why have you NOT yet replaced the ICM ? Code 41 with 62 in Module 7 is the ICM. << At least begin with a new ICM, even a parts-store cheap-o should make it run again. Get a 'stock' coil at the same time. If the ICM has a white or yellow paint dot on it, and "Singapore" on the back, it is the original assembly-line ICM, replace it!

I have no explanation for why replacing the opti makes the car run again. But why are you messing with the opti when there are no opti codes 16 and 36?

The ICM fires the coil by grounding the dark green wire. If the ICM has failed in a way where the green wire is shorted to ground, or the "dwell" time has changed, the coil will run hot, or if shorted, be "on" at all times. This could damage a coil, and it definitely would not run.

I have also observed that peeps frequently report having trouble with MSD components, and the issues are resolved by removing the MSD stuff. All of it, including plug wires.
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It is going to cost some money but the aggravation should end, and you can again trust driving your car and your ownership experience will improve. Do the following:

Purchase a new GM or Delphi coil. Purchase a Delco or Delphi ICM. Purchase a Petris optispark and set of plug wires from him. Abandon all the MSD stuff, replace it with known-quality components, especially the Petris.

 

Unfortunately (because it will entail spending money and time), I agree with all of this. Before I scrolled down to IHBD's reply, I was also going to speculate that an ICM that triggers spark too long or all the time could ruin parts downstream of it. I don't really think it could hurt the actual optical sensor in the distributor, but it might fry the cap and rotor. That said, the labor to verify a good optical sensor and then change cap and rotor is actually more than just replacing the entire opti, and at this point it makes sense to just get the one known quality replacement opti (Petris) and be done with it...after also replacing the ICM and coil, of course.

 

Everything youve said makes total sense i will be buying a new icm very shortly. my remaining concerns are:
- I dont have a good signal to the icm from the pcm. the fsm says i should be getting 1-4vAC and im getting .2. i have good continuity between the wire and the pcm so the fsm says its a faulty pcm or pcm connection. unsure if code 41 could be a pcm/pcm wiring issue and not an icm issue
- I am not getting any tach movement or injector pulse. unsure if code 41 stops the pcm from reading the signals from the opti or if the icm/coil failure damaged the opti. replacing the opti has fixed this issue every single time so very unsure how the opti could be completely okay at the moment. i dont have an oscilloscope
My current vague guess of whats actually happening is i have a bad icm/pcm/wiring harness/wiring harness connector. unsure which of the 4 but either way, one of the 4 causes a "glitch" thats causing damage to the ignition coil which is then damaging the distributor. When i replace the distributor the car runs perfectly well until one of those 4 items "glitches" again damaging the system but mainly the opti causing the no start. this explanation is the only way it makes sense that replacing the distributor fixes the issue, while also explaning all the other various issues and codes. Now i gladly ask that you point out flaws in my logic as im not entirely sure how the pcm functions and if one of my 4 problem areas could be presenting itself as opti failure
-

 

ill be calling msd as im just barely past my 1 year warranty date so i may be able to get certain components replaced free of charge. i just mentioned in my previous reply that i believe either the icm/pcm/pcm harness/connectors is broken. thats causing the coil to fire incorrectly thus damaging it, and the coil is damaging the opti. i dont believe any of my msd components have actually failed but have instead been damaged by something stock to a 31 year old car. ive very much liked the msd 6al box and even the distributor for having the ability to advance timing manually. if i can diagnose which of my 4 potential problems is the actual problem, fix that, then get new msd components for free under warranty id very much prefer that over scrapping a 1k+ system and spending hundreds more on new stuff. if the msd part fails due to bad quality then thats on me but as of right now i dont want to blame this issue on msds quality

 

> The tach signal comes from the dk green wire on the ICM. If the ICM is not 'switching' the coil, there is no tach signal. (NOTE: My 1995 FSM shows pin A13 on the PCM as "TACH OUTPUT". This is a typo error. It is Tach Input. The tach signal comes from the module-controlled dk green wire. )

EDIT: I'm not so sure the above is correct, so disregard it. It looks like the tach signal does come from the PCM (beginning in 1994?) and there is even a Code 99 DTC that monitors the state of this output. My 95 FSM shows a connection between the coil/ICM green wire connection and the PCM tach output. I don't know why these would be connected, and it may be a misprint, but I don't have a car to verify it on.

> There is no injector pulse because Code 41 disables the injectors. (Read "Action Taken" in the Code 41 diagnostic.)

Your symptoms (PARTICULARLY H41) indicate that YOU HAVE A BAD ICM. Replace it, and report back with the results.
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I HAD a Code 41 in my 1992. A half hour into a drive it hiccup'd a few times over about 10 minutes , then just died. The SES, SVC TCS, and ABS lights came on and before it died completely, the tach would go to zero when it cut out briefly. While waiting for the tow-truck I pulled the codes. I had H41 in Module 1 and H62 in Module 7. (Sound familiar?) I knew what it was before the tow truck arrived. It did fire and run a few times after it cooled down, but didn't stay running for more than a minute. I replaced the ICM. My car was running an hour after the truck dropped it in my yard. Most of that hour was how long it took to go get the new ICM.
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Stop guessing. Believe the Diagnostic codes. Believe the two guys in this thread that know what they're talking about and are trying to help you.

Replace your ICM!

 

Couldnt log into my regular account for some reason. Anyways i ordered a new ICM will install it tomorrow. If it starts then lovely. If it doesnt ill run the codes again and report back. My car died very differently than yours. No lights except the battery light came on. The car sputtered once then ran fine for 1 minute before stalling and wouldnt not start again reguardless of temperature. Ive replaced the opti 3 times now and never touched the icm yet the car starts each time. Not debating your knowledge just painting a clearer picture of my sitch. Will reply tomorrow with results of a new icm. Thank you for all your help so far genuinely mean it.