Issue resolved! Thread continued and resolved at this link: https://www.corvetteforum.com/forums...missing-1.html

Update 3: New Opti in from SacCity and still no start. Back to the drawing board. The pistons looked alright from underneath. Maybe I'll take off valve covers and look at the heads. Maybe a compression test. Any ideas appreciated.

Update 2: Looking at the inside of the opti, I think I may have installed the drive wrong and bent the optical disk module in the process. See video in thread.

Update 1: I finally got the timing cover off to find that the chain and sprockets all seem to be in working order. No broken chain or missing teeth, and it is in time. Glad to see it is not mechanical. Now, I am thinking either 1) I installed the opti drive incorrectly so it was way out of time or 2) I need to get an oscilloscope and check the wave forms from the opti and ICM.

Hello, I have a 1994 Corvette LT1 with about 190,000 miles that died on me while driving a couple months back. I towed it to a shop who read the code for No Low-res pulse from the distributor. I have read that this was a common issue, so I towed it home and replaced the distributor and water pump.

Still no start, no spark.

Next, I replaced the ICM and coil. (throwing parts at it is not smart I know, but I figured they needed to be replaced at this mileage anyway.)

Still no start, no spark.

I followed the FSM and found I was intermittently getting a good AC pulse from the PCM to the ICM when cranking, which made me suspect PCM. I spoke to a Corvette specialist shop in town. They said to use a Noid light to check for PCM to injector pulse, and if that was gone too then it is good suspicion for PCM failure.

So, I sent my PCM for testing at SIA electronics. They just sent me a message saying all circuits test as they should... They are suggesting it is possibly CCM. That wasn't on my FSM diagram, so I did not know to check it.

Anybody have any advice for me? I would love to fix my car myself, but I will have to take it to the dealer if I can't get this figured out soon. Thanks to anyone who reads this! You guys are great

 

What is your fuel psi during cranking?
Does it sputter or backfire at all or just cranks and cranks?

 

I would recheck the wiring of the ICM, the Coil & Opti/Distributor Rerun the Voltage tests in FSM, Also where did the Opti/Distributor come from the Chain store ones can be a little Unreliable and it might take a few before you get a winner.

 

Thanks for this idea. I could hear the fuel pump priming, but I need to test the pressure. I will do this as soon as my PCM gets back this week.

It doesn't sputter or anything. Just cranks and cranks.

 

Thank you. I will retry the tests when I get the computer back.

I had also heard they can be dead on arrival, so I replaced the first aftermarket one ("WWP" I think) with a Cardone with no change. Both are supposedly fully tested products.

 

I got the PCM back and rechecked all the voltages according to the PCM. (I will post the charts I have been using below.) The voltages to the Ignition Control Module are all as they are supposed to be. The ICM is new from AC Delco.

When you say recheck the wiring you mean probing for continuity correct? From the PCM to the respective components?

 

I turn the key to "ON" and the fuel pump primes. Fuel pressure gauge shows 45 psi. When it stops priming, it holds at 40 psi. Cranking it stays at 42-43 psi.

 

Here are the charts I have been using: "Engine Cranks but Will Not Run" and "Distribution Ignition System Check."

 

Recheck your connections might just be a loose pin or a piece of crap in there.

 

I just checked continuity from the distributor wires to the PCM and the ICM to the PCM. All good without resistance.

The ICM and coil itself are new so I assume the male pins on those are as good as they look. The female connectors shown below that have been on the car for 190k miles are fairly grimey. After I took the picture I used a wire brush on both and hooked them back up. The terminals don't look corroded to me, but I am an amateur. Think I should look for new pigtails?

The distributor connection is brand new as the Cardone unit came with a new harness too.

 

I tested the voltages at the distributor harness and the ignition coil resistance. Both of those look good.

I just read a post by Shoebox on another forum. It was saying if one of these connectors is loose then you can replace the pigtail or adjust the pins to fit better. I have tried repinning connectors before and had some trouble with it.

Any advice on where to buy pigtails or good quality connector building supplies?

 

It looks to me you have covered everything pretty well. Even though your fuel pressure is showing good, I would do a couple old school tests for chits and giggles. First, pull the tb air intake hose off and squirt some starting fluid in it, not sure if you have even tried this. If it almost fires, or runs for a second, you might have another issue.

 

I had a development today that is making me think the new Cardone optispark may be the culprit. With a multimeter on DC, I tested the distributor connector with the key "on":

A: 5V
B: 5V
C: 12V
D: 0V (continuous with ground

All as it should be according to the FSM.

Next, with the key still "on," I plugged in the connector into the distributor and backprobed each terminal:

A: 0.16V
B: 5V
C: 12V
D: 0V

Finally, I had someone crank the engine for about 10 seconds with terminal A backprobed:

A: 0-1.5V range

Is this all normal? Or is this low resolution signal the problem?

I don't have an oscilloscope, but I watched a ScannerDan video in which he backprobed a distributor like this. From what I have seen, I expected the multimeter to average around 2V when cranking, varying above and below.
Edit: I read on another forum that all the above is normal. If the laser is not shining through it will be about 0V on terminal A and varying around 1V on cranking. B should vary around 2V.
Looks like it's not the opti after all!

 

It's not the CCM. IDK why SIA electronics would suggest that.

When you crank the engine, do you get a tach signal? Can you see a tach signal on a scan tool...or does the needle bounce up off "0" when cranking?

If no tach signal, it's the distributor, or the wiring from there to the PCM.

If you have a tach signal, it's "down stream" of the PCM; ICM, coil, or wiring thereof.

 

I would test the icm and the coil they can be bad right out of the box.

 

Thanks for that idea! I'm working on that now.
I think I'm getting good voltages at the ICM connector with the key "on" and cranking:

A: 12V
B: 0V (1-4V AC when cranking)
C: grounded
D: 12V (variable voltage averaging 10-11V when cranking)

This make me think the ICM is good.

I checked the resistance on the coil (aftermarket), and it is about 8700 ohms at each of the four terminals.

That makes me think it is good, but I am not sure.

At this point, I'm confident the optispark and ICM are good. It seems like the coil should be too... So, does that leave me with a loose connection somewhere, because I have checked all the wires for continuity.?

 

Thanks for that. Nothing else I have read points to that either. You probably just saved me some money shipping my CCM to them, so thank you.

The needle does bounce up off "0" at the start and end of cranking, and my variable voltages at distributor terminal A (avg. 1V) and B (avg. 2V) suggest it is working fine.

Have any other suggestions or links for testing the coil or ICM? I posted just now about what I have tried for that, but there may be more I can try.

If it comes down to wiring, I think I have good continuity everywhere. So that means a loose or corroded connector I guess. I bought new pigtails for the ICM and coil. But, the new coil pigtail doesn't want to stay on so I haven't installed that yet. ICM pigtail coming in the mail next week.

Thanks so much to anyone who is reading this and trying to help me get the old girl running again. Once we figure it out, I will definitely post a more concise overview of everything I have learned to help any fellow DIYers in the future.

 

Most part stores can test them. Some of them do not know what they are doing so since it is out take them to a few places to be sure. Have you tried shooting some fluid in it and see if it farts or starts? Might just be something simple like blown injector fuses.

 

that parts stores can test ICM's. I also agree with starting fluid as a desparate measure. It does work. I don't use it; if you have NO spark, it ain't gonna fire on anything...but starting fluid will add compelling evidence to what you've found so far.


Well hold on a sec....It should come off "0" for the duration of cranking The LT1 should crank ~250 RPM...which should be visible on the tach. If it's not clear....I would want to confirm w/a scan tool. When you confirm or deny that for certain, as shown on a scan tool....you've just "cut the system in 1/2" as to where to chase your problem.


.

 

No I haven't done that yet. But I will pick some up when I go to test the ICM so I can try it.