maranello_man

All,

I am new to the Corvette Forum and am stumped by this problem. Here's what happens:

Car drives for about 10 minutes from a cold start then dies randomly.

Conditions During Failure that I have Verified

• Fuel is present
• Spark is NOT present (spark indicator test light)
• Tach goes away completely upon failure (drops well below zero). Even if the vehicle is still going 55MPH when the failure happens and I don't push in the clutch, the tach completely drops out. The engine is still turning over, the key is still on. I still have an oil pressure reading which drops with speed, which I would expect. This may be a clue to the problem.
• Car turns back on, once I have waited for a while (5-20 minutes)
• Car is more likely to fail when weather is warm
• Measuring spark timing signal from PCM (Powertrain control module) to ICM (ignition coil module) measures about 2VAC (as expected based on the service manual) when running or cranking (when engine is operating correctly), measuring this signal while cranking after engine has died is 0V.
• The above led me to conclude that the signal from the PCM was going away and it was a bad PCM. I soldered a wire to the output pin in the PCM and remeasured the VAC to the ICM while driving to make sure it wasn't a connection problem from the PCM. This measured 2VAC while cranking and running, and 0VAC while cranking after the car had failed.
• Based on the above, I replaced the PCM (ouch), but this did not solve the problem. (grrrr)
• I concluded that something about the ICM was making this signal that I was measuring go away so I replaced the ICM (ouch again), but this did not solve the problem either. (grrrr again)
• Codes stored are the following: C12, H64 (paper clip method). I believe C12 is a "nothing's wrong" code and H64 is an oxygen sensor code.

I need to figure this out soon to save my car and my marriage.

Any help is appreciated!

surfer92

When did the codes show?

In section 1.0, 4.0, 7.0 or 9.0?

Each section is for a different type of code.

1.0 is CCM related
4.0 is ECM and engine
7.0 is SRC
9.0 is ABS

Also, different years have different codes, so what year?

Have to say it sounds like the classic ICM(ignition control module), located at the front of the passenger side head.
Easy to get to and a little pricey, some parts stores say they test them, but I have seen many posts where people have them tested and it shows they are good and they are not.

pcolt94

The signal you are measuring from the PCM that drives the ICM actually starts at the opti. The optical portion of the opti is the main clocking pulse that gets the PCM and all its timing going in relation to fuel injector drive (pulses) and the drive to the ICM.

If you lose drive to the ICM, you will lose your tach also because it's drive is off the coil (and tach filter) and if the ICM does not function, nor does the I-coil.

The ICM will not cause you to lose that drive to it (unless shorted). So your thinking of the problem seem logical to me that the problem is before the ICM.

If the opti was dropping out, I would think it would throw a code 16 for no low resolution pulse. That’s the conflict I have. So I would not jump on the opti yet but would keep it in mind. Since you like to monitor voltages, I would monitor the main 12 volts to the PCM (from ignition switch). If you lose power to the PCM, you lost it all.

I would also check real well the any related plugs connections and pins especially the 4 wire plug (and wires) to the opti.

tombrammer

These problems can be very mysterious. My 93 had similar problems. Start right up when cold, run 10-20 min then die. After cooling down start and die all over again. Much worse when warm. I eventually found it was the connector to the icm. Icm's can be tested by local auto parts store but need to be cycled until they get hot to make sure they are ok. When the motor was running I began grabbing wires and wiggling them, when I wiggled the wire to the ICM bingo, engine died and then wouldn't start. Pulled the connector and bent the little connector ends in the plug to make better contact and problem fixed. Never know it might be something else but worth a try, free fix if it works. Good luck.

MTVette

I've had two opti's go on my 92 LT1. Neither time did it throw a code. The symptoms sound similar to what you're seeing.

pcolt94

Well than OK. I was thinking opti but had no hard proof or classic symptoms. It doesn't surprise me for a minute that this could happen. Just did not want to jump to soon on the opti.

I might possibly do some more checking as indicated in the other posts. If nothing shows up it might be necessary to change the opti. Not a simple or quick job and usually more entailed. Like thats the time to change the wires and should I change the pump while I am at it.

Sometimes parts will not check bad and substitution might be required to find the problem.

PCMs are a good unit unlike the ECMs with weird problems. The Opti craps out much more.

REDC4CORVETTE

I didn't read the thread ,but try a few things.Is the plastic piece that is in the heater hose on the pass side leaking on the starter?coolent?
Is the coil good.Is their any coolent on the opti.Do you smell coolent around the engine compartment?
Did you lock both doors with the key and unlock them after waiting 15 minutes?
Have you disconected the battery for 5 minutes,then hooked it back up?
Never replace the ecm on a 94 corvette they almost never never go bad!!!
It is as rare as pulling hens teeth.Only opti coil go bad on a regular bassis.I hope you don't have a key fob!

Weav's Vet

How old is the coil? It may be getting hot and then dieing on you.

Hope it's not the opti.........been there done that. Good luck!

maranello_man

Thanks to everyone who has replied.

The car is a 1994 LT1 six speed.

The codes are 1-C12, and 9-H64.

The opti was replaced a few months before I bought the car (about 6 months ago).

Yesterday, I disconnected the ICM while the car was in a "good state" and measured the output from the PCM. It was there. When the car failed while driving, I disconnected again, and measured, the signal was gone. This tells me it's not the ICM, or a connection to it.

I'm now monitoring the ignition signals going to the PCM. There are two of them. The car hasn't yet failed while monitoring these. The initial measurement shows between 3 and 4 volts on this line, and it changes while driving. I expected 12V, but maybe this is normal. I am unsure why the voltage changes while driving. This signal seems week, but I need to check the book to see if this is correct. Any ideas?

surfer92

You could have the classic broken wire, hit a bump and the wire disconnects and then while you are playing with it, you reconnect the wire.
Also, if the ICM is not getting a signal and then gets a signal, that could spell a bad opti.

What kind of opti was installed?

MSD's have been dying regularly lately.

maranello_man

Well, I finally got around to pulling out the opti. I've been trying to find a new one, but I'm having a weird problem. All the parts I find at the stores (Advance, O'Reilly's, etc.) have a female style interface instead of a shaft like the one I pulled out of the car. The shaft on mine is pressed into a bearing and cannot be taken out and put into the distributor from the store. Maybe I'm missing something really basic here. See the picture here of an "old" opti like the one I pulled out of my car and a "new" opti like the ones I find in the stores. http://www.screencast.com/users/russ...c-a9334e6dccef

Let me know where I'm going wrong. Do I need to buy a separate shaft?

BTW - I do NOT have the new (95-96) style with the big round shaft. The shaft on the opti I pulled out is small and has splines.

Guitarguy

the shaft on the old 1 should pull out if not you will have to pull the cap and rotor on the old opti and tap it out from the inside, then just pop it in the new 1. it only goes in 1 way, kinda idiot proof.

REDC4CORVETTE

You can only install a 93 or 94 opti,all other years are different
Pace chevrolet has them.You can change the front timing cover to use the newer opti.

jaa1992

I'm going to have to try this. My 92 is acting up when the oil temps are over 200 (water is 190 ish because the fans turn on then)
I've replaced ICM, after ICM, after ICM and its time to figure this out.
Missing 20 min on the track at a good time of the day does not make for a good time trials day.

pcolt94

After looking at the pictures I agree the shaft should pullout. I would say it was jammed in there if the engine was not running and miss-aligned. But since it was running, there is probably some rust in there and you just can't pull the shaft out.

Although at this point I don’t know of it's worth the effort to mess with it. I would just install a new opti and see how the car runs after that. You have a intermittent problem and I would be surprised if something is going to test bad.

surfer92

Did you verify opti getting fire from coil?
And ICM to coil?

maranello_man

Update for everyone.

The shaft on the opti I purchased was riveted inside and was not removeable. I purchased the entire unit (opti, cap, rotor, shaft, etc.) and noticed that the shaft pulled out of that one easily just as everyone had said. I'm not sure why mine was different, but the shaft was NOT transferrable.

Before I replaced the opti, I hooked up a data acquisition board and monitored these signals: high resolution signal from opti-spark measured at the PCM), low resolution signal from opti-spark (measured at the PCM), signal from PCM to ICM (measured at the ICM connector). Here is the video of those signals (http://www.screencast.com/users/russ...d-d49c91d430cf).

What is not on the video is what happens when the engine fails. Both the high res and low res lines go to 5V. I assume the distributor "biases" these lines when it is working correctly and pulls them low.

Since I replaced the distributor, the behavior has changed. No longer does it die at random. Now it takes some time cranking (5-10 seconds) before it will start, but after starting it runs well except for certain situations. When on the freeway in 6th gear and floored, it feels like it misses every now and then. I also have code 36 stored now and did not have it stored before. My guess is that the high res pulse is no longer there and the timing is not as precise as what it needs to be resulting in a miss every now and then. Codes were not stored before because both the high res signal and low res signal would go away at the same time.

Here's the possibilities as I see it:

1. My opti was truly bad, and the new one is also bad, but in a different way (only the high res pulse fails).
2. The wiring is bad

I have kept the PCM I bought when I thought it had to be the PCM and have changed it out again, just to see what happens. The behavior between the two is exactly the same. That's why I don't think the PCM is the problem.

Since the behavior changed when I replaced the opti, I think option 1 is more likely, but I'm just not sure of anything now.

This has been going on for five months. Marriage in danger.

STL94LT1

Double check the opti wire harness.