Hello Again,
I am planning on replacing my ECM with a used one because the check engine light comes on shortly after start up and remains on. I can not retrieve any codes including the 12. The FSM says that the MEM CAL is bad and replace the ECM. Also the display on the dash where the check engine light is located is very flaky, as I can touch or press on it and the light will come on but if I pull it out a little bit the light goes out. Regardless of this I still should be able to retrieve codes, shouldn't I ? Now if I replace the ECM, I read it has to be programed. I called a Corvette shop in town here (Orlando,FL) and I was told that a 1991 did not need to be programed. I understand that the ECM fits many different cars, so any one will work by just replacing, or plug and play? I am a bit confused. Any help out there? I did have a EGR code a few months ago, so at one time I could read codes, but not now. Kim

 

OBD-I ECMs were equipped with the Mem-Cal and Cal-Pak to run the base operations. The programming that your ECM uses for normal operations comes from a PROM that you remove and use in the new ECM...unless that is the problem. Its not unusual for a PROM to become loose or just get dirty pins so contacts are poor.

ECM programming didn't come into play until OBD-II allowed in-car during driving program access and modification. People still set their lap-tops in the passenger seat and drive and make changes at stop lites until they get performance they like...
OBD-I folks have to pull the prom, insert new one and hope its what you want. There is equipment to burn your own and write your own performance programs etc...but most guys don;t go that far because its a few hundred bucks worth of stuff thats good for nuthin else and there are too many places that do custom burns based on your input or data logs.

Look for a spare ECM on Flea-Bay but DO check your prom before buying one. They go for abouit $100-$150 used.

Your symptoms are saying dirty grounds and corroded electrical connections everywhere in the harness. Clean & inspect and repair as needed.

 

Thanks so much for the quick reply! I am a Eectronic Tech. for the USPS and I understand Eproms, as we have cards or pcb's that need the firmware flashed. I am not looking for performance, I just want the light out and everything working correctly so I can see the codes and repair if I desire to. Like I said I could read the codes until just recently. Now the question, is the Mem-cal located elsewhere on the board or is it in the eprom. If on the eprom, then replacing the ECM with the old eprom, will solve nothing. Can I just buy new or used eproms if mine are bad? Kim

 

OK,
I am back, and now it seems my ECM is fine, but my drivers info screen has a ground issue or something. I can push on it and my srs and check engine lite will come on, if I reach a finger under it and pull, lites are off and I can read codes. I am getting a 32 (EGR) and I have the vacuum line plugged as the valve diapham is bad. And now I am getting a 43 (knock sensor). Any common problems with a code 43? Kim

 

Sounds to me like you have a case of "poor harness health" over all.

I'd continue down the same path that has led to your recent success and probe the MANY grounds and electrical connections.

Being familiar with commercial systems, you already know how delicate those are to dirty or loose connections. Tiny amounts of resistence make the difference in what the ECM can see and how it responds. As we know resistence changes with the quality of the connections.
You have several points of interest...
the ground bundle on the block near the oil filter, the power distribution/source bundle on the jumper post or on/behind/under the battery. And the harness entry plug behind the battery.
Since your symptoms are mostly in displays and interior, I'd focus on the harness and display conditions inside the dash and center display. Those all come apart easily and are servicable with some skills and good eyesight. Do a search for C4 proms and you should find some good pics and details that you can look at and get a better understanding of what you're dealing with. GM/Delco made the prom portion quite easy to work on...actually the whole ECM unit is easy.

The calibration chips are on the board...so you'd have to do some board surgery or just replace the whole thing for an unknown (used) unless is was a refurb unit with a warranty. IMO cal paks don't easily fail in this way. When they act up....its major running issues that cannot be traced elsewhere. Major..like cannot rev past 3000 or spits and backfires constantly..or will not start & run at all.

FWIW..
the EGR code to me, says harness. The ECM has no way of knowing when/if the EGR actually operates. It DOES know if the signal to open or related circuits are compromised. So, it sets codes when things in the series get broken...like time/thermo switches (if equipped) or the EGR solenoid that comes from an ugly cluster of splices in the harness.
What will do you the most good at this point is a good FSM with color drawings on the electrical system. The 1st book is mechanical the 2nd book is pure electrical...real drawings in detail with GM diagnostic proceedures. Worth its weight in gold when you have a problem. Parts cost too much to start tossing at "maybes".