My 1991 6 speed ran good all last summer. Then on the day I was parking it for the Wisconsin winter the Service Engine Soon light comes on and SYS blinked on the dash. Now that it is warming up it was time to check out the problem. A couple weeks ago jumpered A to G on the ALDL and got Module 1 (CCM) codes of C41 and H41 and Module 4 (ECM) code H51. When I drive it, the idle is sometimes high and wavers and it can run rough. Then the light goes out, SYS stops flashing and it runs fine for a while. Then the light will come back on along with the SYS message again. The Inst MPG always stays at 0. I disconnected power and reseated the EPROM and tried it again. No change in the problem except when I check the codes again Module 1 still hase C41 and H41 but Module 4 now says Err which indicates loss of communication with CCM.

This is what I think the errors mean:

C41 = Cylinder Select Error (Check wiring at ECM. Possible PROM failure, or incorrectly seated PROM. Reseat PROM. Possible ECM failure.)
H41 = History - Previous logged Cylinder select error
C51 = PROM/EPROM Error (Faulty or incorrect PROM in the ECM. Change PROM with correct version for automobile)

Has anyone else delt with these errors before?
If I need a new EPROM or Engine Computer, not sure where to get one since GM may have obsoleted them.

I am assuming the EPROM is in the ECM which is right on above the battery. I am not sure where the CCM is?

Since the Service engine light and the SYS come and go, can I assume that the problem is intermittant while I am driving, meaning that it could be a bad connection or ground somewhere comes and goes?

I drove it again after cleaning up the battery connections and the ground connections to the body below and slightly inboard of the battery.

When I started it the Service Engine Soon lite came on, it idled funny and the SYS message was on the display. Once I started moving the speed stayed at 0 and the Inst MPG stayed at 0. Then after a couple blocks the Service Engine Soon light went out and the speed and inst MPG began to display and it ran fine. Then a little further down the road the Service Engine Soon came back again and the dispalys went back to 0 with the SYS message flashing occasionally. During this time the odometer did not increment mileage. Other than idling a little funny and jerly shifting a low RPM the car seams very drivable.

I drove it again a week later and the above repeated.

The battery is 8 years old but starts the car fine. I would not think the battery would be the problem though.

Not the Battery - Although the old one was failing a brand new one had no effect on the problem. (5-5-10)
Ordered a used, "tested" ECM on ebay. Hopefully some results will be sean. (5-18-10) I would like to find an EPROM, but they are no where to be found.

That did it. 6-6-10. As soon as I started it up the errors went away. I made a few short drives with good results.

 

Sorry to dig up threads but this is what I think is happening to mine at the moment. All the symptoms are exactly the same.