A typical '81 problem: ECM??? or related? I am lost

A few weeks ago I had, as a matter of routine, checked the error codes, and was pleased to have none: only code 12

Today I went for a short drive, and the "check engine" stayed on, occasionally out for a very short period ( like 1/2 second)
Car drove well.
Checked the codes, and got all sort of weird things, sometimes a steady range of between 20 and 30 flashes.
Got the manual out, and, because I did never get a code 12 at the beginning, I arrived at Chart #6 [page 6E1-16 of the service manual] and followed all instructions to the letter.
I have ended at the bottom right corner "Faulty ECM connection or ECM".

I am now not getting ANY flashes when I put connection to ground either at the test connector, nor at the ECM. Just a steady light.
I have cleaned all terminals from both connectors, and lubed them with Electrolube, also double checked that all pins were fully 'home' in the connector housings, all to no avail.

I never had a short or something, and also thought that the ECM's are bombproof; why now all this?
And I have sufficient experience with electronics to have the PROM back to where it belongs, no errors there!

This ECM trouble is all ONE DAY (today's) experience, but I have done the same thing a number of times, have checked and rechecked all connections, cleaned the ECM contacts on the print and on the plug spring contacts, checked they all all fully home, and the I get no blinking at all when I put contact #5 to ground, just a steady light

Had the ECM out and opened the case, double checked all the PROM pins are straight and not out of line or bent........ all OK

I like to think, that all ECM wiring goes directly from A to B, without connectors on the way; at least that's what the schematic shows.

Following day I removed pin 'G' from the plug in order to isolate a possibility that the Check Engine Lamp is being lit by a short to ground somewhere.
However this is not the case, and contact G on the ECCM is on ground, which lights the CEL

Also confirmed that the bridging of the two test contacts under the ashtray indeed made a good connection,
for this purpose I grounded contact 5 'diagnostic enable' on the ecm directly; also then I could not get the light to blink.
So I have a continuous CEL while driving, and also when I go into the test facility.

Why does this happen? I seem to have progressed from in intermittent problem to a permanent, always present problem; and again, the fault chart #6 indicates a defective ecm...
The car now starts with difficulty, and not at the higher rev's as normal after a cold start.
Also the car is smelling terribly, obviously running extremely rich. The dwell meter, when connected goes completely out of scale.
I contacted a vendor of refurbished units at Ebay, who confirmed my opinion, that the ECM's are extremely seldom at fault, so a unit is non returnable after the sealed bag in which they come has been opened..........

Has anyone a hint where I went wrong, or overlooked something.
I do not mind too much to invest in another ECM, but would hate to think that this does not resolve the fault.
Thanks

Cor

 

Unless you know someone else who has an '81 and will let you swap ECM's to see if that fixes your car...or not..., I think you might have to 'pop' for a new ECM to find out. Identifying the ECM as the 'problem' is a process of elimination: "if everything else is OK, the ECM must be the problem". Thus, there is no definitive test for ECM integrity. The only way to know is to change it and see if the condition goes away.

 

I wouldn't say that the 81 ECM is Bomb proof. I once had a slight arc from the positive battery cable to ground and it fried the ECM. My symtoms were different then yours but it was a bad ECM. I paid 75 dollars for a reman unit from Autozone car has run great ever since.

Good Luck

 

When I got my '81 a couple of years ago ECM was not working and car ran in default mode which was real rich. Installed new ECM from NAPA and had to change out some other sending units too such as O2 and Water temp but finally got all working as factory. I still from time to time get a short Check Engine lite on but it goes back out, still believe a times its the O2 reading....these are 30 years old cars. I would suggest disconnecting the battery to re-set the ECM and that will clear out all codes. Restart and run the car, now it will run perfect until the ECM reads all the sensors and resets itself, mine takes about 20 miles to get back to normal.

 

VetteV8,
I replaced the ECM in my 81 two times. I have just put in a new crate motor and eliminated the computer all together. I got my new ECM from NAPA and one from Auto Zone. They do go bad.

 

I had a similar problem to yours that had been getting worse for about a year. In my case the ECM would not light the "check engine" lamp, so I couldn't retrieve any error codes. I swapped the ECM with a remanufactured one from Rockauto, hoping to be able to read error codes to troubleshoot the poorly running engine. When I did the swap the check engine light worked and the engine problems with mixture and timing advance were also fixed.

 

I was lucky to find a fellow '81 owner [150 miles away from here] who kindly tested my ECM, and it turned out bad; he also checked my PROM in his ECM, and it checked OK
So a new ECM is on its way from Rockauto!
Will keep you posted
Thanks for all feedback.
Cor