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Help! A typical '81 problem: ECM???

VetteV8

Well-known member
Joined
Jan 30, 2011
Messages
82
Location
Netherlands, Drenthe
Corvette
'81 Bright Blue Metallic
Yes , need some help!
a few weeks ago I had, as a matter of routine, checked the error codes, and was pleased to have none: only code 12

This week I had the seats out, the console sides off, and the radio and instrument cluster removed [5 gauges]
All cleaned, polished and with new lamps back together.
All went well, nothing special.
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 am upset to 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!

I am baffled, what happened, and need dearly assistance.
Thanks,
Cor
 

Peer81

Well-known member
Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
Is this from one experience or more then one?
The thing is, I'm somebody that thinks everything is explainable in a rational manner. With my 81 I learned to think, if she still does it the third time around then I'm going to look into it. If you're already at 3, disconnected the battery for some time etc etc the easiest thing is to swap ECM's. Trail and error style.
(at the same time you can pickup your Corvettefevers :D)

Greetings Peter
 

VetteV8

Well-known member
Joined
Jan 30, 2011
Messages
82
Location
Netherlands, Drenthe
Corvette
'81 Bright Blue Metallic
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 :mad

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

Do you have an ECM spare then?
Otherwise Mr Boer in Wezep?

Are there any connectors anywhere in the console; did not think so, but there I have done some work and disturbed something. 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.
Any thoughts?
Car did run fine by the way, this PM, just the darn light being on, almost continuously
Cor
 

Peer81

Well-known member
Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
Sorry I don't have a spare but you can borrow mine for a afternoon to check if that one works better in the car.
You can alway check De Boer or v. Diepenbeek. If I remember correctly the price for a new one is not so steep.
I would say, just disconnect the battery for tonight and try again tomorrow. Maybe just a bad day today.

btw. Do you know if the ECM light has a positive switch or ground switch? The backside of the gauge panel (plastic print) can crack a line or short in another way so the light will be on (if ground switched). If your ECM isn't working your carb will be in full rich mode and will be dificult to just keep it running. So maybe the problem isn't the ECM but the light and print.

Greetings Peter
 

VetteV8

Well-known member
Joined
Jan 30, 2011
Messages
82
Location
Netherlands, Drenthe
Corvette
'81 Bright Blue Metallic
The battery has been connected a few times, even if only because it is impossible to get the ECM out, without removing the battery first; The manual states, this disconnection needs to be only 10 seconds or so; but I also have shorted the + and ground wires [without battery of course] too.
The ground from the ECM is not going through the gauge print; according to the schematic, one is going to engine ground, [wherever that my be,] and then there is a second ground elsewhere. These are contacts A and U on the lettered plug.



Cor
 

VetteV8

Well-known member
Joined
Jan 30, 2011
Messages
82
Location
Netherlands, Drenthe
Corvette
'81 Bright Blue Metallic
So, what has been done today in searching where the fault lies:

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 shorting of the 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 solid, always present problem; and again, the fault chart #6 indicates a defective ecm...
Comments are appreciated.
Cor
 

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