C3 Tech/Performance V8 Technical Info, Internal Engine, External Engine, Basic Tech and Maintenance for the C3 Corvette

1981 ECM/PROM Questions

 
Old 06-18-2018, 03:49 PM
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Hamerdown
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Default 1981 ECM/PROM Questions

Having issues with my 1981 and what I've just come to realize the 81 ECM/PROM is a standalone Computer that is different from 81 and earlier and 81 and later. (lucky me I have the Pink Elephant)
BTW...5 years ago I removed the Air-Pump and Catalytic Converter, plugged heat risers and unused-vacuum ports and the engine ran GREAT with no CEL.
*No CEL coming on...all of a sudden Engine is idling poorly and laboring-down like the Distributor-Advance-Retarded with very strong exhaust odor and again, it's not tossing a CEL.
*MCS...is not clicking (actually I don't recall when it last did, but it did) however when running a dedicated hot & ground to the unplugged MCS and tapping the ground I can make it click.
*with a 12v Test light and key-on, BOTH wires going to the plugged-in MCS are HOT.
*when I test the MCS PLUG-out with key-on, one is HOT the other DEAD...when plugged-in, voltage is running through the MCS.
^^^ also, when I jump the two furthers LEFT terminals on the ALDL and turn key to 'on' the MCS will continually chatter telling me the MCS does function...HOWEVER...the CEL repeats/blinks **-*** pause **-*** until I turn the key off.
*next...I test o2 sensor: at the ECM plug I locate wires 9+14 and with a meter set to DCV I start engine, bring it to operating temp and I get a good reading of .2 -.9
*next...with key-on I force ground that GREEN wire on the passenger side of the carb and the MCS again will click once every time it's grounded.

So....I'm now starting to wonder 'IF' something in the ECM or PROM is not functioning properly???
Questioning 'IF' I get a re-manufactured ECM from 'wherever' and I install my CHIP, will that be the fix for the restarted Dizy-timing, poor Idle, the non-clicking MCS...or...what if my CHIP itself is bad?
Is, there a place that sells the CHIP/PROM for my 81 ECM service #1224810?
Is, there a place that sells the refurb ECM with the proper CHIP already installed?

*** out of frustration and time spent diagnosing this or that, I'm about to rip all the computer stuff out, install a old non-computer controlled Q-Jet (I would prefer a Holley 4150) non computer controlled Distributor and I believe B & M makes something that will lock-up the Overdrive/Converter on my TH350c Transmission and, go old school to 'self tune'.

Feedback...PLEASE!

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Old 06-18-2018, 06:10 PM
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Borrow an old snap on brick scanner , you can read all the data and for some reason it doesn't communicate and there is power and ground at the connector I would say the ecm is bad , I would not just buy one without knowing for sure it's dead .
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Old 06-18-2018, 06:33 PM
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Originally Posted by av81 View Post
Borrow an old snap on brick scanner , you can read all the data and for some reason it doesn't communicate and there is power and ground at the connector I would say the ecm is bad , I would not just buy one without knowing for sure it's dead .
Thanks for the feedback.
Scratching my head trying to figure out 'who' has the old Scanner.
Typical Auto stores only have them for 96 and up.
When you commented ..."there is power and ground at the connector" are you referring to the two ECM plugs on the side? I did try a dedicated ground to the ECM case from Battery Ground...nothing happened.
Then with key-on I grounded the Green connector wire at next to the Carb (used for something) and the MCS clicked.
Of course I don't want to just throw parts at this thing...been trying to self diagnose as best as I can, I do not shine when it comes to electrical issues.


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Old 06-18-2018, 06:57 PM
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I am talking under the ash tray where you plug the scanner in , as long as there is power and ground there the scanner will read data , I would grab the local snap on truck he must know who has an old scanner . If you were local to me I would lend you mine , It works great on the 81 plenty of data .You can short out the two left terminals and read the flashes(check engine lamp) , if it's working right 12 will flash first followed by the trouble code.
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Old 06-18-2018, 07:19 PM
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Originally Posted by av81 View Post
I am talking under the ash tray where you plug the scanner in , as long as there is power and ground there the scanner will read data , I would grab the local snap on truck he must know who has an old scanner . If you were local to me I would lend you mine , It works great on the 81 plenty of data .You can short out the two left terminals and read the flashes(check engine lamp) , if it's working right 12 will flash first followed by the trouble code.
Ahhh here's where there may be an issue.
when I did jump the two left terminals (paperclip) and turned the key to run/on, 12 never flashed!
it just kept repeating **~***, **~***, **~*** over and over until I turned the key off.
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Old 06-19-2018, 09:08 AM
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derekderek
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The computer controlled carbs of the early 80's were a baaaad idea when new that doesn't improve with age. They were trying to get efi emission numbers with a carb. Take that ecm and carb down to the river and float-test them.
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Old 06-19-2018, 09:30 AM
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Originally Posted by Hamerdown View Post
Ahhh here's where there may be an issue.
when I did jump the two left terminals (paperclip) and turned the key to run/on, 12 never flashed!
it just kept repeating **~***, **~***, **~*** over and over until I turned the key off.
What does that mean (**~...), If the ECM is operational and the pins are shorted, it will always blink 12 -- then trouble code. If there is no trouble code it will just keep blinking 12. If the ECM is bad (there are more checks you should do like assuring it has power etc) then they are still readily available and relatively inexpensive.

Removing all the smog stuff will not effect the ECM. the 81 ECM doesn't monitor any of it.

As for removing it -- many have done it. All you need to do is change the carb and the distributor as you mentioned. But the stock ECM setup will also run fine. Usually pretty easy to fix -- follow the troubleshooting trees in the service manual for the fault codes you get.

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Old 06-19-2018, 11:02 AM
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By you typing out the code that you see and typing it as "**---***"

That's telling me you have a code 23.

So...make sure that when you try this again.. It should go " *----**" which is a code 12.

Please respond to this and verify IF it is like I typed it out. ONE flash...then about a half second break THEN two flashes.

THIS IS VERY IMPORTANT!!!!!!!!!!!!!!

Have you played around with your timing at all???...or installed new plug wires???

DUB

Last edited by DUB; 06-19-2018 at 07:11 PM.
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Old 06-19-2018, 12:08 PM
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Originally Posted by DUB View Post
By you typing out the code that you see and typing it as "**---***"
That's telling me you have a code 23.
So...make sure that when you try this again.. It should go " *----**" which is a code 12.
Please respond to this and verify IF it is like I typed it out. ONE flash...then about a half second break THEN two flashes.
THIS IS VERY IMPORTANT!!!!!!!!!!!!!!
Have you played around with your timing at all???...or installed new plug wires???
DUB
DUB
When jumping the two far left terminals at the ALDL and turning the key to 'run' or 'on' the CEL never blinked 12 times...so is that showing an issue within the ECM?
It just blinked **pause*** repeatedly and the MCS was chattering until I turned the key off.
I believe code 23 is for for the MCS however as mentioned above and prior I can get the MSC to click by unplugging and testing with a direct hot & ground wire.
My first post explains in detail (as best as I could explain) what I have done so far and what I have tested...I'm starting to think the ECM may not be functioning correctly and being when I jump the two left terminals the CEL didn't first blink 12 times.


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Old 06-19-2018, 12:16 PM
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Originally Posted by wajulia View Post
What does that mean (**~...), If the ECM is operational and the pins are shorted, it will always blink 12 -- then trouble code. If there is no trouble code it will just keep blinking 12. If the ECM is bad (there are more checks you should do like assuring it has power etc) then they are still readily available and relatively inexpensive.
Removing all the smog stuff will not effect the ECM. the 81 ECM doesn't monitor any of it.
As for removing it -- many have done it. All you need to do is change the carb and the distributor as you mentioned. But the stock ECM setup will also run fine. Usually pretty easy to fix -- follow the troubleshooting trees in the service manual for the fault codes you get.
**~*** means, 'blink,blink-pause-blink, blink, blink' or I'm guessing code 23 which I believe is a MCS.
But for a simple test with the MSC unplugged and a direct hot & ground wires I can get the MCS to 'click' and when jumping the two far left ADLD terminals and key-on the MSC wit chatter.
Also...the ECM never gave a continuous 12 blinking but just started the **-pause-*** over and over.


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Old 06-19-2018, 02:46 PM
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check pawn shops for the old snap on bricks.what issues are you having .
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Old 06-19-2018, 03:26 PM
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I would believe you have a problem in the ECM or wiring because there is a page in the Factory Service Manual on page 6E1-16 where there is a chart that will provide the information to correct the ECM from NOT flashing code 12.

The chart for the M/C Solenoid is specific in the service manual and just because it is making noise means nothing or can mean that it is actually OK but your ECM is shot You are supposed to be checking for voltage also ohms depending how your test results send you in the flow chart.

The Snap-On scanner part number is a MT 2500.

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Old 06-19-2018, 04:54 PM
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Originally Posted by DUB View Post
By you typing out the code that you see and typing it as "**---***"
That's telling me you have a code 23.
So...make sure that when you try this again.. It should go " *----**" which is a code 12.
Please respond to this and verify IF it is like I typed it out. ONE flash...then about a half second break THEN two flashes.
THIS IS VERY IMPORTANT!!!!!!!!!!!!!!
Have you played around with your timing at all???...or installed new plug wires???
DUB
DUB
I took the seat and battery out to work around the Computer area. I can reconnect the battery and allow the Engine to idle-up to temp again and recheck at the far left ADLD terminals (with paperclip) to see 'if' this time it blinks 12 times before tossing a code...but again as bad as it was Idling prior it never threw out a CEL.
I have not altered Timing or new wires/plugs in several years.
I 'may' be able to borrow the older Snap-On Brick scanner but have no ideas how to work the thing...it would be great to be able to speak via phone to someone further.

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Old 06-19-2018, 04:55 PM
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It doesn't blink 12 times , it's one blink pause then two blinks meaning 12 , make sure the scanner has the multi 1 connector .
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Old 06-19-2018, 05:03 PM
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Originally Posted by Hamerdown View Post
I took the seat and battery out to work around the Computer area. I can reconnect the battery and allow the Engine to idle-up to temp again and recheck at the far left ADLD terminals (with paperclip) to see 'if' this time it blinks 12 times before tossing a code...but again as bad as it was Idling prior it never threw out a CEL.
I have not altered Timing or new wires/plugs in several years.
I 'may' be able to borrow the older Snap-On Brick scanner but have no ideas how to work the thing...it would be great to be able to speak via phone to someone further.
I have found that the scanner does not tell you much more than reading the codes by the paperclip method. You don't need to start the car, you should get the code 12 just by turning the key.

With the codes and the service manual you should be able to fix the issue. Do you have a service manual? I can send you a copy of the troubleshooting tree for no code 12 later today.

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Old 06-19-2018, 05:12 PM
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Originally Posted by wajulia View Post
I have found that the scanner does not tell you much more than reading the codes by the paperclip method. You don't need to start the car, you should get the code 12 just by turning the key.

With the codes and the service manual you should be able to fix the issue. Do you have a service manual? I can send you a copy of the troubleshooting tree for no code 12 later today.
No Sir, no 'service manual'.
BTW...I was a member of this forum sometime ago but for some reason my account was deactivated.
With the help of some here I was able to rebuild my TC350c, rear dif, and Trailing Arms. Since a little kid have always had my hands on wrenches, and have worked on Diesel Engines, Drag Boat Engines, Harley's etc but when it comes to wiring/electrical issues I crap the bed.

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Old 06-19-2018, 05:21 PM
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Originally Posted by wajulia View Post
I have found that the scanner does not tell you much more than reading the codes by the paperclip method. You don't need to start the car, you should get the code 12 just by turning the key.

With the codes and the service manual you should be able to fix the issue. Do you have a service manual? I can send you a copy of the troubleshooting tree for no code 12 later today.
not true the scanner reads all the values of every sensor and you can test some with the scanner , if it's interment you can drive the car and make a movie and read the past data from the drive , not sure what scanner you speak of but not the 2500 brick. One issue I saw the flashing codes don't show if you have no tach signal going to the ecm that is a code 12 you won't know cause 12 also is a all clear code . By having a scanner plugged in you can see rpm 0 then you start with an ignition control module to get the rpm reading.
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Old 06-19-2018, 07:22 PM
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I can tell you that using a Snap-On scanner on your car will work but it will not work due to the provision on the need of the cable of the scanner will not go down into the ashtray area. I have a Snap-On one and also MasterTech one and I have to use the MasterTech one.

But this does not mean the Snap-On one can not be used...all you have to do is move the ALDL connector by removing the right side console strip.

You get a scanner I can help you out if you call me. PM me if you are interested.

Waiting to hear from you to see if you get a code 12 liek i wrote in POST#8
Please respond to this and verify IF it is like I typed it out. ONE flash...then about a half second break THEN two flashes.

DUB
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Old 06-19-2018, 08:32 PM
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Originally Posted by DUB View Post
I can tell you that using a Snap-On scanner on your car will work but it will not work due to the provision on the need of the cable of the scanner will not go down into the ashtray area. I have a Snap-On one and also MasterTech one and I have to use the MasterTech one.

But this does not mean the Snap-On one can not be used...all you have to do is move the ALDL connector by removing the right side console strip.

You get a scanner I can help you out if you call me. PM me if you are interested.

Waiting to hear from you to see if you get a code 12 liek i wrote in POST#8
Please respond to this and verify IF it is like I typed it out. ONE flash...then about a half second break THEN two flashes.

DUB
the multi 1 fits without removing nothing but the ashtray , it has 4 or 5 inch colored leads with male spades you plug in one at a time ,
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Old 06-19-2018, 09:05 PM
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Originally Posted by Hamerdown View Post
No Sir, no 'service manual'.
BTW...I was a member of this forum sometime ago but for some reason my account was deactivated.
With the help of some here I was able to rebuild my TC350c, rear dif, and Trailing Arms. Since a little kid have always had my hands on wrenches, and have worked on Diesel Engines, Drag Boat Engines, Harley's etc but when it comes to wiring/electrical issues I crap the bed.

The troubleshooting tree for no 12 code. For good measure I also provided the one for Code 23, but you MUST get the 12 code before you start looking for any other issue.

Fixing the ECM is usually pretty straight forward IF you follow the troubleshooting trees. To many people just start stabbing in the dark and get frustrated.
Attached Images
File Type: pdf
1981 Corvette ECM no codes.pdf (381.1 KB, 27 views)
File Type: pdf
1981 Corvette ECM 23 code.pdf (270.6 KB, 20 views)

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