1981 ecu
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
1981 ecu
I have a new replacement dizzy for my 81, unplugged the 4 pin connector to set initial timing, but when I reconnect the engine stalls. If I try to force a restart it "coughs" gas out of the carb. Idles OK, not great, when disconnected.
I rebuilt the card and the electronics are working as far as I can tell.
There are lots of threads about removing the CCC and going old school but I'd like to keep the feedback system on the carb, computer control of the dizzy and of course the torque converter lockup. I've already de-smoged the car, removed the air pump, EGR, and associated controls. I will gut out the catalytic converter but plan on keeping the O2 sensor to control the carb.
I only found one thread here started in 2013 by a guy wanted to keep his 81 CCC original, but he sold the car before he solved the problem. Some help there and I plan on removing the ECU and cleaning the connections then reinstalling before I shell out another 100 for a new one.
Any help would be appreciated. Based on the numerous "removing CCC" threads here, This seems to be an unresolved issue.
I rebuilt the card and the electronics are working as far as I can tell.
There are lots of threads about removing the CCC and going old school but I'd like to keep the feedback system on the carb, computer control of the dizzy and of course the torque converter lockup. I've already de-smoged the car, removed the air pump, EGR, and associated controls. I will gut out the catalytic converter but plan on keeping the O2 sensor to control the carb.
I only found one thread here started in 2013 by a guy wanted to keep his 81 CCC original, but he sold the car before he solved the problem. Some help there and I plan on removing the ECU and cleaning the connections then reinstalling before I shell out another 100 for a new one.
Any help would be appreciated. Based on the numerous "removing CCC" threads here, This seems to be an unresolved issue.
Instructor
Joined: Aug 2010
Posts: 233
Likes: 18
From: Monrovia Md
I have a fully working ECM and CCC in my 81. Works great. I have not had to do anything to it in many years. Just starts and works great.
For your car -- does the computer blink the 12 code when you short the terminals in the ECM connector. It is a good sign the computer is operational. If you can prove it is working we can go from there.
What are you setting the timing to? It should be 8 deg.
For your car -- does the computer blink the 12 code when you short the terminals in the ECM connector. It is a good sign the computer is operational. If you can prove it is working we can go from there.
What are you setting the timing to? It should be 8 deg.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
The PO had replaced the distributor from an earlier junker with centrifugal and vacuum advance. It was running. It runs with the new correct distributor, but not when connected to to the 4-wire connector. My Code reader returns PROM 0075, and the check engine light blinks rapidly when it is connected, which lead me to believe that the ECU was functional. When not connected the light is always on.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
I have a fully working ECM and CCC in my 81. Works great. I have not had to do anything to it in many years. Just starts and works great.
For your car -- does the computer blink the 12 code when you short the terminals in the ECM connector. It is a good sign the computer is operational. If you can prove it is working we can go from there.
What are you setting the timing to? It should be 8 deg.
For your car -- does the computer blink the 12 code when you short the terminals in the ECM connector. It is a good sign the computer is operational. If you can prove it is working we can go from there.
What are you setting the timing to? It should be 8 deg.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
Unless you can tell us what prom 0075 means then best to short the terminals and give us a 2 digit code or codes.
from what I can find prom 0075 is meaningless for our vettes.
Have you confirmed you timing is correct? A slipped balancer is not uncommon. timing can be retarded far more than you would expect before the engine stops running. Not sure how far advanced you can go. But
As far as removing the CCC system goes many aren't willing to spend the time to learn and understand the system so just ditch the whole thing.
from what I can find prom 0075 is meaningless for our vettes.
Have you confirmed you timing is correct? A slipped balancer is not uncommon. timing can be retarded far more than you would expect before the engine stops running. Not sure how far advanced you can go. But
As far as removing the CCC system goes many aren't willing to spend the time to learn and understand the system so just ditch the whole thing.
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Instructor
Joined: Aug 2010
Posts: 233
Likes: 18
From: Monrovia Md
Code readers for a 1981 are very unique. It was an OBD1 with a unique connector for 1981. They exist, but they are rare. Code 75 has no meaning on this system, Blinking rapidly is also not normal behavior
Agree with just shorting the terminals and see what the codes are . Short terminals D and E with a screwdriver with ignition on and wait for the codes to blink on the CEL. You should hear a click at the carb when you do it -- that is OK.
You mentioned the PO changed the distributor with an earlier one. Is the Computer Controlled Carb still there or has it been replaced also?
Agree with just shorting the terminals and see what the codes are . Short terminals D and E with a screwdriver with ignition on and wait for the codes to blink on the CEL. You should hear a click at the carb when you do it -- that is OK.
You mentioned the PO changed the distributor with an earlier one. Is the Computer Controlled Carb still there or has it been replaced also?
Last edited by wajulia; Dec 6, 2019 at 07:14 AM.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Unless you can tell us what prom 0075 means then best to short the terminals and give us a 2 digit code or codes.
from what I can find prom 0075 is meaningless for our vettes.
Have you confirmed you timing is correct? A slipped balancer is not uncommon. timing can be retarded far more than you would expect before the engine stops running. Not sure how far advanced you can go. But
As far as removing the CCC system goes many aren't willing to spend the time to learn and understand the system so just ditch the whole thing.
from what I can find prom 0075 is meaningless for our vettes.
Have you confirmed you timing is correct? A slipped balancer is not uncommon. timing can be retarded far more than you would expect before the engine stops running. Not sure how far advanced you can go. But
As far as removing the CCC system goes many aren't willing to spend the time to learn and understand the system so just ditch the whole thing.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Code readers for a 1981 are very unique. It was an OBD1 with a unique connector for 1981. They exist, but they are rare. Code 75 has no meaning on this system, Blinking rapidly is also not normal behavior
Agree with just shorting the terminals and see what the codes are . Short terminals D and E with a screwdriver with ignition on and wait for the codes to blink on the CEL. You should hear a click at the carb when you do it -- that is OK.
You mentioned the PO changed the distributor with an earlier one. Is the Computer Controlled Carb still there or has it been replaced also?
Agree with just shorting the terminals and see what the codes are . Short terminals D and E with a screwdriver with ignition on and wait for the codes to blink on the CEL. You should hear a click at the carb when you do it -- that is OK.
You mentioned the PO changed the distributor with an earlier one. Is the Computer Controlled Carb still there or has it been replaced also?
Testing with my ohm meter, I don't get a ground at E. When I jump D to E the CEL blinks rapidly, no sequence as described in the manual.
I'll need to remove the console and clean up the ground for terminal E. Then I'll report back.
BTW my Code reader is an OTC Monitor 85. A guy had it on Ebay with an asking price of $300. I offered him $150 and he took it. I even have the hard case and paper manual.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
Likes: 630
From: Unreconstructed, South Carolina
I cannot KNOW for certain whether a damper's outer ring has slipped (or has Not) without first manually-visually verifying #1 piston is truly at TDC while Zero Groove on damper aligns with Zero on Timing Tab.
Whenever I encounter a puzzling timing issue, I begin with verifying the above.
-addendum-
https://www.crankshaftcoalition.com/..._TDC_Lines_SBC
https://www.crankshaftcoalition.com/...op_dead_center
Whenever I encounter a puzzling timing issue, I begin with verifying the above.
-addendum-
https://www.crankshaftcoalition.com/..._TDC_Lines_SBC
https://www.crankshaftcoalition.com/...op_dead_center
Last edited by jackson; Dec 6, 2019 at 11:08 AM.
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
If your getting that while just using a jumper, then the wiring is jacked up somewhere,
or the ECU is kaputs.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
I'd bet on the ECU being 'toast'. That's probably why the PO swapped the electronic stuff out. He couldn't deal with it. The '81 Shop Manual has the diagnostic testing procedure for the engine control system. The last item, if all else does not fix the problem, is to change the ECU. If you know anyone else with a working '81 C3, maybe they would allow you to put their ECU into your system to see what happens.
P.S. Not so sure about that replacement distributor....
P.S. Not so sure about that replacement distributor....
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
I've located that figure on page 6E1-8 in my shop manual and it's a bit confusing because my connector is 180 degrees off from these figures. But I can see the purpose of the slot, to allow an easy way to jump D to E.
Testing with my ohm meter, I don't get a ground at E. When I jump D to E the CEL blinks rapidly, no sequence as described in the manual.
I'll need to remove the console and clean up the ground for terminal E. Then I'll report back.
BTW my Code reader is an OTC Monitor 85. A guy had it on Ebay with an asking price of $300. I offered him $150 and he took it. I even have the hard case and paper manual.
Testing with my ohm meter, I don't get a ground at E. When I jump D to E the CEL blinks rapidly, no sequence as described in the manual.
I'll need to remove the console and clean up the ground for terminal E. Then I'll report back.
BTW my Code reader is an OTC Monitor 85. A guy had it on Ebay with an asking price of $300. I offered him $150 and he took it. I even have the hard case and paper manual.
The ECM grounds to two locations on the engine. One on the intake at the coolant outlet, one on the back of the passenger side head. (That's where they are on mine anyway). You should be able to find them by following the wires from the two 6 pin connectors on the passenger side.
A bad ground to the ECM could cause all sorts of problems.
There is a diagnostic chart to follow for no code 12 that i can post later but since you have a potential problem to solve first then I'll wait to see what you come up with.
Thread Starter
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
I'd bet on the ECU being 'toast'. That's probably why the PO swapped the electronic stuff out. He couldn't deal with it. The '81 Shop Manual has the diagnostic testing procedure for the engine control system. The last item, if all else does not fix the problem, is to change the ECU. If you know anyone else with a working '81 C3, maybe they would allow you to put their ECU into your system to see what happens.
P.S. Not so sure about that replacement distributor....
P.S. Not so sure about that replacement distributor....
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
If you get it to work, then it isn't the distributor. Until then, I'll hedge on betting on it. I have had some successes with purchase of 'electrical' stuff off the web; but much of that stuff LOOKS fine in the computer-reproduced little image. So many of these foreign-made repros only LOOK like the item, but are much inferior in terms of materials used and dimensional criteria. Just my experiences....
Le Mans Master
Joined: Dec 2006
Posts: 8,837
Likes: 1,028
If you get it to work, then it isn't the distributor. Until then, I'll hedge on betting on it. I have had some successes with purchase of 'electrical' stuff off the web; but much of that stuff LOOKS fine in the computer-reproduced little image. So many of these foreign-made repros only LOOK like the item, but are much inferior in terms of materials used and dimensional criteria. Just my experiences....
Drifting
Joined: Nov 2018
Posts: 1,741
Likes: 636
From: USA
For what it's worth, check out the "Pay-it-forward" stickies on the C3 parts for sale page on this forum. Post # 47, but someone is giving away a factory re-furbished 1981 ecu! You'll need to offer up something to gift, but doesn't need to be expensive, just something a fellow Corvette lover could use! Make sure to read the first post and then find that at ecu and claim it, should help find out if it's the ecu or eliminate it from your possible suspects! Good luck!
Last edited by '78CorvetteS.A.; Dec 7, 2019 at 06:57 AM.
There ya' go!! Merry Christmas early....