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

1981 ECM/PROM Questions

 
Old 06-19-2018, 11:13 PM
  #21  
lionelhutz
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The wrong flashes and the MCS chattering at the same time would make me suspect an issue with the grounds for the PCM. Check you have both good battery voltage and good grounds to the PCM. I'm not sure where that PCM grounded, but the TBI stuff had ground wires that connected to intake bolts. Don't ground the case since that has destroyed other PCM's before.
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Old 06-20-2018, 12:46 AM
  #22  
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Here is the chart referenced in wajulia's post.
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Old 06-20-2018, 11:35 AM
  #23  
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Originally Posted by lionelhutz View Post
The wrong flashes and the MCS chattering at the same time would make me suspect an issue with the grounds for the PCM. Check you have both good battery voltage and good grounds to the PCM. I'm not sure where that PCM grounded, but the TBI stuff had ground wires that connected to intake bolts. Don't ground the case since that has destroyed other PCM's before.
Thanks for the reply...I ran a dedicated ground from the battery - to the ECM metal case...no difference.

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Old 06-20-2018, 11:38 AM
  #24  
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Originally Posted by av81 View Post
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 ,
Waiting for a reply from the fella that said he had the older Snap-On Brick scanner with the "multi 1 connector"
(would that multi-1 connector come as a kit with the Brick scanner?)

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Old 06-20-2018, 11:41 AM
  #25  
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That is not what was meant by 'good grounds' to the ECM.

He was referring to the ground wires that are in your connectors that plug into the ECM are good grounds.

The case does not need to be grounded.

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Old 06-20-2018, 11:43 AM
  #26  
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If I am not mistaken....the M/C solenoid is suppose to chatter/click when pulling up codes.

But not having a code 12 is important.

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Old 06-20-2018, 12:20 PM
  #27  
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Originally Posted by Hamerdown View Post
Waiting for a reply from the fella that said he had the older Snap-On Brick scanner with the "multi 1 connector"
(would that multi-1 connector come as a kit with the Brick scanner?)
Originally Posted by DUB View Post
That is not what was meant by 'good grounds' to the ECM.
He was referring to the ground wires that are in your connectors that plug into the ECM are good grounds.
The case does not need to be grounded.
DUB
Makes sense that the ground would be coming in from the two side mounted plugs...because where the ECM mounts is all fiberglass.
Question...in those two large ECM plugs one has letters stamped on the black lower plug and the upper black plug has numbers...any idea which wires are for a ground into the ECM? With the key-on I could test them for ground.
Also...on the ALDL can you tell me with key-on which terminals should be 12v and ground...I can test that too!

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Old 06-20-2018, 01:17 PM
  #28  
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Black/white stripe is the chassis ground and Orange is the fused ignition power.
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Old 06-20-2018, 01:19 PM
  #29  
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Originally Posted by DUB View Post
If I am not mistaken....the M/C solenoid is suppose to chatter/click when pulling up codes.

But not having a code 12 is important.

DUB

Solenoid should chatter when pulling codes.

Originally Posted by Hamerdown View Post
Makes sense that the ground would be coming in from the two side mounted plugs...because where the ECM mounts is all fiberglass.
Question...in those two large ECM plugs one has letters stamped on the black lower plug and the upper black plug has numbers...any idea which wires are for a ground into the ECM? With the key-on I could test them for ground.
Also...on the ALDL can you tell me with key-on which terminals should be 12v and ground...I can test that too!

Looks like terminals 'a' and 'u' are grounds
Although it looks like terminal 'a' may be unused.

'C' is hot with the ignition on
'R' is always hot
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Old 06-20-2018, 02:45 PM
  #30  
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Now THAT is helpful! And this is fo 1981 correct???
Not pishing my luck but u you send it to my email address so I can print it out.
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Old 06-20-2018, 05:17 PM
  #31  
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YES...that is for a 1981. And you do not have to have the key on when checking for a good ground,

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Old 06-20-2018, 06:04 PM
  #32  
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Originally Posted by Hamerdown View Post
Now THAT is helpful! And this is fo 1981 correct???
Not pishing my luck but u you send it to my email address so I can print it out.
Not a problem. I can send it through once I'm home from work.
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Old 06-23-2018, 11:23 AM
  #33  
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Update...nothing, still waiting for a fella I know to let me borrow his Snap-On MT2500 scanner.
At this point I may just buy one from eBay and something I should have done many years ago or, removed the Computer controlled Q-Jet, and Dizzy.
BTW...I looked into the B+M #70244 that would allow TQ overdrive lock-up on my TC350c...I don't believe there is enough room to install the B&M speedo adapter at the transmission.

Last edited by Hamerdown; 06-23-2018 at 11:31 AM.
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Old 06-23-2018, 03:46 PM
  #34  
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Have you tried everything in the chart?
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Old 06-23-2018, 07:00 PM
  #35  
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Originally Posted by Antz81 View Post
Have you tried everything in the chart?
I am curious also. The chart in post #22 can be done without a scanner.

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Old 06-23-2018, 09:13 PM
  #36  
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Reply to post #34 & #35...not yet, been at my fathers Nursing Home non-stop.
Thinking I want to run the engine again to confirm when I checked for the initial code it did not first flash 12.
Hopefully tomorrow I can squeeze in some time on the Vette.
Question...about post #22 "disconnect ground from TEST TERMINAL, check for blown fuse to ECM check for blown fuse in circuit to ECM term R"
Dumb question...
where is the "Test Terminal" at the ALDL plug or...?
would the 'blown fuse' be under dash in fuse panel or one of the large plugs/wire entering the side of the ECM???
"check for blown fuse in circuit to ECM term R" as in, using the wire schematic in post #29 for "R" = continuous battery?

Last edited by Hamerdown; 06-23-2018 at 09:15 PM.
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Old 06-24-2018, 03:56 AM
  #37  
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Originally Posted by Hamerdown View Post
Reply to post #34 & #35...not yet, been at my fathers Nursing Home non-stop.
Thinking I want to run the engine again to confirm when I checked for the initial code it did not first flash 12.
Hopefully tomorrow I can squeeze in some time on the Vette.
Question...about post #22 "disconnect ground from TEST TERMINAL, check for blown fuse to ECM check for blown fuse in circuit to ECM term R"
Dumb question...
where is the "Test Terminal" at the ALDL plug or...?
would the 'blown fuse' be under dash in fuse panel or one of the large plugs/wire entering the side of the ECM???
"check for blown fuse in circuit to ECM term R" as in, using the wire schematic in post #29 for "R" = continuous battery?
Why do you need to run the engine again? It should show code 12 regardless of if the engine has been run since connecting the battery. (I would not expect to see code 12 if the terminal is grounded while the engine is running)
Yes the test terminal is at the ALDL plug. The fuse would be at the fuse panel under the dash.
Based on your symptoms I would start at the voltage checks after the box that says "does not flash code 12" If there is a blown fuse this will show it up, and since your ECU responds to the test terminal being grounded this is where it will end up anyway.
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Old 06-24-2018, 09:55 AM
  #38  
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Or refer back to post #6.
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Old 06-24-2018, 11:04 AM
  #39  
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Originally Posted by Antz81 View Post
Why do you need to run the engine again? It should show code 12 regardless of if the engine has been run since connecting the battery. (I would not expect to see code 12 if the terminal is grounded while the engine is running)
Yes the test terminal is at the ALDL plug. The fuse would be at the fuse panel under the dash.
Based on your symptoms I would start at the voltage checks after the box that says "does not flash code 12" If there is a blown fuse this will show it up, and since your ECU responds to the test terminal being grounded this is where it will end up anyway.
*why run engine > > > I was under the impression once the battery is disconnected the computer resets and cods are gone (not true?) I've had the battery disconnected for several days and just though I'd start start 'fresh' with running the engine and jumping of the ALDL with a paper clip.
*"the fuse would be under the dash" > > > I wasen't sure 'if' maybe there was a 'fuseable link' you were referring to or something else.
*with all the feedback and great info I'm slowly learning, as mention prior anything 'mechanical' I can eat for a snack. I'm just learning some terms like "ALDL" and not knowing if the "test Terminal" was the ALDL or that GREEN wire on the right side of the Q-Jet or something else...I recall that GREEN wire is where I hooked-up a Dwell Meter to adjust something on the Q-Jet
*when looking down at my ALDL I'm seeing 5 terminals. The two on the far left are where I would jump with Paper Clip.
*being I can't 'see' any wires at the ALDL I'll use post #29 to determine which is HOT & GROUND.
*hoping to get a little time in when I get back from the Nursing Home
****I greatly appreciate everyone's time an patience!
EDIT...I forgot to open the links in post #20!

Last edited by Hamerdown; 06-24-2018 at 11:11 AM.
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Old 06-24-2018, 11:12 AM
  #40  
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Originally Posted by Hamerdown View Post
Question...about post #22 "disconnect ground from TEST TERMINAL, check for blown fuse to ECM check for blown fuse in circuit to ECM term R"
Dumb question...
where is the "Test Terminal" at the ALDL plug or...?
would the 'blown fuse' be under dash in fuse panel or one of the large plugs/wire entering the side of the ECM???
"check for blown fuse in circuit to ECM term R" as in, using the wire schematic in post #29 for "R" = continuous battery?
AS stated previously. Your test terminal and ALDL are the same thing. And GM wants you to remove the tool you were using to check your codes from the ALDL.

If you look in the red circle in the schematic below. You will see the letter 'R' and the ORANGE wire that goes into it that eventually goes to your fuse panel under your dash. THAT is the ECM term 'R' GM is talking about because all the terminals in the ECM connector use LETTERS as a reference for identification.

The computer I am on is not allowing me to upload the diagram. I will get it later today at another computer.

DUB

Last edited by DUB; 06-24-2018 at 11:18 AM.
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