Topic: 81 Computers & Chips
in Forum: C3 Electrical
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When I bought my 81 3 years ago it had this interesting problem where with the ignition on but engine not started the various air control relays would clatter away, but this would stop as soon as I started the engine. At the same time the "check engine" lamp would flash on and off in time with the relays clattering. I ran a post on this a couple of years ago, but was unable to fix it and it made no difference to anything so I just carried on driving 
The TCC (transmission converter clutch) has never worked either.
Well I picked up another computer of Ebay a year ago, and today (!) I finally got around to having a play.
Orig computer & Orig Chip:
- relays clatter
- check engine flashes
- TCC doesn't go.
Ebay computer & Ebay chip:
- relay problem gone
- no check engine light at all (not even lamp test when ignition first turned on)
- TCC works for first time
- BUT noticeable performance drop
Ebay computer & Orig chip:
- relay problem gone
- performance back to normal
- no check engine light at all still
- and no TCC again
So.... I am figuring that it had a computer swap at some stage prior to me, and the original computer is faulty. But the ebay one doesn't seem to be quite right either, so my question is... How do you find out what is the correct computer and correct chip for your 81 ?
Orig computer has label missing
Orig chip = Delco 6079 FO
Ebay computer = GM 4810
Ebay chip = Delco 6092 FR
Thanx !
The TCC (transmission converter clutch) has never worked either.
Well I picked up another computer of Ebay a year ago, and today (!) I finally got around to having a play.
Orig computer & Orig Chip:
- relays clatter
- check engine flashes
- TCC doesn't go.
Ebay computer & Ebay chip:
- relay problem gone
- no check engine light at all (not even lamp test when ignition first turned on)
- TCC works for first time
- BUT noticeable performance drop
Ebay computer & Orig chip:
- relay problem gone
- performance back to normal
- no check engine light at all still
- and no TCC again
So.... I am figuring that it had a computer swap at some stage prior to me, and the original computer is faulty. But the ebay one doesn't seem to be quite right either, so my question is... How do you find out what is the correct computer and correct chip for your 81 ?
Orig computer has label missing
Orig chip = Delco 6079 FO
Ebay computer = GM 4810
Ebay chip = Delco 6092 FR
Thanx !
Kiwi Chris
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Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
GM parts manual shows the same part # for both automatic and standard trans in '81, and that part # is 1224810. The difference would be in the PROMs for the TCC, and emmisions related programs for the CCC carbs.
The 'puter on the '81 only controls the idle fuel mixture, and the ignition timing on the engine.
The parts book also shows the PROM nums, but none of them match the one you list, so it's possible yours is not original for the car. My suggestion would be to get an aftermarket PROM, such as one from Hypertech...it's not likely you'll ever find an new/original one.
hth
btw...the parts book lists SIX different PROMs for the '81 year model...all to be used in the same ECM.Adams' Apple 2008-09-07 12:10:04
The 'puter on the '81 only controls the idle fuel mixture, and the ignition timing on the engine.
The parts book also shows the PROM nums, but none of them match the one you list, so it's possible yours is not original for the car. My suggestion would be to get an aftermarket PROM, such as one from Hypertech...it's not likely you'll ever find an new/original one.
hth
btw...the parts book lists SIX different PROMs for the '81 year model...all to be used in the same ECM.
Joel Adams
C3VR Lifetime Member #56 
My Link 
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
I beleive it is possible to have your ECM tested? I was having proplems on a 90 toyata corrolla and was going to take the computer to a NAPA store to be checked, but I found the problem before having it tested.
The check engine light SHOULD come on when the key is turned on, then go out after the car is started. The only time it should flash is if the ALDL connector terminals A and B are shorted for testing. There are two modes for shorting with different types of flashing, key on not running, and engine running.
It should click when the key is turned on but not started. This is the mixture control solenoid inside the carburator. If it does not do this, there is a problem. Once it is started, the clicking is still there, but cannot be heard without very close attention. After the key is on for a period of time with the engine off, some systems will not stop clicking the solenoid until the key is cycled, or the car is started. Others will quit after a period of time.
I suspect the first computer is better than your replacement. You are probably running without any timing advance, causing bad performance. Did you change the programming (PROM) chip from the OEM to the replacement?
There is no one that can bench test your computer. It can be sent to a rebuilder and tested/repaired, but that is the same as just buying a reman unit. The only way to determine computer operation is to ensure all the wires connected to the computer are okay, all the inputs are present, and correct, and all the output devices are working and connected properly. IF all is good but the operation is bad, the computer needs replaced.
With the key on, and the engine off, short the ALDL pins A and B with jumper and watch the check engine light. It should flash once, hesitate, then flash twice. It will hesitate longer, then repeat this two more times. This is a code 12. Keep watching and count flashes. If it give you 4 flashes then 3, that would be a code 43. It should repeat each code 3 times then finally repeat a code 12.
12 is no reference pulse from the distributor, which of course cannot be there when it is not running. So this is a self check to make sure the system is working properly, then the other codes can mean something. If you don't get the 12, you must repair the system wiring and/or computer first.
Yeah, I'm glad my '79 doesn't have any of that stuff, either!
Barry
When in doubt... spend more money!!! 
I found another 81 computer on ebay and its on its way so I'll see how that goes !
Thanx for the detail Ken.
I figure that the original box is faulty, which is why check light flashes randomly and relays rattle (ignoring the carb solenoid) 2-3 times per second, as this all goes away when the ebay box is plugged in, irrespective of which of the two chips I put in it.
But the no check light at all with the ebay box puzzles me, as I would have thought all 81's computers would activate the check light "lamp test" when you have the ignition on but engine not started irrespective of what chip is in the computer ? So maybe this box is faulty also ?
And how many variations of GM chips for the '81 could there be ? Apart from manual/auto, what other variations of 81 exist which would require other chips? I thought they all had the smog stuff etc. under the hood and in fact under the hood and the electrics etc. was exactly the same content for all 81's ?
Have I missed something here ?????
I found another 81 computer on ebay and its on its way so I'll see how that goes !
Thanx for the detail Ken.
I figure that the original box is faulty, which is why check light flashes randomly and relays rattle (ignoring the carb solenoid) 2-3 times per second, as this all goes away when the ebay box is plugged in, irrespective of which of the two chips I put in it.
But the no check light at all with the ebay box puzzles me, as I would have thought all 81's computers would activate the check light "lamp test" when you have the ignition on but engine not started irrespective of what chip is in the computer ? So maybe this box is faulty also ?
And how many variations of GM chips for the '81 could there be ? Apart from manual/auto, what other variations of 81 exist which would require other chips? I thought they all had the smog stuff etc. under the hood and in fact under the hood and the electrics etc. was exactly the same content for all 81's ?
Have I missed something here ?????
Kiwi Chris
Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
[QUOTE=CeeDee]
And how many variations of GM chips for the '81 could there be ? [/QUOTE]
[QUOTE=Adams' Apple]
btw...the parts book lists SIX different PROMs for the '81 year model...all to be used in the same ECM.
[/QUOTE]
Imma tinkin there are also differences in Federal vs California cars. and possibly high altitude cars...
And how many variations of GM chips for the '81 could there be ? [/QUOTE]
[QUOTE=Adams' Apple]
btw...the parts book lists SIX different PROMs for the '81 year model...all to be used in the same ECM.
[/QUOTE]
Imma tinkin there are also differences in Federal vs California cars. and possibly high altitude cars...
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Joel is right about the differences. Fed, Calif, High alt.
Yes, ALL of them should turn on the light with the key on, engine off.
I've hunted my 81 GM Workshop manual, and my 81 Assembly manual, and I can't find anything which refers to the prom numbers. What parts book are you using Joel ?
CeeDee 2008-09-12 20:15:34
Kiwi Chris
in Forum: C3 Electrical
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