Topic: Check Engine light on '81 help needed
in Forum: C3 Electrical
Lifetime MemberModerator
good Evening, I am new in this forum and I am having some trouble with my vet. I read your advice to a member who had my same issue and that is why I am writing to you. The check engine on my car is on and won't go off. It has been sitting for a couple of weeks and I bought it a couple of months ago. The car runs ok but with a little smoke smell, I will say perhaps rich mixture. Since it is "fat" it does not shift as if it is on D and it does much like if it were in 2. I pulled out the ashtray and jumped the A and B terminals with a clip and light is still on and does not blink. I checked the 10amp ECM fuse and it is ok. I did take the fuse and fired the car and it did start but the choke seemed not to be working at that time. Tried again with the fuse in and fired right up and stayed idling fine. So my question is really how to continue. I ordered a shop manual from ebay and I am waiting to get it. But I do not know if I should replace the ECM, the sensors, or try to clean the ALDL and see if that is why the light does not blink. They seem to have a greenish patina but I think they can be jumped the same. Thank you very much for your time. Roy
I gotta git back to werk now...
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"
TKO500 5 spd.
Borgeson Steering Box
Born 8/1981
Sequence #3975
Click here to see more pics of my Vette on CarDomain.
Lifetime Member #26
Lifetime MemberModerator
I suggested that he clean the corrosion off of the terminals on the ALDL connector, and try to pull codes again. It does kinda sound like the ECM is bad, but he needs to make SURE it will/will NOT pull codes. Seems like every time I've seen the problem of the Check Eng light not flashing codes, it has been a problem with the ECM, whether the PROM is bad/pins bent, or something else in the ECM is damaged. Also possible there is a wiring problem between the ALDL connector and the ECM....so many possibilities....
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"
TKO500 5 spd.
Borgeson Steering Box
Born 8/1981
Sequence #3975
Click here to see more pics of my Vette on CarDomain.
Lifetime Member #26
Thank you so much for your comments. Unfortunately I am in my final exams period and do not have the time to keep up with the car.
However I did order a cleaning spray for the contacts on the connectors and will try to pull the codes. But, as you say, I will order a o2 sensor and change it because the one that the car has seems old.
I also will search for slop in the carburetor and check if there is a vacuum problem.
Nevertheless I will also consider in replacing the ecm. Is it very difficult to do? I do not own a shop, only a craftman set and good will doing mechanics.
Thank you very very much.
Roy
ps: changing to a non computalized system seems like a great idea.
Thank you very very much.
Roy
ps: changing to a non computalized system seems like a great idea.
If it turns out your ECM is the problem, its easy to change. Your Craftsman tools will work fine.
TKO500 5 spd.
Borgeson Steering Box
Born 8/1981
Sequence #3975
Click here to see more pics of my Vette on CarDomain.
Lifetime Member #26
Hello Sarge and Joel, I finally finished my exams and got time to get my hands on the vette.
I changed the o2 sensor for a bosh one. The old one seemed to be very old. The results were as follow: car improved lightly, still feeling it a little "fat" but I feel as I have no fair judgment since it has been a while since I last drove it. But it definitely improved.
nevertheless, the check engine light is still on and I can not pull the codes. Cleaned the contacts with nail polisher and special contact aerosol.
I was thinking in reset the module to see if that clears the code and the light won't show no more. I guess that is be unplugging the battery, but for how long?
Other alternative I was considering is go ahead and change the ecm. When I bought the car, the alternator was overcharging the battery and gases really came out of it, maybe that produced some damage in the ecm since it is right next to the battery.
I could really use some advice as to which steps to take to identify the problem and solve it.
thank you so much for all the help.
roy
Lifetime MemberModerator
An overcharging alternator can absolutely damage the internal electronics of the ECM. Depends on how high the charging voltage got, and for how long. Anything over 16v for very long will definitely fry it. 15v is pushing it.
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"
Yeah, I will probably change the ecm and see if it fixes it and if not change the prom as well.
Now, I have seen some ecm and my best choice is a holley remanufactured one in 69 on ebay. I gues I will get the chip from summitR. What do you think?
Thank you very much.
I ve finally fixed the problem.
By replacing the ECM with an ebay listed holley remanufactured $69 ECM and using the same prong not only does my car ride excellent but also the light is gone for good.
Thank you once more to all.
Merry Christmas.
Roy