So, I'm still learning about this new-to-me '81 that I inherited, and have a couple questions about the emissions and computer control system.

If one or more parts of the emissions system have been deleted/modified, wouldn't that trip the electronic control and send a trouble code or set off a CEL of some sort?

And, more broadly, how would one determine if the computer control system has been modified/disconnected, etc.?

Still learning, thanks-
PV

 

First, make sure the CEL has a bulb in its socket and that it lights. Turn the ignition key to first position (RUN) [not START] and dash warning lights should all come on...including the CEL. If it doesn't, check socket/bulb. If still no luck, someone has defeated the system and the computer may not even be connected any more.

That 'one-year-only' system had electronic carb and distributor (unique to that year). throttle position sensor, oxygen sensor, temp sensor and computer all interconnected with eletrical harness. Any component missing or defective will render the system useless and the computer will default to 'open circuit' mode.

When working, the system was pretty good with better economy and better start/idle quality than a simple carb system. But, when not functional, it is pretty hard to find anyone knowledgeable to work on it. The various components of the system are available and not that expensive; but someone has to figure out how to repair it.

If I had one that didn't work...but all the components were there...I would do my best to fix it, rather than to ditch it. But, that decision is yours. If you ditch it, you will need to replace the carb and distributor with 'standard' versions.

P.S. The computer also manages the lockup clutch in the auto tranny.

 

I just went to the garage and turned the key to the 'Run' position- the only light that comes on is the battery light.

If the computer defaults to open circuit mode, does that mean the car won't run?

 

No. The car will run...but it will run poorly. It will force a 'rich' fuel/air mixture and timing will be on a fixed schedule, rather than 'as needed'.

If no CEL when switch is in RUN postion, check that there is a good bulb in the socket and check it again. If no light, the computer has been disconnected or it is malfunctioning.

To learn more about this computer command system, you should get a copy of the Chevrolet Chassis Service Manual for 1981 (or whatever similar document GM had available for Corvette of that model year). It will have info on how to check out the entire system and find what is not working as it should. That system is the OBD-I type system (prior system to present OBD-II system). Once working, you can get "trouble" codes dumped from the computer to help in diagnosing what component[s] is mis-behaving.

 

Have you had a look in the battery compartment to see if the computer is even there at all? It would be mounted on the wall of the compartment closest to the driver's seat.

 

See, I didn't know that either
Oh well, I'm learning.

Evidently the computer is still there - looks sort of like a hard drive bolted to the compartment. I guess that's exactly what it is.

The reason I'm asking these questions is because it seems as though there have been some 'mods' done to this car over the years before I got it- AIR pump removed, for example. Should that set off a code or light?

I really don't know if the distributor or carb are original to the car or not, and not sure how to make that determination.

I'm not as worried that things may have been done to the car, just really don't know how to track down exactly what those changes might have been. Just so I can get an idea where I'm starting from.

 

AIR and EGR can be removed without any problems. check that you still have the electronic carb and the corrext distributor ( it will have a four pin plug connected to some wires coming out of the back of it). Also check that there is a good bulb for the check engine light in the dash.

 

A pic of the carb with the air cleaner off would help, as would a pic of the distributor wire.

 

here is a picture of the carb off an L81 engine

 

Larry's carb pic shows the electrical connection points that I wanted to see on yours. There should be a wire attached to the choke, blue plastic (metering rods), white (TPS) and idle solenoid on the right.

Incidentally, don't feel bad if your carb doesn't look as nice as Larry's, none of them do. I think that one looks cleaner than they did coming from the factory

 

Here's a pic of the distributor- It does appear to have a 4 pin connector on the rear. Does that look legit?

I'll have to circle back to the thread in a couple days with a carb pic.

 

Short answer is no, I don't think so. Is that a vacuum advance can on it? There shouldn't be one.

 

I went thru these same challenges and was very tempted to ditch the carb and distributor but stayed with it. My CEL bulb was missing and the computer and prom were both bad. Eventually got everything operating correctly and very glad I didn't give up. It's not as complicated as it may seem right now so keep the faith, you will be pleased with the results and much wiser.

 

Keep it, get it running and you will not be disappointed. I went through my 81 computer and electronics from front to back. Pick up a 1981 Shop Manual it has pages full of troubleshooting to help you pinpoint any issues. If you can get a Monitor 85 off of Ebay, go for it. I picked one up with case, instructions and leads for 40 bucks. Best tool I got to diagnose and get the car running at it's best.

The hardest thing I came across when I rebuilt my engine is allowing a local Carb Shop rebuild my carb. After removing it from the engine and returning 12 times I finally took matters into my own hands setting up and adjusting myself and the car runs great now. Not many people know how to set up these one year Vette run electronic carbs, but once setup correctly they start and run great. If you need a carb rebuild, follow the shop manual, get help on the forum and pick up a rebuild kit from Cliff's High Performance. Cliff helped me out over the phone troubleshooting a few issues. The next time I need a rebuild that is where I am getting the kit from and doing all the work rebuilding from scratch.


Here is a clip from YouTube showing the Monitor working.
https://www.youtube.com/watch?v=lx5khHshjVE

 

Reading this thread had me go out and turn my key. All lights come on except the CEL. Everything is original (or new version) and i do have the Monitor 85. When hooked up everything reads fine. Even the CEL flickering as described while checking codes. It actually threw a few which is how I found that the O2 sensor was not working. Fixed it and now zero codes. So, now this has me wondering if I have an undiscovered issue???? I do have a spare ECM and Prom. What does the "run" position CEL indicate? Good grief, this never ends!

I just wanted to add that this morning I looked over the service manual and the trouble shooting charts refer to "R", "C", "U" or term "21". I'm not entirely sure where these "" are located, it does not say?? I'm guessing that these are the terminal indicators on the ECM?