I get an erratic up and down shift between 2 and 3 after driving the car about 15 minutes and believe the problem is in the torque converter clutch. It's the last carbureted Corvette with a complicated system of vacuum and electronic controls and I cannot find anyone to work on it near The Villages, Florida where I live. Garages have recommended pulling the transmission to disconnecting the vacuum modulator and everything in between. Any input and suggestions will be appreciated.

 

When the transmission just shifts into hi range, it can 'hunt' between 2nd and 3rd when going up and down hills or 'rolling' terrain. To resolve this (under these conditions), just put the shift lever in 2nd range until you get to flatter terrain or where you can increase speed. It's just a "quirk" of that pre-electronic control system.

There may be a vacuum switch involved with that '81 trans control system, and it could be 'showing its age'.

 

Your car TCC ( lockup clutch ) is Computer controlled along with the carburetor. I assume it’s acting up between 3rd gear and lockup instead of 2nd and 3rd . It needs a scan tool connected , TPS and several things can cause erratic TCC operation. You need to find an older technician or dealership that can scan OBD 1 system with a scan tool

 

Is the check engine light coming on at all? It might be the computer switching between open and close loop. It could be a temp sensor with a bad connection. Good Luck.

 

Sounds like what you are experiencing is somewhat normal for the 81. Mine shifts similarly at times, quirky. I had my tranny inspected when I bought the car 6 years ago by an older mechanic that worked on these cars when they first came out in 81. He drove it hard, checked all systems and with 96,000 miles on the tranny only recommended doing a resealing job due to leakage, change out the filter and using the latest GM tranny Dexron fluid, otherwise he said it is shifting great and strong, The TH350C is the same as the TH350 but with the electronic lock up torque converter. But as mentioned in other posts there could be a system that can be off that causes the issue. The TPC requires to be adjusted and working properly to put out the correct voltage or you will feel the TCC lock up, then unlock sporadically. The only way I got my 81 to run correctly and check periodically when things feel off is picking up and use a scan tool made for the 81, a Monitor 85. I had my carburetor rebuilt by a local carb builder and after taking it back 12 times he gave up and gave me my money back. I read up on the carb and contacted Cliffs High Performance, Cliff gave me pointers and I rebuilt and adjusted the carb myself, still running strong 6 years later.

Video of the Monitor 85

I bought one on Ebay in the original case and practically brand new for 40 bucks. Good luck, lots of smart members on this forum to assist also, Keep posting.

Good Luck

 

81 has lock up in 2nd and 3rd. There's an electrical solenoid that controls that. I would disconnect that connector, then test drive it. If it gets better, then it could be the solenoid or the hydraulic circuit Just remember, using a scan tool and looking for the TCC command.....based on engine data, the ECM sends the command, and that's it. It has no feedback input to determine if it actually happened. So, it's critical to check ALL of the other sensor input such as coolant temp and TPS to make sure there's no issues there, because the ECM looks to those sensors to decide if TCC should be applied..