At least that's what seems to be going on. After shifting from 2nd to 3rd at 5500 RPMs, the RPMs will drop again after the shift as it pulls from about 1000' to the finish line. I've done the standard TH700 things - shift improver kit, relief valve & servo upgrade, etc. What I suspect is the TCC solenoid isn't flowing enough to keep it from locking, but isn't a problem in 2nd - although I don't know if that's a reasonable possibility given the circuit design. I'm reasonably certain it isn't an electrical lock-up. With the street gears, I could use the extra RPMs in 3rd gear.

Not sure what you are using for an ECM, if at all. However, most ECMs have a MPH threshold to force a TCC lockup. Generally in the 75 - 85 MPG range.

Can open the brake pedal TCC switch and make a run. If it doesn't lock up then it is (was) being commanded to electrically.

RBob.

It's a stock '86 LG4 ECM. It will lock in 2nd at 41 mph and light throttle, so I know it isn't waiting for 4th gear.

I didn't notice the MPH when it was doing it, but the speedo is only indicating 85 mph going through the traps. So, it might be commanding lock-up at 75 mph.

Before I put the 350 shortblock in, the TCC would lock even though it didn't have power to the solenoid. Turned out there was a little piece of plastic in the solenoid blocking the flow path. I made sure this one was clean before installing it. I vaguely recall hearing Vettes used a different solenoid, so that's why I was wondering if 3rd gear flow might be greater, and why it doesn't go to lock while in 2nd.

Guess I'll try disconnecting the power next time I'm out there and see what happens (this isn't a good test to conduct on the street. . . ).

I didn't have TCC lock for 2 years after the V6 to V8 swap. I finally figured out the solenoid wasn't getting any power. So, I ran a separate wire from the brake switch to the harness under the hood, and it started working.

Friday at the track, I snipped that wire and made my runs. It still did it.

Then, this morning, on the way to work and the airport, the TCC lockup was working again. Apparently, the factory wiring for whatever reason started working again.

So, guess I'll have to disconnect power at the brake switch and try again.