My 39 Olds has an Olds 350 engine with a th200 4r trans. I'm running the original 39 Olds rear end which has a 4.30 gear. The transmission is stock, with no performance modifications. I did replace the stator support shaft and installed a rebuilt pump along with new seals, filter and fluid. Although this trans is from a salvage yard and I don't know the mileage on it, it seems to work fine and shifts firmly enough.

My question is about the lock up point. The converter, as it's now wired, locks as soon as the trans shifts to second gear. This seems to make the 2-3 and the 3-4 shift rather harsh or rough. I'd like it to be smoother. Also, because of the low rear end ratio, I believe, the trans doesn't downshift to first gear until the car falls below about 5 MPH, and the converter doesn't unlock. I have the lock up wired through a relay connected to the brake light switch, which unlocks it when the brakes are applied, but when I lift up on the pedal, it locks up again.

My question is this:
My Son-in Law tells me there is an external pressure plug on the trans that can be tapped into which has pressure only when the trans is in 4th gear. He says a pressure switch can be installed there that can be used to apply the lock up only in 4th gear. This seems to be what would take care of my harsh shifting. If this is true, where is this plug? Will it work?

Yes, I know there is a kit I can buy that will do what I want, but if it's possible to do it another way, thats what I'd like. Thanks for your help.