I have a turbo 400 installed in my 1987 camino. It came out of what we think was a '73 box truck of some kind. My dad and I rebuilt the clutches, and installed a B&M Shift Improver Kit. The transmission worked fine for a day or two, but after that it wouldn't upshift from 1st to 2nd until about 40-50 mph. It would drive fine and normal for about 1-2miles, until the engine and tranny warmed up then it starts not shifting. It was like that for a couple weeks (luckily I have a cheap motorcycle I was able to ride until we found time try and fix it). We thought it was the modulator, since that is the most common problem (I've heard), but we replaced that today and after about 20 miles of working perfectly fine it refused to shift again. We are really at a loss for what could be wrong at this point, any help would be greatly appreciated.

Anybody have any ideas?

well,there could be a number of things,let start with the first one
1.governor assembly; governor valve sticking,driven gear loose,damaged,or worn,{check for pin in case and lenght of pin}also check output shaft drive gear for nicks or rough finish, if driven gear shows damage.
2. control valve assembly; 1-2 shift valve train stuck closed. Governor feed channels blocked,leaking, pipes out of position. Valve body gaskets-leaking,damage,or incorrectly installed.
3. Case; Intermediate clutch plug leaking or blown out. porosity between channels.governor feed channel blocked.
4. Intermediate clutch; clutch piston seals-missing,improperly assembled,cut.

I hope this will point you in the right direction..:cali:

If it works on and off, I'd put my money on a sticking valve. Has the trans been sitting a long time? Check the governor valve for smooth operation, as well as the accumulator piston (less likely). If problem persists, I'd drop the valve body, remove the valves one at a time, polish the valve O.D. lightly with 600 paper, flush the bore and ports with solvent and shop air, then re-install. It's not an uncommon problem with a junkyard trans that's been setting for years. Fortunately, 400's are about the easiest tranny to work on, next to a Powerglide. I worked for Hydra-Matic over 30 years ago in the Engineering garage. At one time I could have done a 400 rebuild in a dark garage, blindfolded at midnight. Memory has faded greatly, but I still have some reference material around.

Bill

Thanks for the help.

I'm no TH400 "expert" but I agree with BillyJack. Since it's intermittent and seems to be heat related, I think you have some dirt in the valve body valving somewhere. The Haynes book on GM A/Ts should help you disassemble/reassemble it correctly. One other thought is while you have the VB off, check to ensure the check balls aren't getting "stuck" in their respective holes. Only other two things I'd investigate are a modulator vacuum feed problem, or a sticky governor as has already been mentioned above.

While you're inside the Haynes manual, check out the trouble shooting section too. See if it offers any other reasons for a delayed 1-2 upshift.

Good luck.

One other thing you might check is a collapsing vacuum hose, top or bottom, that feeds your modulator. Also, a small vacuum leak under your dash or anywhere could cause your problem. 400's will always have shifting problems when vacuum leaks are present.