So, my trans has always shifted early from 1st to 2cnd, but it won't shift into 3rd until I'm going about 38mph. I haven't had the vaccum modulator hooked up until tonight, but even after hooking it up it still shifts late. I'm sure that I hooked it up to manifold vaccum, but I can check that tomorrow. Could there be something else? It was built about a year and a half ago.

 

well having the modulator unhooked should have made all shifts late. it should have buried the tach for every shift...

The modulator is what tells the tranny what the motor is doing on the older th350/400's. leaving it unhooked should have made the tranny think your always @ WOT (no vacuume). this leads me to believe you have an internal problem with your trans.

if you hooked it up to a ported vac source it could be causing the ongoing problem too, because it wont get vac at idle or part throttle. make sure your on manifold vac and try again.