I have a 70 Grand Prix with a TH400 transmission. It has been bitter cold in Iowa as of late. Recently, the car would not shift up until it had thoroughly warmed up, then eventually it would shift normally again. I changed the filter, and the fluid was at the correct level, no metal shavings either. The tranny seemed to shift nicely and tight, why is it acting up? I haven't had a chance to test it in the warm weather yet though.
I'm nothing of a tranny expert, but I would say you answered your own question. Cold fluid = thick fluid. thick fluid = higher line pressures causing your tranny not to shift right. When it warms up, it thins out letting it work normally.
