I put 3.70 gears and a tach with shift light in my car over the weekend and now I can't get it to shift into third now. I can leave it in drive and it shifts great , but I need to manually shift it at the track because I loose 1000 rpm from my power band leaving it in drive. When I manually shift into second it shifts, but when I shift in third the shifter shifts to third, but the transmission doesn't. What could cause this to happen?
I wouldn't think a gear swap or adding a new tach would have any effect on how it shifts. I am running the 700R4 which is supposed to be beefed up quite a bit, but I am having my doubts now
I don't think he put a shift kit in either because my shifts aren't firm and it doesn't shift as soon as I shift when I am racing. Any help would be great.

 

Check the fluid level.
Mine was doing the same thing and after slightly
overfilling the fluid level (1/2 QT) it seems a lot better.
It was origionaly, just underfilled by about 1/2 Qt low.
An intermiddently sticking valve in the valve body can cause this
too.
The fluid level is the first place to start looking in any case.