94 cavalier will not shift into 3rd most of time

My 94 cavalier with a 3 speed automatic seems to be stuck in 2nd gear. It will shift to 3rd sometimes if I get the car over 65MPH. Is the transmission on this controlled with an electrical shift regulator or some type of vacuum module? thanks.

Hmm...I really dont know, Im not an auto tranny guy. Im sure there is someone on here who knows though.