94 lumina wont shift to 3rd

I have a 94 lumina with only 57000. Its an actual grandmas car. When u drive it and hit 35 mph it wont shift to 3rd. It goes into like neutral. I cant see it being an inside transmission problem. Just need other opinions. its automatic

Thx for any help
bud

There should be a large steel line coming off the driver's side of the intake manifold, that goes down to the front side of the tranny, probably under the air filter box. There it plugs onto a round canister that has a diapragm in it that modulates upshifts by manifold vacuum. If the rubber hose at either end of the line is split or swelled so it sucks air, or the diaphragm is blown, they will drop out of gear on upshifts...I'd start there. If the line and hose looks good, pull the hose off down at the modulator end...if there's any fluid drips or the inside of the hose is oily, the modulator is blown and needs replaced.

If that all looks good, there could be a solenoid malfunction or a stuck valve body, worn out clutchpack (not likely) or something worse...

Does it feel like it starts to shift and just fails to engage third, or does it just wind up high in second then fall out of engagement?

What happens if you start with the shifter in first, and manually select the upshifts, same thing?

its tacks out in 2nd gear, just before its suppose to shift to 3rd the rpms it just keeps going up but wont shift as if it is in neutral

its tacks out in 2nd gear, just before its suppose to shift to 3rd the rpms it just keeps going up but wont shift as if it is in neutral

Yeah check the modulator first...