Toyota Repair: toyota corolla transmission not changing gears, internal seals, toyota corolla

Question
I have a 97 corolla , that has stopped changing gears on its own. If I ease up of the gas at certain speeds, (25mph, 40 mph) it shifts. I think it is the modulator valve if this vehicle has one , and if so where can i find it . thanks

Answer
It doesn't have a modulator valve,depending on what transmission it is it is either electronically controlled or hydraulic controlled, has the transmission fluid been checked, it may just be low. What you are describing sounds like an internal fluid pressure loss, the transmission needs a certain amount of fluid pressure to function, a pressure loss can be caused by leaking internal seals which would mean a transmission replacement may be necessary.