not shifting into overdrive on 1995 Buick LeSabre

Very rarely does our car shift into overdrive, when it does, it doesn't stay there long. Can you tell me what might be causing this? Is it the speed sensor? If so, where is the speed sensor located?

Asked by for the 1995 Buick LeSabre
it sounds like the clutches are worn out due to the age of the car.

there is a solenoid in the valve body but a shop with a scan tool can verify whether it is working or not.