We have had this car about one year....no problems. Yesterday, on takeoff out of the drive, it shifted a little hard. We drove about 30 miles with no other symptoms. When we got to our exit and slowed to stop, it shifted hard again. On takeoff, it shifted harder still. We pulled off the road and stopped. Each time, the shift was harder up and down. We had a 1998 STS that this same event happened with and it was a sensor or solenoid in the transmission, which we were able to fix ourselves, with this forums help.
Can someone please help me again? Also, on our 98 STS, we could check the codes ourselves. I cannot find a way to check them with this car, so I don't have any codes to help with a diagnosis.

 

O.K.....today, I decided what the heck, and decided to drive my car to work. I only live 3 miles from there. I drove to and from work and all around town during lunch break....no problems at all with the shifting of the transmission. What is going on??? Please help.

 

I would take it to Autozone(or someplace similar) and have it checked for codes. The trans shifting hard is an indication that a transmission code has been set. It shifts hard like that because when there is a code set, the controller will "default" to high line pressure, in an attempt to keep from burning any of the friction elements(clutches/bands).
It may be a simple speed sensor, or it may be something sinister....need to have the codes pulled to determine the direction to go.
What engine does the car have? There could be one of two different transmissions, depending on the engine.

 

Thanks for your reply....it has the 4.6L V-8 engine, it is a STS4, all wheel drive.
I probably will take it by our local auto zone to see if I can get a code on it. I have driven it Tuesday and Wednesday, just here in town, short distances, typical start and stop driving....so far...no more problems. Our '98 STS did the same thing and it was a sensor/solenoid inside the transmission.