Toyota sienna, purchased new. From the beginning I had problems with it downshifting on grades that didn’t warrant it, sometimes even dropping two gears, while in “cruise control”. Took it back to the dealer twice the first year, they connected to the computer and made some adjustments on shift points, but I have never noticed much difference. Can the shift points be adjusted by owner? Would this be covered in a repair manual?
My understanding is that on late-model automatic transmissions, the shift points are dictated by the computer program, and that program adjusts itself slightly to adapt to your driving pattern, typical load, etc.
The first response of the dealer to any complaint of transmission behavior will be to clear computer memory so it goes back to factory default settings. Sometimes that solves the problem.
The shift points are not directly adjustable unless you have the equipment to re-write the computer program.
I would check to see if there are any service bulletins on your model that mention the transmission.
This needs to be addressed by someone who understands how the transmission is ‘told’ that you are climbing a hill. My guess is that it gets a manifold air pressure reading from the engine control computer. Then the car needs to have a diagnostic test that checks to see whether you have a sensor or mechanical component that is bad, but not bad enough to set a failure code in the engine control computer.