The idea that the speedo doesn't work indicates an issue with a speed sensor. That can affect the transmission functions. As far as what is causing the problem, it could be several different things.
The first thing I recommend is to have the computer scanned for diagnostic trouble codes. The best way to do it is via a CAN scan. CAN stands for computer area network. This is how the modules/computers in the vehicle are tied together.
Once the codes are retrieved, we then can focus on a specific area. If that isn't done and we have no codes, it becomes a guessing game.
Here is a video that shows how to scan the CAN bus system of a vehicle:
Once you have that done, let me know what codes are retrieved.
If this is something you are unable to have done, let me know. Also, let me know if you have other questions.
Friday, October 9th, 2020 AT 7:18 PM