BMW Repair: inproper 5th gear ratio on 2000 bmw 328ci, transmission control module, water ingress

Question
Hi i am having problems with my bmw 328ci(2000)  It goes into limp mode on the higway about 80km to 100kn.  I don't feel it on the higway but when i exit and come to a stop, I feel it being hesitant to go.  I took it to aamco and a few other places and the told me it has an improper 5th gear ratio maybe solenoid. I read that if maybe i clean the transmission control module it will help.  I tried looking for it on the passenger side against the firewall with no luck.  I found this unit that looks like a enclosed conatiner on the driver side beside the wheel strut.  It has 4allen screws so i took it apart and sprayed electrical contact cleaner.  Drove it on the highway and the light came on again.  I just bought the vehicle privately and don't have much to spend on it.  If you can  please let me know the exact location of the tranny control unit co i can take it apart and try cleaning it.  Pictures or videos will be greatly appreciated. thank's in advance craig

Answer
Got to admit auto gearboxes are really tricky.Cleaning the connector is fairly straight forward but its under the car and fiddly.
A fault code would be helpful but lets go with the cleaning process.
A round multi pin connector comes off the back of the gearbox and its the only one.It connects the gearbox to the harness that runs up into the car.Find it , unscrew it by holding it and turning the rear section anti clockwise,clean both sides with contact cleaner and then struggle to get the bugger reconnected.Its proximity to the propshaft explains why it gets water ingress as it sprayed all the time in wet weather.

Failing that as a cure it sounds like an internal problem when it goes into overdrive.

Next would be to drop the sump and filter then look carefully at the harness connecting the solenoids for rubbing against the gearbox housing.

This means you will have to refill it with new fluid (dextron 3) and filter available anywhere, this cant hurt.There is a specific procedure for refilling, if you go down this route let me know and I will explain it.

Good luck