I bought this ML320 in an auction and soon after I start driving it; the transmission start to slip and sometimes does not change after the car is driving for about 30 minutes; it drives fine and normal when the car is cold. I took it to the dealer and they told me there is an alarm that is coming out that says "Transmission Control System Failure". they suggested to change the transmission. As you can see; that is not the answer I was expecting because it is not making sense to me. I think it sound like a sensor or something electricly. Anyone can suggest what to do would be greatly appreciated. Thank you

 

make sure fluid level is good first...then next step is to check the trans control unit for trans fluid that has worked its way into the trans control unit via the wire harness.
then also could be the transmission conductor plate, which is very common but will usually throw speed sensor fault codes.