Have a 2002 avalanche Z71. Transmission was rebuilt 33k ago. It currently shifts a little hard going from 1 to 2nd. Real problem shows up between 40 mph and 50 mph. It suddenly shifts into overdrive which drops the engine rpm waaay down and starts lugging the engine.
Took it back to the guys who did the rebuild. They drove it around and noticed that the computer reported no codes. They claimed it was the TPS and said to replace it. I did. Problem is still there.
Took it to a highly trusted mechanic. They said they think the valve body needs replacing but they don’t do transmissions. The guy they recommend drove it around and he thinks it is the torque converter.
I am wondering what causes the computer to decide when it is time to shift into overdrive and I am wondering if there is some sensor that has gone whacko and needs to be replaced instead of starting to tear the transmission apart.