My 2005 Impala will have from time to time a hard shift where there is a big clunk_usually 1st gear. I had it at a transmission shop last year and they could not find anything wrong. Does anyone have any ideas of what is going on.
Transman is the man for this question. Transman?
Here’s a very useful website about your transmission’s common problems: http://www.tripleedgeperformance.com/4T65E_Common_Problems.html
My own experience with one has me seeing them as Jekyll or hyde transmissions. They either work great for a long time or they are a disaster and no one really knows what to do with them when things go wrong. (I also don’t really have any good transmission shops around).
Specify whether or not the shop scanned it for error codes and what the exact codes were, if any.
I have an 04 Monte Carlo that just went through a bunch of transmission diagnostics for a vibration. These cars come with a fluid filled motor and transmission mount. They get old and leak out leaving the motor to bang against the hard pieces inside the mount. This may be some of the harsh shift you are feeling since the motor is not floating on a fluid cushion to the car any more. The aftermarket stores sell the factory type fluid filled and a newer version of solid ribber like the older cars. I have the solid rubber mounts in my car and I can only feel a slight vibration from this mount. I am very picky. What I have noticed is much less movement bouncing around between reverse and drive.
As for firm shifts if the mounts are good, that would be for someone else to diagnosis. I know there are a lot of things inside there that I don’t know about.