Hi ezzie. I don't know for sure if I can help but I'll try. What do you mean by "moving fast"? The car should be moving fast if it's in 3rd gear. I'm guessing you mean something unusual or different.
If you feel a jerk when just about coming to a complete stop, you are probably feeling a "bump-shift". That is where the computer waits a little longer than normal to downshift to first gear. It is the "nature of the beast" and doesn't hurt anything but it can be irritating. There is a list of about ten common complaints related to the feel of the transmission that can be addressed by "flashing" the Electronic Automatic Transmission Controller (EATX) with updated software, but in my limited experience, it usually doesn't help.
I had the same observation with my '93 Dynasty. It's an extremely comfortable car but since new it had that bump-shift. The computer was reflashed but it didn't make any difference. I just live with it now.
A similar problem can occur when accelerating from a rolling stop where you don't quite come to a complete stop. The transmission is caught right in the middle of downshifting when you want to accelerate. To be sure it's ready to go, the computer quickly applies the first gear clutch to be sure it doesn't slip. You might feel that harsh engagement as a clunk or jerk.
Some of these characteristics are more pronounced if the engine mounts are worn, but there should not be any noises associated with the thumping feeling.
Wednesday, March 31st, 2010 AT 4:21 PM