Sorry to revive an old thread, but I thought this may be interesting - -

I have a 2004 Iinfiniti I35 with the harsh 1-2 shift. The car is stored at another location and frequently goes a couple months without being used. Last time the car sat for a few months, my solar battery maintainer failed and the battery went completely dead for a couple months. When I replaced the battery the harsh 1-2 shift was completely GONE - could get it to shift hard if I tried. Then.... after a couple dozen starts, the problem began to return gradually, as if the trans computer was "learning" to shift hard 1-2 again.... This seems to rule out alot of the other suggestions related to the cause of the problem such as the drop resistor, shift solenoid, engine mounts, etc... Thoughts?

 

sorry for bringing back and old thread just trying to see if someone found a solution or possible tried replacing the transmission speed sensor

 

it's the input shaft bearing