Im having trouble with my transmission. Its a 4l60E on a LS1 98 trans am.Every time i move the shift **** from park to drive or reverse, theres a big jerk or pull from the car which causes the rpm to drop and wanting to turn off the car. Another symptom is it doesn't down shift under 60 mph after stepping on the gas pedal. Once the car has been driven for a good amount of time and the engine is hot or running at normal operating temperatures, it down shifts better at a lower speed and when I move the shift **** to park, reverse, or drive, the jerk is hardly felt. What could be the problem? The Tps sensor code is the only code that comes out on the scanner but a local performance tunning shop showed me in the computer that it is working properly.

 

Sounds like a bad seal. I had that problem once on an old grand prix. Once the seals warm up they'll expand and work properly again.

 

I think the TPS sensor code is causing all the problems.
The existence of various codes will cause the trans to go into "safe" mode which includes maxing out the line pressure (the jerk into Drive) and preventing downshifts.
Doesn't matter what your tuning shop says; a code is a code and the trans will react to it. With the throttle closed the TPS voltage needs to be between 0.25 and 0.45Volts; even a 0% throttle reading outside of that range will cause a code.

 

Just because the shop said the TPS sensor is good does not mean that there still isn’t something wrong with the TPS circuit. Could be a broken or shorted wire or a bad connection that is setting the code and putting the trans in safe mode.

 

Building on mrvedit and bbond105, a new TPS is pretty cheap (about $30-40 for a Delco) and very easy to swap out. I'm a fan of attacking the easiest and least expensive possibilities first and it did throw a code.