when the fluid heats up, it thins a bit. if the PCS is on its way out the fluid will start seeping past the PCS when its not supposed to. the puter will "see" this and max out the line pressure in the tranny to try to stop that from happening. and since there is fluid going past the PCS when its not supposed to, and the line pressure is maxed,(meaning even more fluid is seeping past the PCS) it will give you the hard shifts since the tranny cant get the right pressure to shift.
the 4t65e transmissions in the w-body cars are known for the PCS to fail. its a fairly common problem in these trannys.(knock on wood) but i have 114,000+ miles on the original tranny on mine, and wehn the tranny gets hot in mine it starts to whine a lil bit. but i dont have the hard shifts.
if it indeed is the PCS thats starting to fail it will usually throw the P1811 code. but this will not cause the CEL to come on. the code should be stored in the pcm.
one more thing, if it indeed is the PCS and you get it repaired, the tranny fluid will have to be changed. this is a good time to drop the pan and change the filter also. and dont get the dexronIII fluid, get the new dexronVI fluid.
Last edited by milwaukee_01impala; 06-15-2009 at 09:58 AM.