transmission help

[reese5397]

trying to diagnose a transmission problem in a 2000 grand prix gtp with the 4t65e-hd (stock) transmission. the car shifts fine untile the 3rd to 4th transition then it osscillates up and down by about 500 rpms like it cant quite get into 4th gear. if i let off the gas a little then back on it shifts fine. happens randomly it seems too. ive checked all the fluids and they are okay at this point. ive heard getting the ecm re-flashed might fix or it may have a bad selonoid or somthing of an electrical nature. also heard it was some what of a common problem too. looking for a little insight. thanks

[jsparrow]

This is a common problem. My 2000 Grand Prix GT has lost 4th and TCC lock up. Otherwise it runs/drives fine. It has the original transmission as well and has just under 154,000 miles on it.
From what I've read on the Internet there's a shaft inside the transmission which has splines on it that break off. The PCM "thinks" the shift has occurred so the "Service Engine Soon" light does not come on. The ballpark price to fix it is around $1500.00.
Since this car is my" toy" and I don't drive it every day I just keep it in 3rd and it's fine. Uses a bit more gas because the engine revvs higher at cruising speed but otherwise runs great. Hope that helps. -Jim