Quote Originally Posted by Scimmia View Post
I don't care how many you've owned, you obviously don't have a clue in this case. As for explaining how something mechanical will simply stop, the splines wear away over time and eventually give out. When this happens, the trans will no longer shift into 4th, and that's it. No other symptoms. This is why GM started heat treating them for the 2003 model year, the hardened splines don't wear away. This has come up more times than I can count over the years on the various wbody/bonneville boards that I'm on. Extremely, extremely common.

What "piston" are you even talking about? I can only guess you're talking about the accumulator piston? Not a very common failure at all on these transmissions. Pretty common problem on the 4L60 with the plastic pistons, but that's a totally different setup.
I'm sorry but I agree with this. Its a VERY VERY VERY VERY common issue. There doesn't have to be anything wrong, they splines can just go away without any warning or without any noise. Thats why I'm saying a simple task of just driving on the highway with the car in drove, pull the selector back into 3rd and see what happens. If nothing happens then 4th gear it gone, in which case the most common issue is the splines. If it does go up then you would be correct in assuming its not locking the torque converter.

Lets stop *****ing at each other and just fix the problem, hows that sound?

Try what I said and report back here and we can asses the situation from then.