Ok First off its not in a 3rd gen but its Still 700r4. I have a 88 K1500 305 v8 700r4 4x4. Just Did a motor swap took motor from my 1988 firebird 305 and put it into my k1500 because im building a 350 for my firebird. Engine runs great transmission is full transfer case works perfect. Cannot get the truck in reverse and when i get it in drive it seems like it slips in and out of drive while it slowly accelerates. i can floor it nothing, i can lightly start off and it moves somewhat. When i get moving it will shift into 2nd gear but i cant get over 20 mph. Transmission is full with clean new fluid all lines are hooked up no leaks. i tried the adjusting the T.V cable and that seems to have no effect on it. any suggestions will be helpful.

I have a similar problem, but I just put in a shift kit. The book told me that the pressure regulator valve c clip could not be seated properly, or it's incorrectly put together, but another problem would be interesting to see..

dude iv got the same issue, except my trans dosnt appear to want to shift into second, it just drives as long as i dont hit the gas, hittin the gas does nothing dosnt appear to be slipping but it also isnt really doing anything, i mean i have drive but thats it.

Ok got a chance today to check and see what the problem could be. i started off by checking fluid - full. then checked linkages - ok. looked for any leaks or lose wires - none. So i dropped to the pan off the transmission and what do i find or should i said dont find..... No transmission filter!!!! the guy who rebuilt it forgot to put one in before he put the pan on. Partly my fault for not checking before i put the new motor in but hey i figured its a professional shop they should know what they are doing WRONG!!! Anyways i put a filter on and filled up the transmission and low and behold it works fine. Wasn`t working before because it had no way of getting the fuild lol. oh well goes to show everyone can make mistakes i guess.

Mine was the Pressure Relief valve or something, had a spring, 2 pistons and a long rod. Put it back in with the c clip, and boom, reverse, no slipping, and it shifts like a crazy bugger.