I have a 93 s10 blazer 4wd auto 4.3 vortec. it needs some work but i dont have alot of money to fix it right now, but this is what the problem is. It shifts fine from 1 to 2 gear but when it shifts to third the motor revs up like it slipping in other words it doesnt go into 3rd gear at all. how ever if i let off the gas it shifts into 4th gear and when i push on the gas it revs up a little then it catches and goes but it still sounds like it is slipping or the rpms are still a little high. i replaced the speed sensor and the trans filter but no change. One of the codes that came on when i had them read was tcc stuck on or off not sure which. the guy at tranny shop said it was electrical.

 

Your tcc could be not locking, but chances are your going to need a rebuild soon anyways, ive had that happen to three trannys, they all died soon after. it doesnt matter if it is the tcc or not if you keep driving it like that, the slipping WILL kill the trans!

P.S. Almost all.. Actually... All tranny mechs are scumbags! word to the wise dont try to save money on it just take it to whatever shop is the highest rated with the BBB!

 

I have a 98 blazer and having the same problem but it happens when going into 2nd gear. I wasn't sure what it was but now I know I will have to replace the transmission