Chevrolet Blazer

I have a 95 S-10 Blazer, 4X4, 4.3L V6, automatic w/OD, 170000 miles. The problem is that the transmission will not shift into 3RD or 4TH gear. Sometimes it will shift fine and then when I stop or slow down to where it has to shift back to 2ND it will not shift back into third. Sometimes it will shift if I build up enough speed going down a hill and let it drift without giving it any gas for a good distance, and than sometimes not. Once it shifts into 3RD it always shifts into 4TH just fine. There is no CEL on. I had a shop road test it with a computer and they said the trucks computer is calling for 3RD and 4TH gear but it was not shifting. I was told it could be a solenoid not activating or just hanging up. Could this be the problem? If so, where could I find a diagram of the solenoids, which is which, and how do I go about testing them? I was planning to change the filter and fluid this spring any way. No reason I couldn’t do it now while it’s only 40 degrees instead of 80. The cold air is good for the lungs, just not the back. Thanks for all your help.
Gm problems2
March 9, 2007.

I would sugest looking it the tourque converter it sounds like one of the pressure plates is fouled.

Mar 12, 2007.
Torque converters have vanes. Not pressure plates. Standard shift ie manual trannys have 1 clutch with pressure plate.

Mar 13, 2007.