hello, first time poster long time reader/problem hunter. Sorry in advance for long post

Vehicle is 1999 s-10 blazer 4X4 automatic with 4.3, engine has 272,xxx on it and is a daily driver. problem started with a notice in the tach jumping around some, so i did a tune up with no change. Drove for a few months with the tach jumping around the check engine light never came on and then it decided to not have 4th or overdrive. Time for a different trans, changed trans with one i had laying around, worked great for a week then started dropping out of overdrive, did a little research and decided to put a momentary switch on Pin 10 of the blue connector at PCM, (TCC control), i could now MAKE the transmission lock in over drive after it unlocked itself and now a CEL for (TCC circuit). Got sick of this and thinking the trans i had was a dud, bought a used trans from salvage yard. Thinking i got lucky with the new purchase as it had a newer torque converter in it, was excited to get installed, with new trans, filter, fluid. A week goes by with it working great, then the gremlin comes back doing the same thing as the previous trans.

So question is what is making the TCC unlock, is it an engine miss causing this or maybe ECU?

Any input as in what to check or try would be great, im not ready to give up this vehicle and would like to try an figure it out in case this comes up again on something with less miles

 

cant believe no one has a suggestion, for anyone having the same issue i did find that i have low fuel pressure, and i am guessing that it is causing the computer to go into limp mode (causing no overdrive) now to figure out what is the cause of the low fuel pressure