I have a 93 Chevy 1500 pickup that is not wanting to shift well .I has to rev high to shift and don't want to down shift when I slow down...I've tried to trace vacuum but can't find vacuum lines on tranny...there is a busted vacuum lines directly behind the throttle body that I've no clue where it goes. I did the code trick with the paper clip and got a 12... Any help/guesses

 

Have you tried a "flush"? I would start by removing your tranny pan, draining all the old fluid, filling with new fluid (to specs) and driving it around a bit to get the new fluid pushed around nicely. If you have anymore questions, please message me. Have a good day.