A few months ago my 97 Blazer started having the infamous harsh 1-2 shift, 1870 code, and no lock up when transmission was warmed up. I looked into numerous ways to fixed the code:
the Teckpak TCC valve-https://blazerforum.com/forum/engine-transmission-35/alternative-p1870-fix-so-far-52746/

the Sonnax kit that requires boring out the PWM valve body with a reamer tool or a new valve body, but here is a simple fix that I found that works just like the Teckpak valve-
97 4L60E bolt directly into a 95 car? - LS1TECH

Basically you are eliminating the PWM "pulse-width modulation" that GM came up with in '95, and your "lock up" of the converter is on or off. This mod you don't have to take off the valve body and works just the same as installing the above mentioned valve, but it costs nothing but your time to do. You simply make sure the valve is pushed against the stopper on the valve body and take a screwdriver and "stake" the valve so it cant move this makes the lockup on or off no more PWM. The link on ls1tech shows the pics and explains it. I have had this mod for sometime now and my trans works as it used to, also my gas mileage went back up cause lock up now works again. All this cost me was the fluid and filter kit, and no more 1870 code. This mod may work for some as it's a simple alternative fix if your valve body PWM is worn, but wanted to share it because it worked for me.
https://blazerforum.com/forum/attach...1&d=1341243000