85 S10 Blazer tranny issue
 

First off hello to everyone, I am brand new here and have never owned an S10 anything, but my step daughter does. I recently was walking by hers and noticed 1-2 dozen gallon jugs (empty) of transmission fluid in the back end of her rig, upon asking what was up? I find that she was going thru 1/2 gallon a day just driving the 50 miles round trip to and from work. After looking underneath I see that its just dripping away. No big deal, I may not be savvy with an S10 but I know when its time to replace the filter and gasket (although I wasnt tickled about that freaking cross member). I finally get the pan off, wiped out all nice and clean, reinstalled, topped off with fluid, and took it out for a test drive. Up until this this point the only problem the tranny had was its inability to hold fluid. I get about 9 miles and all the sudden the engine revs and thats it, it has dropped out of gear ( I assume ). I turned off the engine, restarted (with fingers crossed) put it in gear and away we go,....for about 1/4 to 1/2 mile when it does it again. I repeat this process about 10 more times getting it back to the house. I have had a few friends tell me "its the vacuum modulator (or something like that), its on the side of the transmission, check to make sure its connected, easy fix." Yeah right, I dont see anything and the parts stores are not listing any part that I am finding.
Please all you 1st gen gurus, what am I looking for, missing, yes I am totally clueless at this point :(

Wash the transmission off really well. Take it for a spin. see where its wet again....and go from there.

When it does again shift it manually. Does it move in 1st and 2nd. Sometimes on older transmissions your better off theaving the old fluid in. the old fluid full of dirt and crud is whats holding the clutch packs from slipping. New fluid has a detergent in it which cleans the worn clutch packs causing them to slip. I`m fearing this is what happened and a rebuild is in store.

There is no vacuum modulator on 700R4 or 4L60 transmissions. Trans was toast before you became involved. Replace it, rebuild it, or junk the vehicle. Those are your choices. Sorry!

I know this is a few months old now. Thought I'd chime in since I've been tackling my share of trans problems. OP if you're still trying to figure out here are a couple things I'd consider checking.

It sounds to me like the circuit for the forward clutch pack may be leaking(internally). . . Assuming of course that you didn't leak out a bunch of ATF. Being low on fluid can cause a no movement situation.

Could also be the pump going out and not building enough pressure to hold the forward clutch pack. Forward clutches could be toast also.

One other thing, being an 85 and not electronic, I believe a loose detent/kickdown cable could also cause low fluid pressure issues. Not 100% sure on this though. . .