Hey so here's my run down.
first issue, slipping transmission, no violent, but hoppy anywhere over 40. Fluid was up, no debris, was rusty brown. Realized i hadn't done the filter in a minimum of 150k, possibly stock filter, currently 226k. 4wheel drive 4 door 2001 GMC jimmy, 4.3/4l60e.
So here's where I'm at, and confused, every gear was working, just occasional slips, normally under 2k rpms, if i loaded it, no slip. Self diagnosed that the filter must be clogged so time for a change. Swapped the filter, pan was clean, no debris, decent bit if black gunk on the magnet, but no actually filings or shiny, i assume 200+k of clutch wear, reminded me if brake dust. So changed the filter, topped off fluid, added some Lucas stop slip, nice smooth shifting 1-3, and reverse. But now, no drive/4th/overdrive. Runs like a gem up to about 35 in 3, don't wanna push it harder than 2500 rpms. I'm wondering if maybe i killed a solenoid, or hit a wire plug somewhere? Any thoughts? If any thoughts call me out on the idiot i am. Sorry if this has been covered, but i searched and couldn't find anything directly relating to a fresh filter nuking drive. It does seem to have an issue going into 3rd if i start in drive, but if i do it manually golden. All the gears worked, so should have to be relevant to the dropped pan. I did have to wrestle it past the cross member a little, but i didn't think i was catching anything. Need to drive this 30 miles tomorrow, so hoping for input, thanks guys, and sorry for the long winded post, any more information needed just ask. And for reference, no codes on obdii.

 

Last early spring my transmission started to slip when shifting from 2 to 3. I thought it might have been that I was due for a transmission fluid change and new filter. I went through my GM manual and checked everything I could trying to avoid the reality that it probably needed to go to the shop. Sure enough, and long story short, it needed a new clutch pack. I burned it up towing my 2,200 boat and trailer.

The symptoms came on pretty quick, in about a week.

All this happened on a 1999 Blazer, 4WD at about 130,000 miles.

 

Guess my question is, how could changing the filter n fluid go from eh it slips at low rom, to nothing at all. That doesn't add up. Something screams i did something wrong on filter swap...

** Very odd update.
So it sat for 8-9 hours, at a park n ride, had to limp it home. On a whim while running it in third (capped out around 35) started going downhill and said hell with it, tried drive, gravity got me going 45 or so, gave it gas, now it will shift into 4th/overdrive, uphill downhill, got it to 60 no issue. Any thoughts on this? Had this truck 10 years, over 150k put on it by me, rather attached, most work done by me learning as i go, was not ready to let her go. I'll know more after she sits overnight and try drive from cold. Wonder if somehow something got stuck and the downhill roll freed it up? Thanks for the time reading all this.

 

I guess there could be a correlation/causation going on here. With such a complicated system, if the components are not totally shot, you could have intermittent functionality due to the particular conditions at a specific moment. Or there could be no correlations at all and what you are observing is random. What popped into my mind after reading your update was if the viscosity of the ATF was higher after sitting for a while (and cooling off) with that facilitating the transmission's shifting.

You might have to "bite the bullet" and take it to a transmission shop to get it properly diagnosed. I seriously considered doing a transmission replacement at home with a mail-order transmission instead of having the local shop repair mine once I learned that my clutch pack was shot. Cost-wise it would have been close to "a wash" with doing it myself saving a few to several hundred bucks. However, even though I've pulled a motor before (a '71 Dodge Dart) I was not up to working on the much more complicated and compact Blazer.

Hope these thoughts help.

Good Luck

 

Agree with Christine, need a new 2-3 clutch pack. The new fluid has detergent in it. That cleaned the crud holding your clutch pack from slipping. Need a rebuild. Basically on older vehicles if the trans fluid changes were not kept up, you better to just leave it. Doesnt make sense but thats the way it works

 

I agree. I replaced the filter/fluid when I got mine 30,000 miles ago. Someone had been in the trans before ( broken off cross threaded pan bolt in hard to get to position by crossmember ) Recently I replaced the front T/C seal and while I was there I did a fluid filter change. Next time driving after about twenty miles I started slipping in third/no OD and then lost it altogether, I had to return in manually selected second gear.