Hello everyone,
So I am pretty savvy when it comes to working on cars and trucks, I just dont have much knowledge when it comes to transmissions. My uncle gave me a 1988 F-350, 2wd, with the 460 and a C6, and said I can take it if I get it running properly. Now when he parked it 4 years ago, everything ran near perfect. But starting it up recently he noticed it had a miss and was pissing ATF all over the place out of the overflow. So I began with pulling the pan, dumping out whatever fluid was left (which was a bit milky, maybe some water got in there?) and then pulled the trans lines off the cooler and blew them out with an air hose. One of the lines felt slightly clogged at first but cleared out for the most part, (was still building up a little pressure but not much). Put the lines back on, swapped out the filter and bolted everything back up. After, I threw in some fresh ATF and we went for a test drive.

Now we let it warm up and then put it in first and did some pulls and revved it out a little trying to clean it up a bit, and then went through the gears. Everything felt great, it pulled hard, shifted really solid into each gear, then while turning onto a street in first gear, slightly inclined, all of a sudden it felt like the trans lost pressure and just started free revving as if it was in neutral. We got out, checked the fluid level and everything seemed to be fine, got back in, put it back in drive, and felt perfect once again, for a few more minutes of driving, then did the same thing all over again. Now any suggestions on where to begin to determine the problem? I was thinking about bypassing the trans cooler to see if that helps (maybe there is a bit of a clog in there), but anything else I should try?

 

I believe yours trans had a down shift modulator ran by vacuum. Maybe the modulator is bad or vacuum line broke off.

 

Hmmmm ill have to look for a popped off vacuum line.... Hopefully it's as simple as that!

 

Alright so I'm working on the truck now..... So I jumped in it to pull around the black and everything felt great again, but once it started warming up after about 10 minutes of driving it started to disengage again.... Usually it is when I'm stopping at a stop sign or anything it doesn't want to start going again, I shut off the motor and then fire back up again and it goes right into gear, for maybe a minute or so... I let it cool off for a couple hours and it drove great again, then bam, back to its old ways once again. I emailed my trans guy (builds my th400s for my offroad truck) and he said to check motor and trans mounts because I broken one could cause the shifting linkage to pop into neutral but everything seems ok, and also I don't see any vacuum lines missing. Any help would be great appreciated! I don't have a problem dropping the trans to be rebuilt but I do want to diagnose the problem before doing so! Feel free to call me with any advice as well, 949-637-fivetwotwo1

 

So after more searching, someone got back to me in another forum and said that there's a good chance it could be the valve body.... My buddy had one for a C6 wrapped up in his garage that was new that I could try, so I dropped the pan, and there was a pretty good amount of sludge at the bottom of it, way more then I expected since I dropped the pan when I first got the truck (about 10 miles before) and cleaned it all out. I swapped valve body's and it seemed to shift well, just wouldn't go into 3rd at all.... I'm thinking it was the wrong valve body. Is it a bad idea to pull apart the one that was in there before and try to clean out the internals?