Thought I'd repost this and possible save someone much grief with the GM trans. factory screwup.
"Cheap Fix for TCC/4th Gear Problem
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My 1995 Transport developed "the common" problem of losing TCC/OD when the trans. fluid got hot. First I installed a small trans. cooler in front of the rad/AC cond. Tried piping it both ways-with and w/o the rad. cooler. It worked much better in series, with the fluid going thru the add-on last.
But the problem remained-it just took longer to lose the TCC/OD. Did a LOT of research into the problem. Seems some moron trans. engineer in GM managed to design the TCC apply valve such that the slightest leakage which got past the valve (steel valve sliding in the soft alum. valve body) would cause the TCC to disengage. The "stupid computer" would then sense the RPM mismatch-it had commanded the TCC to apply-so it then decides to prevent 4th gear (OD) from working. I live in Phoenix and the trans. temp. sensor never got hot enough to set a code-the problem was the trans. fluid getting thinner as it got hot.
I believe Trans-go or Sonnax makes a kit to replace this valve and add a steel bushing (what the trans. should have had originally), but it appears that the trans. has to be removed to install it. But there is another answer.
I decided to take a chance and use a thicker trans. fluid. I knew I was going to have to pull the trans. anyway if it didn't work, and then I'd have it rebuilt with the stronger heat-treated stuff etc. that also should have been used originally. I also wanted to regear it lower, because even with a good-running 3.8 the trans. was way overgeared-at least for me.
So, after making MANY phone calls, etc. I bought a 5 gal. pail of 50 wt. "Powershift" trans./hydraulic clutch fluid from a Caterpiller dealer. This fluid is used in earthmoving equip., and also comes in 30 and 10 weight. I dropped the pan and refilled the trans.-you can only get ~50-70% of the fluid out. Refilling with the 50 wt. gives about 30 wt. with the fluid that remains in the converter. What all this means is that I gained ~30-40 deg. F on the viscosity of the trans. fluid, and the problem disappeared. It has worked fine for ~4+ years and 30,000 miles so far.
The fluid is not expensive-I think I paid $35 for the 5 gal. They don't sell it in smaller amounts.
Several cautions besides the "do this at your own risk". Because I live in Phoenix, the lowest temp. is ~30 F. That means the trans. fluid is about like reg. ATF at 0 or -10 F--no big deal. But if I lived where it got colder I'd have used the 30 wt. for ~15-20 F "advantage".
The trans. works fine now, but I still put up with the gearing that I don't like. I'm just waiting for something else to break so I can remove it and rebuild it right!"