No reverse, OD, or auto shifting

[TechmanDan]

Howdy!
I've got a 2002 4.0L V6 4x4 Automatic, 160K miles, with some transmission issues.

Short version, symptoms:
Place selector in "R", no movement, akin to being low on fluid.
Place selector in "D", no movement, akn to being low on fluid.
Place selector in "2" or "1", movement in the correct gear.
Start in 1st or 2nd, turn OD off (button) and shift into D, everything works fine. Stop, proceed to move again, no movement, must shift into 1 or 2 to take off again.


In chronological order:
Bought truck at 74K.
100K: Flush and fill at local trusted mechanic. No shavings, no debris, no issues. Everything ran great!
130K: Flush and fill at same local mechanic. No shavings, no debris, no issues. Still ran great!
159K: Noticed that trans was slipping when shifting from OD to 4th or 3rd, as in passing situation on freeway. Instead of grabbing lower gear, it would just high-rev as if in nuetral. Manually turning off OD got me into lower gear for passing.
After a month of that (I'd say it happened 10 ten times in that month), I was on my way home, stopped at a stop sign. Proceeded to pull out, truck revved, but no movement. Shifted into 2nd, moved along fine. I was able to get it home, and it's since been collecting dust and occasionally running to the hardware store for project materials that don't fit in the car.
Local trans shop says it's the forward sprag. Fluid is good condition, good level. No CEL, all sensors show good reading.

Question:
1) What do you think is wrong?
2) I've got engine teardown and general mechanical (shadetree) experience, and i'm not afraid to tear into this, but as a newbie, what are the risks of opening it up myself?

 

[adsm08]

The front one-way clutch could cause the issue, but I don't know that it's the problem. It is certainly something to look at if you are going in. A one-way clutch is simple to test too. Just grab the component in front of it and try to turn it both ways. If it is working it will turn one way, but not the other.


It seems that a lot more could be wrong in the pump or valve body that would cause your symptoms though. Bad gasket, sticking valves, etc. Make sure to take a line-pressure reading before taking the trans out.

With that kind of mileage I would say the biggest risk you run opening it up and looking is finding out that enough stuff is worn out to warrant a total overhaul while you are there.

To be perfectly honest, and this is not trying to dissuade you from attempting this repair, I rarely bother trying to do specific repairs to transmissions with that many miles. It's generally more cost effective to do a total overhaul or replacement, plus in the case of my customers they get a better warranty.

 

[TechmanDan]

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Thanks for the info, I appreciate it.
Since my first post, I learned about a neat trick involving adding a touch of brake fluid to the transmission to make the seals swell, so I tried it.
After 2 hours of runtime, the transmission worked perfectly; smooth shifting, I could start in "D", and I had reverse!
Sadly, it reverted back after a day, and i'm told there's a limit to how much brake fluid you can put in.
Based on the 'fix', I think it's safe to say the issue is one, or more, of the seals inside, so I think my next trick is to buy a used transmission, swap the two, and use the existing one as a tinker toy to see if I can fix it. I think it'd be neat if I could!