4EAT: no 4th gear?

[davew833]

This question relates to my '92 Legacy turbo AWD auto, not my SVX, but I'm not getting much action on this topic over at legacycentral or usmb so any ideas are welcome! All Subaru phase 1 4EATS are basically the same, right?

The problem is the car never shifts into 4th gear, ever, or even tries. I can get up to 4500 rpm and 75 mph on the freeway and no shift out of 3rd. Doesn't matter if it's cold or warmed up. Seems to shift through the other gears just fine. It doesn't seem to slip or flare in other gears. No torque bind.
I did a stall test and it seemed to be in normal range. It had a previously stored code for VSS1 which I replaced with one from my spare SVX trans. That didn't fix the 4th gear problem. Do you have to clear stored codes before the TCU realizes the problem's been fixed?

I've only had this car for 2 months. There's evidence it's been parked for about 10 years previous to that. It's got about 137k miles.

About a month ago, the trans started slipping badly in any gear so I dropped the pan and found the filter clogged with brownish gunk. I didn't find an abnormal amount of metal particles in the pan, but the filter was so clogged I had to replace it. I drained and filled the fluid twice during that process. That seemed to restore normal driving in the lower gears. I had never driven the car on the freeway previous to that so I don't know if 4th gear has ever worked since I've had it.

Any suggestions as to why it won't shift into 4th, and are there any easy fixes I can try before I start pulling and swapping trannies?

Someone suggested brake band adjustment, but I've got no 2-3 shift flare. Someone else suggested TPS, but they are a PITA to adjust on a Legacy compared to the SVX, and I get no codes for that.

Also, I have a chance to get a low-mileage 4EAT out of a non-turbo '92 Legacy from the Pick-N-Pull for cheap. Does anyone know how the gearing compares? My research says the final drive is the same at 3.90 but I don't know how the other gears compare.