Approaching 200k on the odometer, it started going into "limp mode", first occasionally, then more and more often, until it was MOST of the time. Mechanic went through diagnostic steps until he got to "Substitute a known good TCM", which he didn't have. I bought a used TCM on ebay - worked for about 5 blocks, then back to "limp". Seller sent another TCM - worked for MONTHS!...hundreds of miles, city and highway. Then, SLOWLY started having problems. For a while, we could tell it was shifting properly by the fact that the D4 light was off (even when D4 was selected). I could leave home, drive a few miles, then suddenly the D4 would come on and the tranny would shift to 3rd gear. It might go back to normal in a few seconds..or it might be a few miles. The return trip home from work might be the other way around - start out in limp mode, then change to normal halfway home. For a few weeks, I tried to find a pattern, but it seemed quite random. Finally, though, it seems to have gotten stuck in limp mode.
I put the old TCM back in - OK for a couple of blocks, then limp.
**The weird thing is that, with the old one, I have to shift to 1st to get it to start off in low gear, but with the replacement, shifting into 2nd position gets it into the lower of the 2 gears that are available.
When it works, it works well. Any suggestions as to what I should do next? We need to sell the car...and it would be great if someone needed it for non-highway use only...but if another TCM might fix it, I guess I would give it a try.

 

Start the car and wiggle the TCM connector(s) and see if does anything. If it does you need to fix the connection. If that's not it check grounds.

 

The 90-93 AT car TCUs are known to have bad caps and resistors from the factory, they wear out and burn or blow out. Click on the H4A link in my sig. *crap it looks like the page is down* A couple bucks for replacement resistors and caps may get you back on the road no problem. That is if you can solder.