93 2.0L mystery problem with transmission

[cbc58]

We have a 93 automatic transmission problem which no one can seem to figure out... maybe someone here knows. Have been to the other boards but no luck.

Symptoms: Car shifts hard from 1st to 2nd and transmission hold light is blinking. I know that some will say it is going into limp mode, but the local transmission shop doesn't think that's the case.

Over the course of a few weeks, the hold light started to blink more and more frequently until one day it came on all the time. The hard-shift started occuring after that.

Was told by some people to check the diagnostic box and do a self test using the TAT pin. TAT pin was empty and someone mentioned that there might be a single wire under the dash for testing behind the PCM. Ripped out the lower dash, can't find it - looked extensively. Regular diagnostic test shows codes 23 & 28.
Can not find any other diagnostic connector in the car. Someone mentioned we probably have a EEC-IV unit (?)

Transmission guy thinks that its electrical. His theory is that something is signaling a boost to the line pressure, but since there is no way to check the transmission codes (that we have found yet), we can't determine what it is.

Really a mystery and I know there are others out there with the same predicament and hopefully you know the answer to what this might be.

Thanks.