This is what I found on your P1600:
The ECM receives various messages from the TCM via communication line.
The ECM monitors an abnormal message signal due to open circuit in communication line and TCM failure.
DTC SET CONDITIONS
Two seconds or more have passed immediately after the engine started.
Abnormal communication line (TCM to ECM) with the TCM.
The most likely caused for this code to be set are:
Automatic transmission control system failed.
Open or short circuit or loose connector in communication line between the ECM and TCM.
....so, if the engine seems to still be running fine, but the transmission is unresponsive, I'm guess that your TCU is toast or you have a short between it and the ECU that you can't see... or the ECU trace for it is burnt. You'll probably need to open both of them up and look, then do continuity checks on the TCU wires to the ECU.
95 GSX fed by an 18g