The TCU gets RPM and other signals from the ECU, vehicle speed sensor, TPS, MAF sensor (if it has one) and then decides when to apply or remove voltage to the Lockup Solenoid that operates the valve to send oil pressure to the torque converter for lockup or unlock.
That is a lot of parts and connectors to check through.
As stated above, swap the transmission with a known good used one. They almost always come with the torque converter attached. Measure the diameter of the bolt holes on the torque converter, just incase there are two different ones like I found on our 95 Legacy.
On the bellhousing above the starter is a sticker. The first 6 digits MUST match on the replacement. It is best to have the 7th digit (engine) match also.