.....well...a test can be sone to verify if problem is in or out of transmission.
As said, the ON command comes from the computer. But the solenoid is connected this way: one of the wires is +12V...ALWAYS present with key ON. THis is normal. The other wire , other side of the solenoid coil, is GROUNDED by the computer to activate TCC. There is a remote possibility that the ground is made inside the transmission...faulty/chaffed wiring, as I say, very remote possibility. THis can be tested with an ohmmeter, one probe on either of the 2 wires of the transmission TCC connector and the other probe on ground/transmission body: a low ohm reading (0 to 30 ohms)would tell a ground fault inside the transmission. Open Ohm reading or very high ohm reading which is without a consequence (possible through lightly conducting fluid) tells everything iselectrically OK inside.