[Phantom]

Assuming that you have a newer car the PCM might have to be fooled several ways to get it to work. For TCC it most likely will look for the lock signal and determine how it is functioning by looking at the output shaft RPM vs TCC slip. If you command the TCC to lock below 70MPH but mimic the unlocked signal and the converter slip is read as 0-50RPM it should throw a code.

What I would try to do first is locate a company that sells a tuner solution for your car (I use HPtuners but they only support GM, Ford, and Dodge.) That is how I tune the shift points, TCC lock, shift Pressure and time.

If there is not a tuning solution that you can find for car you could pay for a chip or build a solution like you are thinking. In building a solution it would be best to build a controller to go between sensors, trans, and command wires. In that solution you would need to be able to log the current conditions so you can trick it into thinking it is running normally.

IE. Manually command TCC follow this logic "If MPH<70 then interrupt TCC slip and send represented value of 500RPM slip, other wise pass original signal."

You will need to know the pulses per mile for MPH and probably the same for TCC slip and build a frequency generator.

Hope this helps and does not discourage you.