Not sure exactly how they are wired on your model, but usually the aux
fan comes on when either the coolant temp exceeds a certain temp or if
the temp at the AC dryer/receiver exceeds a certain temp.  AC Pressure
should have nothing to do with it, as long as the AC compressor is
running.   On older models these were both just simple temp switches,
not actual temp measuring sensors and they were connected to a relay
which then controlled the fan.  You could easily check the operation by
just shorting the temp switches to see if the fan came on.  I wouldn't
just randomly replace components when it's easy to test and find the
actual problem, which could be just a bad connection somewhere.

Suggest you get a wiring diagram for the car, which is going to be well
worth it not only for this problem, but for the future if you are going
to do your own work.

There might be a resistor connected in series.. to reduce the fan
speed....this resistor was blown in my car and did not turn on the radiator
electric fan when aircond was on but will turn on when engine at high
temperature...