Well... I have built several AC systems. I do know that the high side should be somewhere in the neighborhood of 250 and the low side should be about 1/5 of that. And it ultimately comes down to the performance of the system - refrigerant boiling approximately 2/3 of the way through the evaporator...

And I do know that the AC system spins the aux fan for a moment when you start the car, then stops and checks for the voltage produced by the freewheeling of a properly working fan before it will let the compressor run - which let me fool the AC by starting the car while driving on the interstate so the fan was already freewheeling.

And I know that the only specification needed in order to manufacture a fan that will work in this car is that it respond to a PWM signal - be it whether it's a BLDC motor with a computer driver thay determins the proper speed via the width of the pulse or a simple DC motor that is current limited by MOSFETs or IGBTs with the gate being driven directly by the DME signal without so much as a filter.

And if it were a modified version of the second part of that - DME driven MOSFETs with a voltage divider resistor circuit - a weakened signal from corrosion (due to a pierced wire casing) could cause the fan to not get enough power to spin at short PWM signals while longer ones could push through.

But you are correct... I don't know enough to troubleshoot this car - namely which wire from the DME goes to the fan.