I believe you will find that the aux. fans have a dual funtion/speed that works like this:

Low speed-controlled by the high pressure sw.at the reciever/drier of a/c. The purpose is to aid the condensor at extreme high side readings. The resistor is how the low speed is attained.

High speed is controlled by the aux fan coolant temp sensor
and cuts in around 105C. This funtion is for engine temp
cooling assist and is independent of a/c system.
Different relays for each system.

May vary from model to model, but this is the basics.