dr bob

A reminder that on S4+ cars, the fan (and cooling flap) operation is while AC button is pushed is based on refrigerant pressure. Since refrigerant pressure and temperature change with a known relationship, this pressure reading is a very effective way to determine the residual heat left afte the condenser. Note that the sensor is an analog device, with current flow proportional to high-side pressure. If there's no or almost-no pressure in the system, no need for extra fan flow for the AC system needs.


For those reading along at home:
With regards to Jon's suggestion that you "bridge the two connections on the top of the pressure switch on the receiver dryer", I'm not sure that will fool the system into thinking there's more AC system load. Regardless of whether it works or not, there's no real-world condition where the resistance through the pressure transducer is zero.


Remember that the sensor at the bottom of the radiator runs the cooling fans only enough to present cooler water to the engine. Regardless of actual amount of heat coming in or waterflow, it just runs the fans to help maintain consistent return temperature to the engine. Ultimately, the thermostat decides what temp the engine coolant runs at. So long as the radiator and fans can keep up.