I know that the Aux fan is definitely controlled by the AC high pressure transducer in most BMW's. It may also be controlled by other factors as well, but I know for sure if you increase or overcharge the AC in one of these cars the Aux fan will start to work and much faster.
Mine never seemed to come on, my AC was a bit low, once charged up to what appeared to be the proper level, the fan started to run faster and be louder. Very similar system in my GMC Yukon as well. Is also partially to to the behavior of the AC systems and the variable output compressor as well.
One thing I find over and over again, many fan manufactures do not get how to make a quiet fan. Even width and spaced blades are WAY louder than partially staggered blades that usually have 1 blade a bit wider. It is unbelievable how much quieter a properly designed fan blade can actually be. No more school bus/trash truck sound from under the hood.
If anyone has their original fan, you may want to carefully compare blade spacing and widths.
Also not sure, but INPA may give you insight to the Aux fan speed, I do believe it will give you AC high side pressure output?
+1 on that.
From the technical manual -
Auxiliary Fan Control
The Auxiliary Fan motor incorporates an output final stage that activates the fan motor at
The auxiliary fan is controlled by the MS S52. The motor output stage receives power and
ground and activates the motor based on a PWM signal (10 - 100 Hz) received from the
Similar to the aux fan in the E46 with MS 42.0 control, the fan is activated based on the following
• Radiator outlet temperature sensor input exceeds a preset temperature.
• IHKA signalling via the K and CAN bus based on calculated refrigerant pressures.
• Vehicle speed.
• Battery voltage level.
When the over temperature light in the instrument cluster is on (120OC) the fan is run in the
overrun function. This signal is provided to the ECM via the CAN bus. When this occurs
the fan is run at a frequency of 10 Hz.
Since your fan is coming on early and staying on - I'd say there is something wrong with the refrigerant pressure sensor.
I've replaced my Aux Fan with an ebay special - it is louder - but not too bad.
I agree that there is a lot of science behind the fan noise.
With my SPAL and the Aux fan on - it can get quite loud - but no more so than most modern cars in the summer.