If the fan is faulty it can be read with software that is specific to BMW. It will not trigger an OBD II code since it is not emission related. The fact that your fan does come on means the fan is working. You can command to run the fan if you have the right software.

But to answer your question when the engine is started the DME sends a command to the fan to turn it on ( it may make 1 or 2 revolution). The fan sends a signal back to the DME that it has turned on there are no fault.

The fan only comes on when there is insufficient air flow to cool the condenser. If you are driving at speed, the fan is not needed to cool the condenser and may not come on (depending on the outside air temp).

The speed of the fan is controlled by the DME. It uses a pulse signal to vary the speed of the fan. That is why there are three wires to the fan, a plus wire, a negative wire and the signal wire. The longer the pulse the faster the fan will run.

__________________
2006 Infiniti G35
2001 BMW 3.0I E53 X5 Build date 08/2000 SOLD
Lotus Europa 1970 Destroyed by fire
Lotus Europa 1970 S2 Renault Powered
Lotus Type 52 1970 Twincam Webers Powered
PORSCHE 911 Targa 1982 The Garage Queen
Audi Avant donated to Kars for Kids
BMW 525IT Sold
Audi 4000CS Quattro Sold
Jensen Healey Lotus Powered Sold
Opel 1900 Sold
Triumph Spitfire 1971 Sold
Triumph Spitfire 1968 Sold
Plymouth "Cuda" 340 Six pack SOLD