On an M50 engined E39 the viscous fan should spin slightly at cold, and the clutch should engage and spin the fan (with torque) once the engine is up to the correct temperature. A quick test is that you should be able to stop the fan with a cold engine with a rolled up newspaper.
The reason why the fan shouldn't be fully engaged at all times is because it will constantly cool the engine, which will send a signal to the ecu via the temp sensor to map the engine as if it is cold - increase the fueling; hence why it will kill the MPG, and potentialy cause bore wash, which isn't good for the engine.
? err no, it will constantly cool the RAD the stat will keep the engine at operating temp and close if it gets too cold, if a locked fan is overcooling the engine to the point it's running on a cold map the stat is faulty, the higher fuel usage is due to the extra drag from the fan constantly pulling air through the restriction of the rad core.
There are differant types of "viscous fan" as said
www.haydenauto.com/featured%20products-fan%20clutches%20and%20fan%20blades/content.aspxthe normal type used in RWD fords pretty sure it's the same as BMW style as i have used ford ones in BMWs in the past ?
www.haydenauto.com/upload/HaydenAuto/Documents/Cat_Hayden/Fan_Clutches_How_They_Work.pdfAnd lastly why they spin faster for a while after first started
www.haydenauto.com/upload/HaydenAuto/Documents/Cat_Hayden/Hayden%20Fan%20Clutch%20Design%20Types.pdfAnd finaly taken from the ford factory manual descibing the viscouse fan operation
PRINCIPLE OF OPERATION
Temperature Sensing Viscous Fan
The temperature sensing fan drive operates a shear type fluid coupling which controls the fan speed by transmitting the drive through a film of silicone fluid and has the ability to idle the fan at all engine speeds, whenever the engine cooling requirements are low.
The interior of the drive is divided into two chambers. The rear chamber contains the rotor (K) whilst
the front chamber forms the fluid reservoir (Ll. Fluid is pumped into the reservoir by the rotating action of the rotor and the use of a ram pump (C). This pump consists of a hole (called the discharge port (A), and a "weir" (B), which is a depression in the pump plate designed to cause a pressure rise in the fluid. This has the effect of transferring the fluid from the rotor chamber to the reservoir
while the engine is running. The ram pump is located near the circumference of the pump plate (P). The rotor is directly 1inked, via the drive shaft (M) to the engine and with the engine running the rotation of the rotor retains the fluid in the reservoir.
As the engine cooling demands increase, a bi-metallic element (E) senses the air temperature behind the radiator, and operates a control valve (G) which progressively (as demand increases), opens the
intake port (H). This allows fluid re-entry into the rotor chamber, which in turn increases the fan drives' torque capacity. Hence, as the engine coolant temperature increases the fan rotor chamber
fills with more fluid, increasing the friction factor, which increases fan speed (within its design
parameters), thereby increasing the air flow and cooling capabilities.
As the above description explains, the speed of the temperature sensing viscous fan is related to engine temperature and not engine rpm.