Friction coupling devices and fluid coupling devices that drive radiator cooling fans for over the road trucks, such as class 8 trucks, are generally of two types, dry friction clutch assemblies and viscous drives, respectively. 
Dry friction clutch assemblies tend to have two operating conditions “ON and OFF” referring to when a friction clutch is either fully engaged or fully disengaged.  When a friction clutch assembly is providing cooling the clutch is fully engaged and not slipping.  When the friction clutch assembly is not providing cooling the assembly is fully disengaged and slip speed is at a maximum between a clutch plate and an engagement surface. 
The dry friction clutch assemblies generally have low thermal capacity, since they typically do not incorporate fluid flow cooling mechanisms.  Thus, the clutch assemblies have minimal cooling capability and are unable to cycle repeat in short durations of time.  Also, because of low thermal capacity, the clutch assemblies are also limited in torsional capacity, such that they are incapable of engaging at high engine revolutions per minute (rpm) or high engine speeds.  The thermal energy that is generated during engagement at high engine rpm speeds can “burn up” or cause the clutch assembly to become inoperative. 
Viscous drives, on the other hand, have become popular due to their ability to cycle repeat, engage at higher engine speeds, and have varying degrees of engagement.  Viscous drives have an operating range of engagement and are generally less engaged at higher engine speeds and generally more engaged at lower engine speeds.  Viscous drives are never fully engaged for internal viscous shear purposes. 
Unfortunately, viscous drives are also thermally and torsionally limited.  Viscous drives are always slipping to some degree causing them to be incapable of turning at fully engaged peak operating speeds or at higher speeds than originally designed.  Since viscous drives are continuously slipping, they are continuously generating heat, unlike friction clutch assemblies.  Viscous drives are further limited in that the more engine cooling needed the larger and more costly the viscous drive and cooling fan that is required.  Thus, for increased engine cooling requirements viscous drives can become impractical in size and cost. 
Due to increased engine cooling requirements, it is desirable that a fan drive system be capable of not only providing an increased amount of cooling over traditional fan drive systems but also that it have the associated advantages of a friction clutch assembly and of a viscous drive, as stated above, without the associated disadvantages.  It is also desirable that the fan drive system be practical and reasonable in size and cost and to be approximately similar to and preferably not to exceed that of traditional fan drive systems. 
To overcome the disadvantages of both of the aforementioned traditional fan drive systems, a new fan drive system has been developed which can be referred to as a solenoid actuated hydraulically controlled fan drive system.  A housing assembly is provided which is typically in the neighborhood of 12-16 inches in diameter.  To minimize parasitic losses, the housing is not completely filled with hydraulic fluid, but is typically filled with the hydraulic fluid such that there is only 1-2 inches of the hydraulic fluid spaced around a circumference (assuming that the housing is being spun).  The fan drive system is engine driven via a belt or chain driven pulley.  A stationary bracket rotatably mounts the pulley to the chassis of the vehicle.  The pulley is fixable connected to the housing assembly.  A clutch assembly within the housing assembly is selectively engaged to connect the rotative fan with the housing assembly.  To actuate the clutch, there is a need to provide hydraulic pressure.  To provide the hydraulic pressure, a pitot tube is utilized.  The pitot tube is fixably connected to the bracket.  The velocity of the fluid, which is rotating within the housing, is utilized to generate pressure by entering into an aperture of the stationary pitot tube.  The pitot tube is also fluidly connected with a piston engaging circuit which through a clutch friction pack engages a fan hub which is rotatably mounted to the housing assembly.  To control the amount of engagement of the fan hub with the housing assembly via the friction pack, a hydraulic control arrangement is provided.  The hydraulic control arrangement controls the pressure within the pitot tube by selectively connecting the pitot tube with a reservoir sump.  The reservoir sump occurs due to the void of fluid in the center of the housing assembly.  A solenoid actuated relief valve is utilized to selectively cut off a fluid connection of the pitot tube with the low pressure sump formed within the radial center of the housing assembly.  To ensure a full engagement of the rotating fan hub with the housing (fan locked in position), the pitot tube interior is blocked off from the sump thereby causing full pressure to be utilized to actuate the friction pack which torsionally connects the fan hub with the housing assembly.  To allow the amount of torsional connection between the housing and fan hub to vary, an electrical controller system is utilized to selectively open and close the solenoid motor thereby controlling the amount of pressure which is exerted on the clutch the friction pack by the piston. 
As mentioned previously, for partial fan operation, the clutch of the fan drive is continually slipping.  This continual slippage generates a high amount of heat.  In many situations, the amount of slippage heat generated is greater than that which can be transferred to the surrounding air by convection from fins located upon the fan drive housing.  Accordingly, in many prior fan drive systems, operation has been limited to fully on or off state of operation.  It is desirable to provide a fan drive system with a hydraulically actuated clutch which is more tolerant of partial clutch engagement.