The fan clutch on a Ford 6.0L diesel engine works by using a combination of a mechanical fan clutch and a fan clutch adapter. The fan clutch operates as a thermostatic engine cooling fan, which means it can freewheel at low temperatures and engage when temperatures increase: Freewheeling At low temperatures, the fan clutch freewheels to allow the engine to warm up faster. This reduces the load on the engine. Engaging When temperatures increase, the clutch engages and the fan is driven by the engine's power to move air and cool the engine.  Due to this operation, the fan can be calibrated to operate at varying temperatures based on Engine Coolant and Engine Oil Temp. 

The fan system is called an electro viscous fan clutch. It differs from a

standard fan clutch simply by the control. The standard clutch uses a

thermostat on the front side to rotate a small shaft which increases the

fan speed depending on the heat coming from the radiator.

The ECM controls the electro-viscous fan clutch engagement. The

control module regulates a 12-volt pulse width modulated signal (PWM)

to the cooling fan relay. The PWM signal determines the ON time of the

relay. As the commanded state of the fan clutch increases, so does the

ON time of the relay. This ON time directly effects the amount of time the

solenoid, which is internal to the fan clutch, is energized. When the

solenoid in the fan clutch is energized, it opens the spring loaded valve

and allows fluid to flow from the storage chamber to the fluid coupling of

the cooling fan clutch, increasing the fan speed. When the solenoid is 

de-energized, the spring loaded valve closes, and blocks the path of the

fluid to the fluid coupling of the fan clutch, reducing fan speed.

The fan has the ability to create a feedback signal, so the control module

has an actual fan speed input. This is done with a hall effect sensor

internal to the fan clutch. The control module supplies a 5-volt reference

and a low reference to the hall effect sensor. The hall effect sensor

returns a signal pulse through the cooling fan speed signal circuit in

response to the reluctor track passing by the magnetic field of the hall

effect sensor.

Diagnosis of both fan clutches is similar for the mechanical aspect. 

If there is any lateral movement in the drive, then the clutch is in need of

replacement.

The standard fan clutch will typically spin abnormally free when it fails,

and should spin with some resistance normally.

The electro-viscous clutch being electrically controlled had DTC's that will

set if the speed is not what is commanded or there are open circuits to

the sensors.

Leaks on either unit indicate failure of the clutch.