I was always curious on how the clutch fan works. Bismic and others had described the inner workings, but could never really understand the "how". So I decided to cut up my old failed clutch fan to get a visual of "how it works.
Here is a copy/paste describing the functions of the internals.
OK. Below is a description of the 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.
Read more: How does a vistronic fan clutch work and how do you diagnose one - JustAnswer http://www.justanswer.com/car/1zsxa-...#ixzz2492V6KLP
After disassembling/destroying the old fan, I finally got how it works. Here are two videos.