Fan Clutch Tutorial - Part 2 Two Types of Fan Clutches Non-thermal Non-thermal clutches operate based on the shaft speed of the water pump. At low and idling speeds the clutch allows the fan blade to turn at almost a 1:1 ratio. At high speeds the silicone fluid contained in the clutch will lose its ability to transfer the energy from the shaft to the fan clutch body (and therefore the fan) and the fan is then allowed to almost free-wheel, removing its load from the engine.  Thermal The thermal fan clutch is a little more sophisticated and operates, as the name implies, in response to underhood temperatures. When the engine is cold, the fan is disconnected; when too much heat is generated, the fan is allowed to cool the engine. A bi-metal thermostatic coil mounted at the front of the clutch senses the underhood temperature and operates the clutch accordingly. Next - Fan Clutch Tutorial - Part 3