From
http://www.haydenauto.com/Featured%20Products-Fan%20Clutches%20and%20Fan%20Blades/Content.aspx Non-Thermal Fan Clutch
* Low cost alternative for some standard thermal clutches.
* Always engaged, less fuel savings than a thermal clutch.
* Spins at about 30-60% of the water pump speed.
* Shorter life expectancy.
* Cannot replace a heavy-duty clutch.
* Identified by the smooth, steel faceplate, (without a thermal spring assembly), on the front.
Thermal Fan Clutch
* Varies the fan speed with temperature of the air behind the radiator.
* Engaged (high speed) operation provides maximum cooling.
* Disengaged (low speed) operation provides fuel savings and noise reduction.
* Greater life expectancy than a non-thermal clutch.
* Briefly engaged at cold start-up.
* Engages at about 170° radiator air temperature, (about 30° lower than coolant temperature).
