fan always on

[hump]

Bought my truck last October, and never paid much attention to the fan but it seems that it is on all the time. Start truck cold and fan is on. Any ideas on what it would take to repair this? I assume it is a clutch of some sort. Thanks in advance.

[whatnot]

What do you mean "always on"?
It should always spin as long as the engine is running. It doesn't spin full speed untill the engine coolant gets hot.
Without the engine running, does the fan turn easy?

[hump]

Just showing my ignorance, I read many posts about the fan cutting in so I assumed it didn't kick in at low temps. I towed a small camper up some serious grades last weekend though, and sure enough I heard the fan roaring as the temps climbed. So I guess it works, anybody got a simple explanation how higher temps cause it to speed up? Just curious.

[Barry Nave]

The fan clutch has fluid inside and an orifce
vavle controled by a spring in the center of the clutch. As the spring heats up to a set * vaule the orifce opens up and the fluid coppling takes place to drive the fan at the pulley speed.
As it cools down the cooling system the spring will release this fluid coppling to less then the pulley speed.
Eng. gets hot and the fan cycles again.

[DmaxMaverick]

In addition to Bnave95's details, the heat comes from the operation of the thermostat(s). When the stat opens, hot engine coolant passes through the radiator, heating the air that contacts the fan clutch. The heat causes the results described by Bnave95.

Not all fan clutches have the bi-metal spring on the front. Light duty clutches rely only on the heat changes of the silicone fluid inside the clutch for operation. They are not responsive or aggressive enough for the needs of a heat sensitive engine, such as the 6.5TD. I tried a "bargain" clutch (no spring) on a 6.2 once. I might as well have left the fan on the bench. By the time it spun up, temps were beyond the point of no return.

[hump]

Thanks guys, always like to get a bit more info on my rig. The fluid coupling is similiar to the recent all wheel drive set ups I guess.

[rjschoolcraft]

The fluid coupling in a fan clutch works on the viscous shear principle. Introduce fluid between two close clearance surfaces and drive torque is generated by shearing the fluid. Good clutches have interlocking grooves and lands to increase surface area for shear.

Fluid couplings in transmissions (torque convertes, etc.) work off of fluid momentum from centrifugal pumping. A different theory altogether.

[hump]

Thanks Ronniejoe, my ignorance is showing again, but I don't mind, I learn a lot that way. It's cool that people will share their knowledge with others they don't even know. Amazing that an engine is the common denominator. Thanks again