HMMM
wrong is relative... The fluid in the fan clutch is silicone oil. Silicone
oil has the unique property of getting thicker as the shear forces increase,
somewhat the opposite of hydraulic oil. When used in a Foettinger coupling
(turbine style impeller and housing) the fluid will drive the coupling
"tighter" as speed increases.) Silicone oil has a very flat temperature vs.
viscosity curve, shear forces aside. The intent here is to increase fan
speed as engine speed increases, but to leave some "slop" in the coupling to
reduce noise and power robbing. A thermostaticly controlled clutch has the
valving situation as described below. SO... If the fan "Roars", a couple of
things could be wrong. The valve could be stuck in the "hot" posistion,
locking the fan (more or less) to the shaft. The fluid may have leaked out,
increasing fan and engine temp, and frying the bearing, causing a lockup
condition (sounds unlikely on paper but two of my 3 fan clutch failures were
caused by this mechanisim.) I have never seen a "stuck" valve, but my
experiences are somewhat limited...
2cents Bridges
At 08:55 AM 6/3/98 -0500, you wrote:
>> << The viscosity of the fluid clutch increases as the temp behind
>> the radiator rises. If the clutch doesn't get as hot, the fluid
>> won't get as thick and the engine won't have to work so hard
>> to turn that windmill. >>
>>
>> WRONG!!
>> The viscous coupling fluid does not change viscosity with temperature (at
>> least not with the intent of spinning the fan). The fan is basically a pump
>> internally. On the front of the clutch is a Bimetal element shaped like a
>> spring. The center of the element is attached to a small shaft. As the
>> element heats, it twists the shaft which is attached to a valve inside the
>> clutch. As the valve closes, it causes more restriction in the fluid path,
>> and therefore more engine torque is passed on to the fan blades.
>>
>
>NOW when the fan is cold as in when first started, the fluid IS THICK and
causes
>the fan to turn fast even though the valve is open. After a minute or 2 the
fluid
>warms, becomes thinner and since the valve is open the clutch free wheels.
>Thetas is why the cold fan noise. It is NOT a malfunction therefore no one is
>required to fix it. Dodge is not the only make whose fans do this. Every
Ford or
>GM I ever owned did this too. Learn to live with it. If you have a clutch fan
>and it is working normally it may do this. If the clutch is worn out it
may get
>quiet but now you got other problems to worry about besides a minute or 2
of fan
>noise when the engine is cold.
>Jim Miller (26 years in a radiator shop)
>Waco, Tx
>Still Dakless (only God knows where my truck is)
>
>
>
>
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