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Old 03-17-2004, 10:25 PM
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Location: oakland ca usa
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Correct operation of clutch fan on 1991 300CE

Hopefully someone can clarify the correct operation of a clutch fan on a 1991 300CE. A buddy of mine just had the clutch fan replaced on his 1991 300CE and the owner (mechanic) that claims to have 32yrs of MBZ experience at Silver Star Mercedes in Oakland Ca on 17th and Wood Steets says that the 1991 300CE has a Viscous fan that is normally locked up at all times until you reach higher rpms or freeway speeds than it disengages. I was told by another mechanic that the 1991 and the other CE models have a bi metalic type of fan that engages at a preset factory tempature of approx 95 degrees centigrade. He claims that the fan is normally disengaged until the bi metalic spring reaches a preset tempature. Then the clutch fan engages.

Which mechanic is correct?

Thanks for your response.


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Old 03-17-2004, 10:38 PM
Ali Al-Chalabi's Avatar
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Location: Knoxville, TN
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If it is like almost every other Mercedes fan clutch made, it engages based on temperature around 95C.

The fan will always be spinning with the engine running, but when it gets hot enogh the bimetallic strip bends and pushes a pin that directs fluid to get a tighter grip on the fan and thus makes the fan turn faster. It also cuts out above 4,000-5,000 rpms engine speed.
Ali Al-Chalabi

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Old 03-18-2004, 10:47 PM
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Hi..thanks for the quick response...very much appreciated

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Old 03-18-2004, 11:29 PM
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Location: Evansville, Indiana
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Actually, while the fan spins all the time, this is the way to check:

It should roar for a few seconds when you first start it since the viscous oil is all stitting at the bottom of the mechanism. It will then slow down as the oil runs out of the gap between the disk that drives the fan and the slot it is in. It stays up in the "reservior" so long as the hole in it is covered by a plate pushed in by the bimetallic strip.

When the bimetallic strip gets hot enough, it pops out, releasing the plate (actually a simple valve) and the viscous oil runs back down into the working area and the fan speeds up.

The fan never sits completely still, and it never actually runs at engine speed, either -- close at lower speeds, but never as fast. At high speeds, the oil runs out too fast to drag the fan at engine speed, and the drag of the fan is too great for it to speed up more.

When engaged, the fan will be running near engine speed from about 900 rpm to around 3000 rpm -- above that, it usually won't go much faster than 3000 rpm no matter how fast the engien runs.


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