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This is from W140 Introduction into Service Manual, 1992 Model:
119 engine
"Visco-fan clutch
The visco fan clutch is maintenance-free and turns CCW. The visco-fan clutch is temperature and rpm controlled.
Identification:
Yellow label with number 119 200 00 22.
Upon reaching an air temperature of approx. 92 to 100C at the bimetallic spring, the fan smoothly engages.
Up to an engine speed of approx. 3600 to 3800 rpm, the fan speed is proportionate to engine speed, whereby slippage in the visco-fan clutch will not allow fan speed to exceed approx. 3000 rpm.
The safety disengagement of the fan occures audibly with increasing engine speed via centrifugal force at approx. 3800 rpm, at which point fan speed drops to approx. 600 rpm.
Reengagement occurs audibly after engine speed drops below 3300 rpm."
Attached is a crossection of the visco clutch for 120 engine:
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