I've been puzzling the method whereby the pin causes the engagement.
I notice that at rest., warm or cold when starting the fan is always engaged for a few minutes. I envisioned that the fluid had flowed to the bottom and starts dragging the driven part. What is puzzling to me is what does the snap action of the bimetal do? Does it divert fluid from a storage compartment to the clutch faces? Is there a piloted scoop passage to perhaps remove fluid from the outside to a central storage which is released to the outside by the bimetallic operated pilot? Haven't been able to find any diagrams so far.

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Retired Marine Engineer
2005 E320 CDI
1987 300 Turbo Diesel @ 300,000 kM going
1974 240 D 300,K Miles when given away gone
1970 200 D gone
1969 190 D gone