Clutch fans rely on the air hitting their bimetal element to be hot enough to enable the clutching mechanism. If the radiator or A/C condenser are plugged with junk in the air path leading to the clutch bimetal, it will engage much later or not at all. Repeated failures suggest either cheap off-brand parts or a blocked airflow path. If the clutch isn't Sachs, ACM, or OE Mercedes, you bought the wrong one.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]