That is not the Aux Fan.....it is the Viscious Fan and it is fluid coupled free wheeled until the ambient air temp gets high enough to lock it.

http://catalog.peachparts.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleAAA&cookieID=2FG17RJ3A2GD01E3YM&yearid=1989%40%401989&makeid=MB%40%40MB%40%40X&modelid=300%2DTE%2D001%40%40300TE+Wagon&catid=G%40%40Cooling+System&mode=PA&subcatid=G5010@@Fan+Clutch&source=www.peachparts.com&clientid=catalog.mercedesshop

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A Dalton