You talking about the aux fan ???

If yes , then check this sensor with an ohm meter for open or check the plug for a bad connection.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AO123CS42EH10NA0O&year=1991&make=MB&model=190-E-004&category=R&part=A%2FC+ACC+Temp+Sensor&appEngines=_any

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A Dalton