Other possibility is a blasted open monovalve diaphragm, or a bad engine coolant temp sensor. The fact that the fan comes on right away would indicate the CCU thinks the coolant is hotter than it is.

Could also be a CCU problem, but I would start checking both interior and coolant temp sensors, and verifying the monovalve is OK.

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-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition