gidpor,

I had a similar climate control nightmare that plagued me for months on my U.S. spec '88 300SEL However I had a combination of problems: 1) On defrost only, fan blower would cut out intermittently. After Push Button Control Unit replaced x3, found wire damaged at plug on firewall just directly behind fusebox. Found this by tapping fuse box area with screwdriver handle. 2) Had constant cycling of hvac doors. Replaced switchover valve inside r/h side of center console. 3) Had uncommanded and sometimes unbearably hot air flow into passenger compartment. Sometimes I had split hot/cold air combinations. Hot air coming from top side vents and cold air coming from bottom side vents. This problem would only occur after driving approx 20 to 30 min. Interior temp sensor replaced x1, Aspirator fan x1, The Monovalve insert replaced x2, Monovalve housing x1. After the Monovalve housing was replaced all was fine for approx. 2 weeks. My problem came back after starting an 800 mile journey with my family packed into the car. Not a happy camper! I again searched these threads and could not find a similar problem. My thinking was that there was some sort of a signal loss problem to cause the system to default to heat. After my not so pleasant trip, my mechanic took out the complete Monovalve assy. again and took it apart. There is a check valve located in the bottom of the assy., if the check valve sticks it could let hot water enter into the heater core. The check valve in mine was found contaminated with sludge/debris that probably made the valve stick open. The coolant system was flushed when I bought the car in Jan/04, was flushed again (approx. 7 mos. later) when the original monovalve housing was replaced. The check valve within the housing seems to be very sensitve to any type of contamination. The monovalve assy was cleaned and replaced and the coolant system flushed again until positively clean. No reoccurance with uncommanded heat for 3 mos. now. Hopefully, problem is gone for good. Also hope my experience helps you.

Brgds,

BENZSTER