Unlike all other Mercedes, the ML doesnt have a heater valve to keep water from flowing thru the heater core!! The heater box has flaps that are supposed to keep the heat from getting to you in the fresh air & a/c modes, somewhat like domestic cars. Guess it's not working as well as designed, haven't seen anything about this problem yet.

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS M.B. TECH