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Hi guy's I have a 00' ML320 and the heater/windshield defroster doesn't want to work. All fuses -the relay (MB already replaced) with new one, engine water is hot, blower motor works fine. Regardless of climate settings, blower settings, air coming out of vents is either ambient or super cold. Is there an interlock that can be preventing this from activating? I know some other MB models have a ac/heat control valve in the engine compartment. Is this possibly the same problem? Can't afford to leave my only vehicle at MB for a day or two to have them inspect it. I suspect my vehicle has this valve and it is stuck. Does anyone know the location of this control valve? Thanks for any help.
 

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Yes, W163 forum must have better knowledge, perhaps someone with experience about the same issue.

If I got it right, the ML isn't using water temp regulation but air mix control to reach the correct heating temperature. There is a circulation pump on the heater water line but I did not see any water flow control valve. You could check that the return water pipe from the heater core is hot when the engine is hot (and the blower on). If it is, then it sounds like one of the air flow flaps isn't moving to allow hot air flow from the heater core.
 
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