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Discussion Starter #1
my '85 300SD when you turn the heat on for the first 20 min.s blows hot ,hot. no matter where you set the dial. then it works like it is supposed to. why does it take so long for it to regulate and is there a fix for this without costing an arm and a leg?
 

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Could be a few things. Is the automatic fan setting working? Fan motor in automatic mode should change it's speed to compensate for the change of temperature. Also if your temperature sensor is not working this could be a cause of the problem. Do the different buttons give you the different functions they should? First hit sounds like the ACC unit has a problem though you want to make sure everything else is working first before you go replacing it. The temperature dial turns on the heater in white zone, in the blue zone?
 

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Discussion Starter #3
yes the auto fan works, the AC is toast as of this past summer, but even when the AC worked i had this problem. with the setting on EC i should be able to regulate the heat with the dial. i can have the dial set as low as possible with out turning it off and the fan on low and it still blows super hot air for about 20 min.s then it gets cold and you have to move the dial up to around 22 and it works fine. sure i have heat, but it gets annoying when driving the short distance that i do to work every day. what about the monovalve? can it cause this?
 

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What about if the temp dial is turned down to 'coldest' setting - so it clicks. Still hot for ages?
Could be monovalve, they can create odd behaviour. Or your sensor by the dome light. Is the sampling fan working? You can sometimes hear it running with the ignition on an engine off. You can hold thin tissue over the grille by the domelight to see if you can tell if there is a gentle suction going on.
 
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