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  #1  
Old 10-07-2017, 02:41 PM
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1984 300CD, Blowing cold air at freeway speeds.

Once the speed gets under about 45MPH, the heat comes back hot. Over about 45MPH it blows rather cool. Any ideas why?

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  #2  
Old 10-07-2017, 07:41 PM
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Is this a problem of the A/C not working at low speed? Or the heat not working at high speed? Your description isn't very clear...

If the A/C isn't cold enough at low speed, you have a bad condenser fan and/or fan switch. Or the condenser fins are all bent closed from years of small debris hitting it.

If the heat isn't hot enough at high speed, your coolant thermostat has failed. They almost always fail in the open position, resulting in an engine that runs too cool (and no heat on the freeway).
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  #3  
Old 10-07-2017, 08:54 PM
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If it's a heating issue, it could also be one of the crappy MTC monovalve cartridges has been installed. Seems to be a very common issue with them.
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  #4  
Old 10-08-2017, 06:15 PM
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X2 - Its the Monovalve moving magnet assembly.

If you haven't replaced it its a torn diaphragm in the existing unit.

If you have replaced it with a MTC, get rid of it and get an OEM (Mercedes or Bosch) replacement.
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  #5  
Old 10-31-2017, 06:12 PM
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The heater blows cold air when it's set to the heat setting. But, it only blows cold when at freeway speeds. I just replaced the monovalve. It still blowing cold.
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  #6  
Old 10-31-2017, 06:38 PM
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Which monovalve assembly did you replace it with? If you used another aftermarket one, you'll have the same problem, sorry to say.

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