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Hi, everyone. I normally don't worry much about the heater on my 1990 420SEL, because it's usually warm here in Dallas. But the last couple of weeks have been cold and a problem has come up. When I start the car cold, the heater will not come on, but after the engine warms up a bit, the fan starts blowing cold air. If I stop, put the transmission in park, and shut the car off and restart it, it works fine. Sticky monovalve? Sticking diverters? Or something with the Climate Control Unit? Anyone ever experience this?

Regards to All,
Dave
 

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Yeah, I've got sort of same behaviour on my 500SE. It blows warm, but not hot air until I restart it (blows very hot air when engine is off and key in 2nd position). I'm at loss to explain it - sticking monovalves would cause the air to always be warm (they always fail in open position). Sticking diverters would just cause the hot air to end somewhere else....

But I think you might have an electrical problem - start the car cold, then open the fuse box and press the blower motor fuse cap (I think it's number 15 or 16). If you can see it spark that means there's a cold connection somewhere, either the lower part under the fuse is corroded or the fuse is old. Replace ;-)
 

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It is correct for the heater not to work when the engine is stone cold. That is the cold lockout. You can override that by pushing defrost, then the system will operate immediately, you won't get any heat yet since the engine is still cold. As for the stop and start thing I would suspect either low coolant or a frozen aux water pump.
 

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It is correct for the heater not to work when the engine is stone cold. That is the cold lockout. You can override that by pushing defrost, then the system will operate immediately, you won't get any heat yet since the engine is still cold. As for the stop and start thing I would suspect either low coolant or a frozen aux water pump.
Yeah, what he said. In addition, this "no fan on cold start behavior" only works with the "AUTO" fan speed button pressed.
 

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I'm experiencing the same problems with my 85 380SE. As an aside, MB is forcing me to do what I've avoided for years, ie: dealing with temperatures in Celcius! I want to be reading gauges in psi, not bar, F, not C! arrrgh!
 
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