Defroster Control ... HOT air only

[Washbush]

Gents ...

I recently purchased my first MB, a 1989 300e. The heating/ventilation/air conditioning controls seem to be working normally, with one exception ... with the temperature selector wheel set at full cold, pushing the defrost button produces only HOT air up on the windscreen and from the left and right dash vents. The temperature selector wheel, although set on full cold, does not seem to have any effect on the temperature of the air coming out if I adjust it during operation. In fact, the output air seems to be full HOT. Any suggestions on some trouble shooting steps I can take myself?

Thanks very much ...

John Washbush
Heltonville, Indiana