I'm not intimately familiar with the SL's, but I'm quite confident that there is a coolant flow valve of some sort in the climate control system. I would be willing to bet that your problem is related to it. Either an incorrect signal, not likely, or a malfunctioning valve.

Hope this helps.

__________________
2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution