Monovalve?
 

Wife complains that her 1990 300SL is acting funny with regards to heat.

The heat is fine, but when she slows down to idle, it starts to blow cold. Fan speed or air volume doesn't change significantly -- just the temperature of the air it is blowing gets colder and colder. Obviously the system still thinks it's blowing hot air!

Haven't driven it yet, but have done the following:
-Checked coolant level (Level is fine. Car has never lost or burned up coolant)
-Checked fuse #7 "Climate control/circulating water pump/etc" (Fine. Replaced just to be safe)

Any ideas on what to check next? Can it be the monovalve? Everything I have read says the torn monovalve problem is the OPPOSITE of this (blows hot all the time). Can it be stuck?

Not sure if this helps, but the heat wasn't used all summer and it only started to get cold recently. Got cold a month ago, but it wasn't freezing so the problem probably existed but wasn't noticed.

Heat and everything worked perfectly last winter and nothing has changed since.

not a pro on the gasser engines but if the cooling system has an auxiliary pump, to pump coolant when the engine is idling, it might be failing.

I believe The Don...
 

...is probably correct. That engine has an aux water pump that keeps circulation up at idle.

Elsewhere in posts here, I think I recall some horror stories about the aux water pump frying the CCU when it goes bad.

Regards,
Kevin

I had same problem last winter and it was a monovalve. The pump is a usual suspect, but it was working fine. You can feel if the pump motor spinning or not. The monovalve was sticking in closed position and the water pressure at idle was not enough to overcome the friction. At higher RPM the valve would work OK. The new one fixed the problem. You could also try to take it apart and clean armature and inside of the coil. But first check you aux. pump.
Mike