Get a vacuum schematic and a vacuum pump/tester; many owners use a Mity
Vac (may not be spelled correctly).
There's a vacuum pump on the front of the engine. You can rebuild the
pump with a kit.
Vacuum is used for:
Transmission's shift modulation
Engine shut down
Your car has a significant vacuum leak or a bad vacuum pump. Start with
the pump (source) and check if it's making vacuum, then work your way
through the system.
Climate control has a "cold engine lockout" a bimetallic vacuum switch
on the very, very bottom of the climate control servo that's supposed to
keep the heater OFF until the heater core is warm enough to provide
heat. The bimetallic switch can be jumped. Blower's ON/OFF is also
controlled by a vacuum switch.
Climate control system has two major parts: its logic board "amplifier"
and its valve body, including vacuum, "servo".
If you find or are told that the climate control "servo" is bad I
suggest you consider a digital retrofit kit that's sold by
www.performanceproducts.com. I put one on my '80 300SD.
They also have a repair parts catalog for your model that includes
exploded parts diagrams and may also include a vacuum schematic.
You may find manuals at: http://www.tmcpubl.com//mercedes.html or