Hey King,

I've got a '73 220D, W115 - same system most likely.

The in-dash fan is used for heat, de-frost, and for moving fresh air into the car. To change where this air goes you use the two blue slides on the control panel. You can send air to the floor, up the wind shield, and/or out the dash vents. Using the slides in the control panel in combo with the vent slides you can come up with various ratios of heated vs. fresh air coming into the car through the various vents.

The fan in the console is for AC only. If your system is working properly it will come on when you turn the AC on, and the in-dash fan will shut down. Vacuum is used to move flaps in the system to "connect" the various vents to the AC system (while disconnecting them from the fresh air intake). And like in "fresh air" mode, you can use the control panel slides to direct the cooled air through the various vents (defogging the windshield vs. blowing at you). The air for cooling is drawn into the system from inside the car, through the sides of the console, I think.

You can allow fresh air in at any time by opening the central vent on the dash, with the top-most slider.

I'm not sure if you can run AC and heat together, since fan speed between the two motors is controlled by a single switch.

Hope this helps!

I lack an owners' manual, and this is what I've figured out so far in my car. I have not been able to experiment with the AC since mine does not work, yet! ;-)

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- Greg -
1973 220D, The Prodigal Benz
1974 240D