I finally pin-pointed my problem cause.
I had little air from front vents and my defrost vents were always staying open sending most of the air up to the windshield. My visual check of opening and closing of flaps in the center vents suggested that it worked fine and I had to look for problem in defrost flap system. I want to attach word document with 2 pictures in it to show what I did. But I dont know how to since the size of the document is about 380kb. I found the defrost change over valve to be faulty.
Hope this helps. Its a long document but I wanted to give all details. Post if you have any comments.

Following description is for my MB 1985 300D Turbo. These tests were done while engine was running.
Please be careful not to hit the shift by accident. Apply parking break, put big stones on 1ll 4 tires. I am just a bit over conscious about safety)
Vacuum change-over valve. There are five of these valves behind the ACC control panel. Removing the ACC control then removing the optional switch panel (panel that has antenna, sunroof, rear light and defrost button on it) gives more room to test or change this valves. I also removed the lighter and then Radio for a much better room. Facing the panel the left most valve (labeled in mine as #1) is change over valve for controlling the defrost vacuum element which in turn controls the defrost flaps. This defrost vacuum element is located under the left (driver side) kick panel. After removing the kick panel you can see it right to the steering column. In mine it is pink in color. There is another vacuum element which too is pink in color on the floor under the defrost vacuum element and towards the wall -this one is "Vacuum element for leg-room flaps".
Under economy mode temp dial set on minimum (engaged), the defrost vacuum element is under vacuum which closes the defrost flap. Therefore
1. Any failure of this vacuum element or
2. Leaking line providing vacuum to this element (at the bottom vacuum connection of the element) or
3. Failure of defrost change-over valve
will cause the defrost flaps to stay open. This results in most of the air coming out of the top defrost and little or no air from front vents when dial is engaged in MIN. and economy push button (this one is the one left to stop button) is pressed.

I had some air from front vents and my front vent flaps were working ok so I decided to start the problem solving with the defrost flap.

I have heard that its mostly the problem with defrost change-over valve. There are simple tests in MB service Disk for testing these valves. A very simple though not 100% accurate test is to feel the switch-over valve bottom with one finger while pressing various push buttons on the control unit. If you feel clicks and clacks on this valve then it is probably ok. But I do not suggest this test. DO the simple MB test for 100% accuracy.

I am also told that if the car was not exposed to extreme weather conditions, the vacuum element will not fail (told to me by a senior MB mechanic). He says it's almost always the change-over valve because they are electrical valves and with years of constant work tends to fail.

To check the defrost vacuum element, remove the bottom connection to this element and apply vacuum by a hand vacuum pump it should close the defrost flaps with enough vacuum and should hold the vacuum.( I attached a rubber rube on it and sucked on it and created enough vacuum to close the defrost flaps, slide my tongue over to close the end and wait for a 15-20 sec to see if it lost any vacuum and it did not telling me the vacuum element is good.)

The end that was detached from the bottom of the pink defrost element should create a suction when dial is engaged in MIN. and economy push button (this one is the one left to stop button) is pressed. I had no vacuum to this line telling me either my change-over valve is no good or connection between that valve and the defrost vacuum element is no good. In my case it was the defrost change-over valve toasted. Lines were all very good condition.

Note: The engine may not shut off if one or more vacuum change-over valve is disconnected. So if you need to shut off the engine. Put them back temporarily and turn off the engine. Then remove the faulty one and replace. I almost lost my mind when I could not turn mine off.

MB 1985 300D Turbo. Locations of Five change over valves. Behind the ACC control unit.
Radio, Lighter and optional switches removed for better access. Each of the five valve is attached to a metal plate by one screw. Also some models the electric connector are labeled - yellow tags- 1 thru 5. In this picture I have an extra ling rubber tube attached for testing purpose( one that’s hanging ouside) and also the valve # 1 which is defrost change-over valve is removed.

My image files are posted by another members in following posts


MVK

1985 300D turbo

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One penny saved is three penny earned.

1985 300D Turbo 179,000miles