Hi James:
Well lets first say this My advice in previous post is only for my model or comparable. My car is MB 300D Turbo 1985- 123.
This car uses the ACC Type 3 or third version.

Your car uses ACC type 2 or second version. So things like location of vacuum elements and location of change over valve will be different.
I am not a mechanic and I have never owned anyother Benz so I have very limited experience with your model.

So it would be not a good idea to give you a wrong advice. But as a rule of thumb you start trouble shooting with simplest things first.
1. Vacuum line attachments and connections in engine compartment.
2. Visual check of flap movement in center vent. This is what you do. Engine running., car at normal temp( about 60-80C), set to the highest heat and look with a flash light into the center vent,while looking in there change the temp dial to cold. If you see the flap opening( does not matter is air flow is very little) then mostly(99.9%) the vacuum element that controls the center vent flap is good and I will not mess with it.
3. If your legroom flaps open and close as they are supposed to during the heating operations, then the vacuum elenemts for those legroom flaps ar fine as well (99.9%).

If # 2 and # 3 above are passing the test then

4.Start trouble shooting defrost vent. Defrost vent in my car stays open if you do not apply vacuum to it. So if that vacuum element was bad due to internal leak, then it will never close and most air will go to the defrost vent on top. Second assuming that defrost vacuum element was good but still stays open, then your problem is with the vacuum not reaching to the defrost vac element. and no or insufficient vacuum to the defrost element means it cant close and stays open. so most air will go to thar defrost vent.
Why the vacuum not reaching to the defrost vac element.
a. Bad connection - leaking or detached line between the change over valve anf the defrost vac element.
b. non functional change over defrost valve. staying closed so that no vac goes to defrost vac element.

If a. in above a abd b is good, then b. is your problem.

There are simple tests for testing vac elments and change over valves. Check the benz cd. Its available from ebay for about 20-30 bucks. Its worth buying. It will make your life very very easy. It has a whole section on ACC. covers many different model years. look for 123 benz cd in ebay.

Well thats all I can say hope this helps. I am going out for 10 days.

MVK

__________________
One penny saved is three penny earned.

1985 300D Turbo 179,000miles