Sorry, don't have 116 specific info, but on a 126 (81 300SD)...

There are two pods connecting to the main air flap. When the ACC is off, vacuum is routed to the pods to completely close the main air flap. When the AC is on but temp wheel is not min temp (not max cooling) there is no vacuum routed to the pods for 100% fresh air, and 20% fresh air when the temp is set for min temp. With access to the pods you can connect your MityVac to the pods to see if they operate with vacuum.

But this probably isn't your problem. The flap should only determine if outside air or inside air is pushed through the system. The rest of the flaps route the airflow to the specific vents. When in defrost, all the vacuum pods have no vacuum to them. It's a default mode. And I'm assuming your fan is working, yes?

Do you have defrost when the heat is on? if so, then you need to check the operation of your switchover valves. A strong vacuum leak might cause this symptom (may cause problems shutting off motor). Check to see that there is voltage to operate the switchover valves.

My recommendation: Check the main vacuum line to the switchover valves when everything is off. It should hold vacuum. Then check the electrical operation of all the switchover valves, and that the pods they operate all move and hold vacuum. If they all work and your vacuum lines are sealed properly, then you need to check the operation of the control unit.

Do you have a Climate Control Manual for your car? I can help you with 126 diagrams which might be close - otherwise you'll have to depend on the kindness of others.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)