The following applies to the 1981-1985 300D, 300CD and 300TD with automatic climate control:
The vacuum line that leads to the "Center Flap" actuator also draws vacuum from the "Defroster Flap" actuator on the SHORT RUN connection (in other words, the vacuum line has a 3 way connector near the Center Flap actuator which feeds another vacuum line that leads to the Defrost Flap actuator Short Run). The Defroster Flap actuator is known to fail early (the replacement actuator is an improved model). So....before going through all the effort to replace the Center Flap actuator, it needs to be tested. This can be performed by either blocking off the vacuum line to the Defrost Actuator Short Run connection where the 3 way connector is located (next to the Center Flap actuator) OR disconnecting the vacuum line at the Defrost Actuator SHORT RUN which is located on the left side of the black heater exchange box (behind dash). This requires removing the vertical carpet panel on the left side of the console for access and lower panel which the steering column passes through. I think it would be easiest to block it at the Center Flap actuator 3 way connector so only vacuum is drawn from the Center Flap actuator. You could then block the line to the center flap actuator and see if the Defroster Flap actuator SHORT RUN is working.
In case you are not aware....the Defrost Flap actuator has "two" vacuum connections were are independent of each other (this is the only climate control actuator with this configuration). When no vacuum is applied to either connection, air blows fully out of the defroster outlets. The LONG RUN pulls the Defroster Flap completely shut, while the SHORT RUN pulls it partially shut. When the climate control system is cooling the car (with the middle button engaged...not "EC" or the two towards the left) and the desired temperature (set on the temperature wheel) is more than 3 degrees than cooler than the actual interior temperature (or the temp wheel is set at "MIN"), the system is trying to cool MAXIMUM and blow all air through the dash outlets only. The vacuum switch to the Defroster Flap actuator LONG RUN is engaged, which pulls the defroster flaps completely shut. Vacuum is also applied to the Center Flap actuator and the Defroster Flap actuator SHORT RUN (since they are on the same line)....except the long run overrides the short run on the Defroster actuator. As the interior cools down and gets within 3 degrees of the desired temperature set on the "temperature wheel", the vacuum switch to the Defroster actuator LONG RUN shuts off. Now only the SHORT RUN works, which allows a small amount of air to release from the defroster ducts, along with the center ducts. At this point, the system also allows fresh air (versus recirculated air) to take over.
I hope this somewhat makes sense, as I am writing all of this from memory. I am at work and don't have my manual with me.
