this may get you started
Bob. On my '85 300D I have gone through some of the actuators with the mighty vac. The following actuators work: center vent, defrost main and recirc. I have not tried the foot area actuator but need to. I ended up replacing the recirc actuator. Funny thing about the recirc actuator - after replacement it works but if you put the tester on it it "bleeds" off the negative pressure. It is brand new so I guess it is designed like that. Keep me posted on your troubleshooting and I will do the same. I did find the linkage disconnected on the center vents. Also, get a copy of the "Electrical Troubleshooting Manual" from the MB 800 number for this car. It has all the schematics for the vacuum system too.