W123-center vent diaphragm replacement
 

I already have the new center vent diaphragm for the replacement. Are there any tricks that would be helpful for the job? Do I have to take the dash off to get to it? If so, is there a writeup somewhere, or is it easy enough I shouldn't have to worry about it?

Here is a write up that includes removing the dash (you don't need to remove the center console):

http://www.peachparts.com/Wikka/W123Evaporator

The center pod can be switched out w/o removing the dash but it is tricky and time consuming....others can pipe in on how exactly to do it. I had to switch out my dash so I did it at the time. Good luck!

The manual says it's as simple as pulling the vacuum line, then "remove clamp and remove locks by means of a screwdriver".

It is that simple, but the only hitch is getting to the center vent accuator. Its a really tight squeeze to get to it. I would suggest doing all of the pods. My center vent pod went bad a couple years ago. I pulled the dash and replaced the diaphragms all at once so I wouldn't have to do it again. The dash is pretty easy to get out. The service manual lays it out pretty well. And it makes the whole job a lot easier.

Center vent woes...
 

Saw this thread & had to throw in my 2 cents worth as I am doing 2 pods of my own on the 300D...If you pull the glove box liner and passenger knee bolster ....you can remove the center vent rubber duct to the nozzles and then the nozzels....this will let you see the vac pod and let you decide how you want to get at it ....I find the little grey/black parkerized spring steel clips almost impossible to get loose without breaking the plastic stud prongs on the pods...Then you have more trouble.....A small dremel tool with a emery wheel disk will slice the clips once you can get the tool in there...then work them off with a screwdriver.....use lots of light and mirrors to see behind the hard plastic duct the actuator rod passes through....usually gets done with a lot of flavorful part re-naming.....
It shall become the (Pod of many names).....

I hear you can get more access by taking the cluster out as well.

The trick is to dismantle the pod while the black half remains locked in place. Once youve installed the new diaphragm, you can fit the pieces together and use your mytivac to overcome the resistance of the coil spring in the pod assembly and suck it all into place. If you can't draw a vacuum, the diaphragm isn't sealing and needs to be fitted a little better. Once it seals up, you can a) clip the black and pink halves together with ease and b) rest assured that the pod is fully functional because youve tested it in the process of putting it together.

Follow the web instructions through the glove box, it's a PITA but worth it. Doing it the second time was easy.
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While you are in there, replace the foam hose from the blower to the temp sensor. Made a world of difference for my case...

ok english please
 

Excuse my ignorance ,but will this allow center vents to blow a/c?

This allows the center vents to blow at all. More importantly, it gets rid of the vacuum leak...well, at least one of them.

Yeah, I have to do this too, the center pod in my '85 just died. I may end up wiring the center vents open all the time and block off that vacuum line.