I know that this is a total neophyte question, too, but I've read a lot about the vacuum door (and trunk and everything else) system woes. What I'm not sure of, though, is how it is supposed to work when it works normally. Understanding this would sure help me understand the troubleshooting and repair procedures.

Sorry again for all the very newbie kinds of questions. I promise I'll settle them down.

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Cam
1982 240d

It's pretty simple, when you lock and unlock the driver's door (from inside or outside) the other three doors, the trunk, and fuel door all automatically do the same thing.

The lock system has two main circuits; lock and unlock. There is a selector valve in the driver's door which routes vacuum to one circuit and simultaneously vents the other circuit to the atmosphere. The atmospheric pressure entering the vented circuit acts against a diaphragm and moves the actuator to the selected position. A vacuum reservoir is provided to allow several lock/unlock cycles while the vacuum pump in not in operation.

the vacuum is "fed" through the yellow lines off the main vacuum tee. there is an egg crate looking vacuum pod either under the rear window, or in the front fender area or both... fun fun fun to chase down leaks.

If you are gonna chase vacuum issues ... you really need to get a Mighty Vac or similar.

On the 300D, on the left side of the drivers seat there is a connection point for the lock and unlock for all the rest of the doors, trunk and fuel door. Putting a Mighty Vac on the leads there makes trouble shooting much easier.

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

Yeah, I had hell with my vacuum locks. I had so many leaks I stopped looking. I ended up plugging off the door lock portion and installed an electric set up for all doors and trunk. I bought a kit off of Ebay ($40) with a couple of extra solinoids for gas cap and trunk. Now I can unlock the doors, trunk and gas cap with a remote key fob kind of like an alarm. I like the new system so much I'm going to install a kit in my VW Golf next.
No, it's not original, but it works mighty fine.

Just a thought in case you get tired of the vacuum set up.