I'm having a problem with the central locking on my 78 450sl. If I use the key to unlock, both doors unlock. Whne I'm in the car, pressing the drivers lock button will lock the drivers door. But... When I lock the car with the key (on the drivers side), only the drivers door locks. I have to walk around the car and lock the passenger door. Anyone have an idea on this?
Much appreciated.

There is a separate chamber in the vacuum actuator and a separate line to open and close each door. It is possible that there is a leak in the line that locks the door. It is probably yellow with a red stripe. (Yellow with a green stripe unlocks the door).

Sometimes a line will break or slip off the connector or the connecting tube will leak where the line goes into the door. But most likely to fix this you will have to take the panel off the door.

The lines are made of a hard, flexible plastic and they are connected with black rubber connectors. Most problems, they say, are due to a break in the actuator. The next most likely problem is a connector leaking. The hard plastic lines occasionally break.

I have heard that the actuators can be repaired by sliding a couple of condums over the torn bellows and tightening them on with cable ties.

The hard part is asking for especially narrow, yet extra strong, condums.

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Thanks for the reply. I thought of that at first but manually locking the drivers door from inside the car will lock the passenger door. If there was a leak or break in the lock line, that wouldn't be able to happen - or am I missing something?

Most likely the actuator (vacuum element) on the passenger door is gone south.