Car: 93 300D 2.5 turbo. I gather I have some kind of vacuum issue somewhere but need some advice on where to start. The door locks have been working intermittantly for the last couple months. All other vacuum related things seem to be fine (brakes, transmission shifting, shut off). When I turn the key in the door, I hear a humming sound but nothing happens sometimes. The humming (like a small electric motor hum) lasts for maybe 20-30 seconds then stops. The locks have kind of come and gone. Today set me off since I couldn't get the fuel door open. I found the manual release but would like to fix the bigger problem. Can anyone point me to either a thread for a similar set of symptoms or just give me some straight suggestions? I did a search but didn't see quite what I am experiencing so thought I'd just hammer out a thread. Thanks in advance for any help.

The central locking system of your car has an electric pump under the rear bench on the passenger side. This system is independent of the engine vacuum system. You might have developed a leak that over time diminished the effectiveness of the pump. I don't have a circuit diagram but you can apply vacuum and pressure to each of the lines at the pump to see which line if any isn't tight. When the system works, do you notice a lazy lock button? Likewise, put vacuum and pressure gauges on the pump and how it does. I don't have any specs but I'm sure someone has a known working pump to provide a baseline.

Sixto
87 300D

Thanks, that is good info. I just went through a near catastrophe a few months ago with the engine vac pump so I'm actually glad to hear this is a separate system if for no other reason than just thinking of the engine pump sends chills up my spine.

Any directions on exactly how to access this thing? Through the trunk or does the seat actually come out? Just wondering before I remove something that isn't meant to be removed .

There's a red tab on either end of the rear seat cushion where it meets carpet. Push the tabs outward towards the doors then lift the forward edge of the seat cushion. Pull the seat cushion forward and set it aside. The central locking pump should be visible close to the right door.

Sixto
87 300D