Heres the deal. I have a 1979 Mercedes 300SD. When I lock the door, Only one goes down. I know there all supposed to go down at the same time. I also know that this is a vacuum issue. How do I go about diagnosing the problem? Please help.

Maybe someone has a good tutorial on this that they can share. There are several treads on this topic...some better n others. Here is one result using the search tool
1991 190 E door locks anomaly


I'll holler if I run across something very descriptive... please do the same!

Good luck

You'll need about 3-5 golf tees, a vacuum gauge/mityvac. One set of lines is for up, one for down (laymans terms, it's a little more complicated but that's the gist of it). need to work from front to back, drivers side first, then to passenger side, then rear passenger, fuel door and then trunk. Pull up floormats and carpet to find the vacuum lines and plug in. if vacuum drops you need to follow that line to when it's leaking and when it's not. Just need to be methodical and shouldn't take more than 30 minutes.

That's the non graphic version. The service manual has the graphic version. I only have 123 series manual so best to find your chassis version from someone here, but in the alternate, PM with e-mail address and I'll send diagram.

__________________
1982 300TD 210K miles ("The Replacement" aka "The Anvil") - SOLD
1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
1995 Ducati 916 (SOLD, sniff)
1999 Ducati 900SSie (SOLD)

... and I'll send you a diagram I made after repairing multiple problems on my Son's 1980 300D. I suspect they have essentially the same vacuum plumbing!

Last chance for a FREE "COLOR" diagram of the door lock system on 1980 W123 models. Get your's while they are HOT! The diagram also has a couple of hints about common problems and repair techniques.

Sam