Okay Guys Heres the Story-

Since I've had my '84 300SD the central locking hasnt worked. When I did some work on it, I re-attached the OEM alarm horn that sits in the inner fender well on the right side (drivers perspective) and the alarm actually works amazingly even though I do not have the alarm key that has the little red dot on it. But the central locking is a no go. I have ruled out a vacuum issue because I hooked up a 12v power supply to the pump in the trunk and it sucked all the locks down and pushed them all up again including the trunk and the gas door- all of them except the driver which is normal. Presently, I have the driver door panel pulled off and I have checked the continuity between the No.13 fuse and the three pin plug on the inside of the door on the Red wire with a black stripe and it was a no go. My thought is that the alarm system has affected the central locking because I do not hear the pump even trying to actuate when I turn the key at the door. Does anyone know if the wires for the driver door switch make a direct line to the fuse box or does it go to the alarm on the OEM alarm system. I know it cant be the fuse becuase the hazard lights and some other things are on that same circuit and they are working-
In the mean time I have ordered an alarm key that should get here wednesday so I can rule the alarm system out!
Thanks guys - Pat

I had the same problem on my 87 300D. I remember thinking it was the alarm also! Seems theres a wire connection in the passenger side rear floorboard where the wiring harness corrodes because water gets back there inevitably. What I mean is pull up the carpet on the rear passenger floorboard and look for the wiring harness that runs back to the vacuum pump. Theres a metal connector that corrodes.

This drove me nuts for a year until someone told me the fix on the Forum. i went right outside and saw it and I was elated because a 1/2 hour later I had it fixed! It all clicked because once I drove through a rainstorm and suddenly my vacuum locks worked for several days thereafter.

A Mercedes is not a Mercedes until yoy have the vacuum locks running!~~~~~~~Please tell me if this helps!