On my 87 300D, my central locking system is mostly nonfunctional.
Oddly enough, it has come to life once or twice for several days in the half year I have owned this car. Once I locked my trunk and to my astonishment the system worked.
I have tested the vacuum pump iaw with info I got from other threads.. In fact, I have two now. I know they both work correctly. I have some understanding of how the sytem is supposed to work; i.e. the vaccum pump kicks on when it gets the right signal and kicks off when it gets the right signal.
The reason I would like to know this is because I have my driver side door panel off to fix the door lock which was vandalized and i am about to put it back on, and I am thinking of replacing the door actuator. After all, that in my mind would be the key link to the system. Can I jumper it to test the system?
This is (hopefully) the last time I take this door panel off.
I cant think of anywhere else to look. If I dont fix it this time, it will have to wait until the spring...

I would apply vacuum to the actuator using a Mityvac to see if it works. If it is OK, would check all other circuits to find the leak.

The actuator has a switch that generates a signal, even if it responds Ok to vacuum , the switch might be at fault. Also the pump can be at fault sometime. to reset the pump, disconnect the fuse (I think C) and reconnect.
Change the actuator since you are there (if it is original) ~$40. also check the door strap (door check) $15. Do not forget to install the vapor barrier (plastic sheet) it keeps moiture out of the door cavity.
If you fixing the locks, each has another switch for activating/deactivating security system (the S. system receives a siganl from the doors actuators - or from the pump confirming that they are closed)
I hope this is will help

toknow