Easy to test when out of car. See Kent's manual to learn how to test without removing door panel.
Doors won't lock or unlock. It could be a failed door lock actuator. Check out Kent's YouTube video. It will explain how they operate and how to test them with a vacuum hand pump tester. This tester is essential if you are going to attempt to fix your door locks on your own. You can save hundreds $$$ by doing it yourself.
Sometimes it is just a leaking actuator. Other times it can be a combination of problems that can drive you crazy. See Kent's manual on how to troubleshoot and repair the system on a 1977 to 1985 240D, 300D, 300TD, 300CD, 280E, 280TE and 230E
Problem & Solution
Common among these chassis:
W123