ML320 Automatic Locks don't work

[barryd]

I checked the fuse and interchanged the working relays for which the parts numbers match. Door locks still do not work. The liftgate opens and closes just fine. However, the 4 doors and the gas flap neither open nor close. I can feel the correct relays open and close when I push the corresponding button on the key.

Still looking for ideas. Since none of the 4 doors work, it can't be individual actuators (unless they all 4 burned out simultaneously). Are the connections to the doors electrical or vacuum? Where is a central point to start the search for the fault?