My 94 E320's central locking system does not work correctly. I pulled the vacuum pump out and tested it on the bench applying 12V/GND to the power connectors and sent a 12V or GND signal to the 3 pins and can confirm the pump is actually working.

Measuring the power input to the vacuum pump from the car gives me correct 12V, but measuring the signals when I turn the key only shows me 8V from driver's door, passenger door, and trunk lock.

Should I take a look at swapping out the convenience control unit from behind the driver's seat or is there something else I need to look at?