You'll need the manual to find your way around the bowels of the dash. I don't know where to find the pertinent sections in the 123 CD manual but there should be a whole category dedicated to climate control.

To check the actuators, look for the ACC vacuum switching valve bank and test that each output circuit holds a vacuum. Note that one or more circuits might be tied in together so you'll have to test downstream of the switching valve bank. The manual will give you the spec but essentially you're looking for each actuator to hold at least 1/3 of a MightyVac gauge sweep for a few seconds. Maybe up to a minute or 30 seconds? I don't quite remember.

To check the cabin temp sensor, make sure that the hose to the aspirator motor is in place and intact (I don't know the layout of a 123), and check that the voltage or resistance is within spec per the manual for the ambient cabin temperature.

Sixto
91 300SE
87 300SDL
83 300SD