1991 300d with a 2.5.

Does this pump run all the time? I am going to test and see if it is working. is it a 12 volt continuous? How is the best way to test? Can I clamp off the hose to try and diagnosis the a/c problem? On my 91 is it right to the left of the brake booster and hose goes through firewall on the drivers side?

Heres is the problem. I think it is 2 fold. Center vents do not work. No cold air, no hot air. Vent problem is probably pods or vaccume or actuator etc...

But I am trying to tackle the no cooling issue first. It is South Mississippi BTW..... Low pressure, 40, High 180 comppress off is 90 each side.
Compressor runs continuously. All CCU buttons appear to work as far as on/off . Wheel make no change in temp either hot or cold. Maybe reostat problem or thermostat?

There should be a way to get check the system by issolating each element and a time and what to start with the lack of cool temps out the vent. The one thing I can think of is stopping the hot water going through the heater core. But I doubt this is the trouble as when the engine is cold, I still get no A/C. How about the reostat in the CCU I will pull it and test the ohms. Does the compressor run continuously and the temp is controled by the aux water pump/valve? Where is the thermostat in the system? Thanks for all the help.

Hot in Mississippi.