Guys: yes, the CCU puts a ground on pin 10 through the low pressure switch. With one side of the relay coil on Klima pin 5 for power and the other side on Klima pin 10, the relay will operate when the CCU is turned on and outputs the ground to pin 10. And if freon pressure is low, the low pressure switch will break this ground connection and shut off the compressor.
I've read some Mercedes material that says the compressor runs all the time when the CCU it is set for heat OR A/C and that the temperature of the air to the interior of the car is adjusted automatically by adjusting the amount of heat to mix with the air flow. This would explain my compressor being off when the "0" button is pressed but is on with every other setting. I didn't observe the behavior of the compressor with the original Klima installed to confirm this to be correct. I will. Anyone else observe this?
I am now researching the diesel kickdown behavior that turns the compressor off under hard acceleration that MXFrank mentions.

The compressor should be off when in Economy (EC) mode. Every other function it should be running unless the coil gets cold enough to operate the evaporator temp sensor at which point the compressor starts cycling. The air should never be colder than ~42˚ if the system is working properly.

The microswitch on the accelerator linkage will also drop the compressor out at WOT

The overheat sensor in the head will drop the compressor out when it operates. The trigger point varies on year and model.

The Klima also checks the speed sensor on the compressor and will drop the clutch out if it senses that the compressor has stalled.

The Klima as designed also adds a delay when the engine is started so that the compressor does not engage until 600 RPM is reached.

So the Klima actually does do some important things, but most of the functions can be replaced with simple relays.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
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Thanks, man! Also, I'd like to direct this discussion back over to the one you and Mxfrank were having about this topic. I've attached my version of your diagram of the dual relay set up to add some protective diodes. Let's go back there, okay?

Go here: http://www.peachparts.com/shopforum/diesel-discussion/386849-compressor-speed-sensor-2.html#post3735539