There are several interrupts along the circuit that enerizes the compressor clutch. The two easiest to check are the 105C/115C switch that cuts off the compressor when coolant temp gets to 115-120C, and the low pressure switch on the drier that cuts off the compressor if there isn't enough refrigerant pressure in the system. It's safe to temporarily bypass the 105C/115C switch when the engine's cold and probably safe to momentarily bypass the low pressure switch.
Beyond that there's a relay that compares compressor revs to engine revs. One input is compressor revs, the other is engine revs. Even if there's something wrong with either or both inputs the compressor should come on for a moment. If your tech is familiar with MBs, this is probably the device he's trying to locate.
Then there's the ACC temp control unit and push-button unit that consider inputs from you, the cabin temp sensor, the temp control wheel and the evaporator temp sensor to determine whether or not to engage the compressor.
What I'd do is disconnect the 105C/115C switch to see if that does it, then jump the low pressure switch to see if that does it, then check for an appropriate input to the AC relay to see if the problem is inside the cabin. The rest will depend on the results.
BTW, I'm not familiar with W126 V8s so some of this is guessing.