Nick -
The Volvo Wiring Diagram Service manual for 1987-88 240 shows what you describe.
There is a microswitch in the circuit that is attached to the rotary switch. Some of the connections are to it, not the rotary switch.

The microswitch connects BL, power from fuse #12, to R, which goes to the a/c relay terminal #61/D+ which, on the relay base, is a double red wire terminal. The other red from that relay base goes to the ECU for idle speed adjustment. The relay is what gives the 10 second delay in a/c engagement after the alternator begins charging.

The two rotary switch terminals have a single green on one, a double green on the other. One of the double green wires goes to the engine bay and thence the pressure sensor on the receiver dryer and thence the compressor clutch. The other of the double green is not shown, but could end up at the heater fan switch terminal #5 via a connector in which it changes to R-W.

The capillary tube has to do with controlling the size of the orifice in the expansion valve. I think it is filled with Freon, and once severed will allow the system to freeze up and damage itself. The 1978 system shows a rocker switch on the panel and a thermostat mounted separately inside the panel somewhere. Same function - keeping the evaporator from freezing over.

What I know about the later models is that I don't know them at all. Some use a variable orifice tube in place of the expansion valve, and there may be some differences in controls.

Good Luck,

Bob

:>)