I have an 85 XJS HE. The aircon relay is always pulled in, irrespective if evaporator fan has been turned to off ( the off position turns the blower fan off and seems to work correctly. Clutch relays works fine). A switch has been fitted between the aircon clutch and the relay that turns the aircon clutch off.
1 Please confirm the blower cabin rotary switch should turn aircon clutch relay off. 2 Would there have been any reason why the car has been modified or why the rotary switch would have been discounted from clutch relay?

 

Basically, the a/c compressor runs whenever the RH knob is in any position other than OFF. Thats the factory design.

The evaporator fans are the cabin fans. The Condensor fan is the one on the left in the engine bay, and on some cars is wired to come on whenever the compressor is energized. Others have a strange set up, triggered by the Temp Sensor at the water pump bottom hose Inlet. Most of us have deleted that, and switched that fan the way we want it, thermo, or compressor.

Compressor by pas, yes, most do it so the system can be run as a "normal" car, without the a/c running, just outside air. I did all mine inside the cabin, using one of the Yellow wires at the evaporator thermostat.

Faults, oh boy.
Behind the fascia panel, after removing the radio and 2 system knobs, and that fragile fascia, you will see 2 small hex heads. These are long bolts that hold the micro switch pack in place. They come loose and those switches move around and do NOT do as designed. Place that unit in the OFF position, and reach through the aperture, and "feel" for the other end and the nuts, then CAREFULLY tighten the 2 bolts. Dont go silly, just firm them up. This has solved 90% of system issues for me over too many years.
After that its one item at a time. Putting it back as designed is always the best, so you are not chasing Freddy Fiddler issues.

Delanair booklet attached.