Thanks, Bruce.

But it seems that there are some differences between my '93 240 and your 940, particularly the two-speed fan that your 940 has; your dual relay to drive it; and two pressostats.

I'm looking at my Green Book wiring diagrams, and it shows a Pressure Sensor (on that T fitting I described, perhaps equivalent to one of the two you said yours has at the bottom of the condensor), another Pressure Sensor (on the dryer), and no temperature sensor (as you described in your radiator) at all. My fan relay also shows only one output direct to the fan (one power setting, apparently).
And the flow diagram for the current indicates that the fan works when the pressure sensor (the one off the T fitting) closes.

But it also has a caption, "When Extra Cooling Required", so maybe it is supposed to be only occasional.
And it's promising to hear that your fan also doesn't work continually -- "...FWIW, the only time I've ever known my fan to run was during long idling on a hot day when testing the system...."
So I have to thank you, and answer your "FWIW" with, It was Worth a Lot! Thanks, it was reassuring to hear. Perhaps there's really nothing wrong, after all.