<<In any event, my low-speed fans DO NOT come on every time the AC is on but do cycle on and off as refrigerant pressure rises.
Which is the norm.
The high side pressure switch has a cut-in point that has to be met in order to turn the low fan on. It is a safety switch.
The design of the cut-in point spec. is to let the a/c do it's job
until there is a demand to keep excessive high side pressure down.
[ air across the condenser does this]. This happens when the ambient temps and humidity are high enough to tax the system.
So, it is quite possible to have normal a/c and NO fan.
An example would be a/c operation on a cool day.
However, if you do have taxing conditions [ hot/humid, etc] and the fan does not run, before condemning the sw ,
[ assuming the fan runs with the jumper test], it is very possible to be JUST short on freon. This shortage will limit the compressors ability to ever reach the pressure switch's cut-in point.
So it is important to check freon level [ either with the high-side eye-sight or better yet, gauges].
But , you say "my a/c seems to work, so I must have freon".
True, but not enought to trip the switch, but still enough to cool on a NORMAL day.
The engine coolant fan [ high speed] kind of works the same way, but is eng. temp sensored. Again, it is a safty device and only comes on when the cooling system is TAXED beyound normal load. [ hill climb, towing, etc.]
I think a lot of the confusion on the fans comes from the fact that the a/c is actually a physical load on the engine and it's operation in itself will cause the engine coolant temp to rise enough to trip the coolant high speed fan. [ around 105-107C].
A fairly good DIY a/c high side pressure test is to let the car idle on a fairly warm day with a/c on and watch for low fan to cut in. This may take a few minutes. If it does not , take a peek at the eye-sight on top of the drier. Lots of bubbles/foam ???= low freon
That's the basics.....