Hi. My problem seems to be that the electric auxiliary fan (in front of the radiator) never turns on.

First, I'd like to know if the fan is supposed to turn on whenever the A/C compressor is working, or if it only turns on under particular (dire) conditions.

Here's what I can tell you so far:
My A/C system seems to cool (almost normally) -- subjectively, it might not be cooling as much as it should, but I can't measure it objectively.
Certainly, the compressor is cycling on and off as it should, and the belts seem tight.
[and unfortunately, there is no more "sight glass" as there used to be in older models to ascertain refridgerant supply by 'looking for bubbles', so I don't know if there's enough refridgerant.]

Critical Point:
I've also checked the auxiliary fan's electrical supply and all seems well, up to a point (more on that, next) -- meaning that if I take the plug off the pressure sensor (the one directly beneath the lower radiator hose that is on a "T" off the refridgerant line) and jumper the freed plug's contacts with a paper clip, the fan comes on with a roar.

But it seems that the pressure sensor is never closing the circuit (as I did with the paper clip). So I have to wonder why. I cleaned the contacts with an electrical contact cleaner (in fact, two different products) and they appear shiny.

So I'm left wondering -- when, or under what circumstances, is that pressure sensor supposed to close the circuit (which it isn't doing)? Is it whenever the A/C is working, or just sometimes under particularly hard-to-operate conditions (extraordinary hot temperatures)?

And what are the reasons that the pressure sensor might not close the circuit? Only that the sensor is faulty, or also if there isn't enough refridgerant in the system (to generate enough pressure)? And if the refridgerant content is low, is there any way an amateur (someone without refridgerant knowledge or equipment) can confirm this, such as by feeling the refridgerant lines' temperatures (a trick I used to do with older cars)?

Thanks, all.