The aux fans come on when the a/c high side presssure starts to climb up there...so, with no additional airflow to combat that extra thermal heat burden, the radiator gets passively heated from the a/c condenser and the engine o-heats..usually at low rpms or traffic.
Here is the switch that activates that circuit and there is a simple test I use to verify that circuit in the event that you do in fact find that you have no low fans w/ac ..
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You will notice that it is referred to as a temp sw , but in reality it is pressure sensor type sw and monitors the high side refrig pressure .
So, the test:
If you have no low fan w/ac On and a good hot ambient temp condition, you peel back the insulator sleeves on the pigtail end of that sw [ it is located at the reciever/drier ]. and jumper there with anything ...a screwdriver blade is fine... low fan/s should come ON ..if yes , the circuit is verified ...if NO, you have a fuse or relay problem ..or even a bad fan motor..
Now for the tricky part...
There is a time when the fan circuit passes the jumper test , but it still does not come on with the a/c on...the reason for this is usually b/c the a/c system is slightly low on refrigerant..not enought to trigger the fan,but still enough to give you a/c [ altough not max] ..so you wind up with no fan , a moderate a/c output, and an O-heated car....tricky diagnosis that fools many a seasoned guy. Just a few ounches light of refrigerant will bring on this condition, so something to watch for if your fans are not comng ON , but they do pass the jumper test..
Of course, you could also have a bad sw...but the refrig level is always the first suspect b/c they seep over time ... no matter what.
Check the fans.........