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This KEY wording here is : "..on a HOT day".

Aux fan is just that....an aux added fan to AID the system with increased air-flow when the sensors detect a "Higher than normal Thermal Load".
A/c is used in winters to aid in clearing windshield of humidity, but you will never see a low fan when it is doing so... b/c you have NO High Thermal Load.
Car travelling down the road at 45 MPH, a/c On , no aux low fan.. b/c it is not needed... NO high thermal load b/c travelling vehicle has plenty of airflow to keep condenser pressure in check. But come to a stop light or slow traffic and On comes fan..b/c of rise in Thermal load and Sensor Demand for more air-flow..
So, the word HOT here is the trick, coupled with low airflow..
Do not expect to see your low fan activated on a cool day just b/c your a/c is ON..it only comes on with Thermal Load Demand .. it is ONLY an aux airflow aid device.. mostly activated at idle, car at rest, hot conditions...and it is actually possible in cooler climates to never come on .

So, how can you actually determine if the low speed fans are doing it's job? With the car stoped and a/c on, you can see the fans runnig but when driving how can you tell if they did turned on at the right moment? I can actually hear when the hi speed fans turn on when temperature is like 110 or something but, can you hear the low speed fans when driving?

And the other question is, how can you test the resistor?

Thanks for this great info.

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Juan
1995 E320 Sedan
2008 Ford Escape
RC Helicopter

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The correct Moment is if the high side pressure switch is calling for the aid of aux fan..if there is a very high thermal load , they come on, if not, they don't..so, if they come on at idle w/ac on on a hot day, they are fine.
When the car is moving, they may come on or may not come on..that is dependent on demand of high side pressure, which is determined by the intensity of thermal load. If they are called for to check pressure and they do not come on ,then the high side will get to a dangerous pressure and the compressor cut-out pressure switch will deactivate the compressor and you will have no a/c.
All fan activation is on pressure demand..the pressure sw does not know if the car is moving, what the temp are , or any other variable..it only knows that SOMETHING is making the high side pressure get out of spec, so it comes on to bring that pressure back to efficient working condition. [ that is what the airflow does..it drops the condensers pressure.]

The R15 resstor can be checked with an ohm meter or a test lamp on each side of the circuit..if you have low fan, you do not have a bad resistor..high fan does not use the R15.