Check the archives for a few articles I wrote on that exact scenerio..
No low fan is one of the most common, mis-diagnosed , causes of high coolant temp complaints.
Once the high thermal load caused by a/c passive heat exchange from condenser to radiator starts to creep on you , it's over...
I tell guys their problem with high coolant temps w/ac is caused by a slightly low refrigerent level and and they think I am crazy. [ until I top it off and the fan comes on at spec]
FIRST check for this coolant temp condition is LOW FAN , Problem is, the test used is they jumper the high side pressure switch and the fan comes On ..So,sure enough, they wrongly condemn the Switch.
It is almost [99%] always the refrig level, not the switch.
I have only seen 2 sw go bad ..and one was physically broken, the other had a short and ran the low fan all the time.
The best system is to run 2 wires off the drier pressure sw to a cabin sw [ on/off] that shunts the system so you can get a low fan on Demand manually. This is great also for towing. If the sw is OFF , the system just goes back to stock configuration b/c the sw is in parellel with the circuit.
The resistor bridge at the coolant temp sensor/thermistor for High fan does lower the cut-in of that sensors 105/107C value, but in doing so, it also lowers the CUT-OUT ..sometimes to the point of never turning off the fan. In other words , it has a differentail . With proper refrig/low fan operation [ and a cabin sw], you are controlling/checking thermal loads on the eng temps way before they gets to these higher temps.
And w/cabin sw , there is no cut-in/cut-out differential , so you don't have to worry about a HIGH fan on all that time [ not good for fan motor longevity or charging system]. Basically , I use the correct thermistor value as a coolant temp safety cut-in rather than trying to controll the engine temps with the High Fan. So, I keep that cut-in/cut-out on the high range, as it's main purpose is engine protection at higher than normal temps.
I also keep them at a higher cut-in so they turn back OFF after they have checked the condition ..that simple requires different R values. If one is in constant hot/Southern climates, then you may just have to use a lower cut-in of high fan for temp control, but that is another story..the large V/8 engine seem to benefit here also.
I actually use BOTH systems.
An added note to all this is that armed with this info, it is also easy for one to see that their a/c system is low on refrig simple by noticing a NO LOW Fan condition..you are in reality using the high side pressure sw as a monitor for rerig level.. [ just a casual observation]
The manual sw is not needed to control the temp if low fan is working, but it is a neat add-on for manual /over-ride control, as it can also be used without the a/c being ON [ like towing,etc].. I have always has a cabin sw on all my Benzs over the years , just in case that one time comes when I don't like the rising temp gauge staring at me in a traffic jam and I can't do a thing about it [ except throw that switch...