You did not mention the year and model of your vehicle. I have the official MB manual for the heating/ AC system for the W123 models (230, 240D, the various 300 models, 280E & CE - covering the 1977 to 1982 model years) and this info may be similar to other MB year/model.
The supplemental/auxiliary fan located in front of the AC condensor operates when the refrigerant temperature reaches 62 degrees centigrade or 52 degrees C. for the 4 & 5 cylinder engines. The sensor for the refrigerant temp is located on my '77 300D at the AC drier on the high pressure line (this line runs hot when the AC is operating - as Steve wrote about) and is probably located in tha same position on other model years since this is the obvious/best location. The supplemental/auxiliary fan will also operate when coolant temp reaches 100 degrees C.
The MB manual has various graphs showing the relationship between refrigerant pressure and temperature, the higher the refrigerant temp the higher the pressure - as Steve discussed.
Find out if you have a refrigerant leak. Since you have already added refrigerant, then I suggest monitoring the low side pressure with a gauge to see if it drops (low pressure line is the larger of the two AC lines looking at the metal tubes/flexible lines and the place where refrigerant is usually added). If you do not have an AC gauge then monitor the AC temp coming from the center vent inside the driving compartment. Insert a thermometer a short way into the center vent when the AC is on, run the engine at a speed slightly higher than idle (say 1,000 - 1,500 RPM) until the AC temp stabilizes, record the temp. In a few days or a week or two (perhaps a month if the leak is very slow) re-check the AC temp for an increase. These tests should be done when the outside temp is about the same (do not do it one time when the temp is say 65 degrees F. and then another time when it is say 90 degrees F. because you are not duplicating the same condition as before and the results could be off enough to not show if you have a refrigerant leak) - if you live in a hot climate then a bit of shade will help.
Some types of AC compressors loose refrigerant very slowly over time through the front seal that is located behind the AC clutch and is driven by the fan belt - a line of oil or staining can be seen on the inside of the hood above the AC compressor and/or on other parts located close to the compressor.
Sometimes a leak can be found by visually inspecting the AC lines/components for oil leaks (since there is oil mixed with the refrigerant that lubricates the AC compressor). The oil will look slick and dirt/grime will accumulate slightly away from the actual leak (unless it is a very slow leak, then the dirt/grime may hide the leak) - so you may need to clean the AC lines/components to get rid of the normal dirt/grime that is picked-up over time (unless you keep your engine compartment clean).
I have seen a lot of leaks at the AC lines usually at the connection where the flexible hose is attached to the metal tube that connects the line to various AC components, but on my MB the leaks were a serious of very small pin holes along the flexible line. If this is the case, then many AC supply houses have the equipment and parts to replace the flexible hose - at a fraction of the cost of buying a new hose (some MB AC hoses cost around $200 ea from aftermarket suppliers and much more from MB). A few phone calls to AC supply houses will yield one or more that repair AC lines.
There are also tools (sniffers) that sample the air around the AC lines/components that will locate refrigerant leaks. If the leak is small/very slow, then a sniffer may be the only way to find it. If this is the case, only AC professionals typically have these tools - meaning you will need to seek out a repair shop.
Last, the various sensors (coolant & refrigerant) may be defective causing the aux. fan to run more often then it should. The coolant sensors can be checked by immersing them in hot water that is at the temp where the sensor operates and using a volt/ohm meter to determine if the electrical circuit has closed (completing the ground side of the circuit) and allowing the aux. fan to operate. However, your problem seems to be related to the amount of refrigerant in your AC system - as Steve wrote about.
Hope this helps.
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1977 300D: 300,000+ miles
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