Most often mis-diagnosed over-heat w/ac on that chassis is low refrigerant level.
The high side pressure switch on the drier/reciever does not get to cut-in b/c of slightly low refrig, so the car still has a/c operation, but no LOW FAN..this results in passive heat thremal load to radiator . By the time the coolant temp sensor reaches 107 cut-in , it is too late .
The tesy is to jumper the pigtail ends together at the high side sw and see if the low fans come on...if yes , then the low fan circuit is verified....now see if the low fan comes on when a/c has a thernmal loaad on it ...if No, you are low on refirgerant.
If you are in a high ambient climate zone, you can bridge the coolant temp sensor [ blue , 2 wire thermistor one at thermo housing] with a resistor to lower the 105/107 cut-in for high fan to approx 100C..this is a common mod on these dual aux fan seups where one needs the extra airflow.

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A Dalton

Define what Thermal load it needs to turn the low speed on for me Arthur. How can I duplicate that situation? I can't seem to get the car to go over 82C today sitting there in the drive way. With AC on too. It is 60F ambient temp today here. Should they come on immediately or do they have to see more load than that? Mine are not low speed functioning when at 82C with AC on. And I think they should be running immediately when AC is put on. Is that correct? So that relay is bad or the level is low. At first I was leaning to the relay due to the fact the AC puts out very well but I read a few thread says this is wrong way to think about it. Could still be it is just slightly low enough to do exactly this. And that is more likely the issue here. Something to look into first I guess.

They will come on at near 90-100C (not sure when but have observed them both running at one point) in low and they will come on High (no mistaking that noise when they do that) at 107C and then they hold temp there, if not drop it a tad. This is all with AC on or not. So I know from that the Fans are fine so I imagine I should look into the AC relationship even though I have not seen any problems yet... but then again, it is winter and not harshly hot this time of year. Good time to go into that I guess.


Added on edit, From pictures I have seen of this Bridged Resistor, it appears they have just lodged each end of the resistor lead into the plug? Of course I would need to know which two too shove it into if that all it is....

I will write a short tutorial on thernmal load conditions and low fan activation..you can then have a better understanding and most likey be able to answer your own questions with that info.

Low aux fan is A/C acitvated ONLY.. the relay is fused and it is located in the relay box. [ behind the fuse box...there are two..one for low and one for hogh fan] The sensor that triggers the low fan relay is the HIGH PRESSURE switch located on the reciever/drier, at the condenser. There are two sensors located at the condenser ..the one for aux fan cut-in is the one with the 6" Pig-Tail wires coming out of the sensor...NOT the one with the spade connectors.
So ..do the low fans work is the first question when one has a hot running engine w/ac ON.
The first thing we want to do is test the integrity of that fan circuit..this is quite simple to do by simply jumping the two pigtail wires together [ thereby by-passing the high pres sw] ..this should result in a low fan activation if all componenets in the fan system are OK [ fan/relay/fuse /droppping resistor/etc]... Once that is verified , then you are now down to only two possibles ...the sw is bad OR there is not enough high pressure to trigger the sw cut-in spec. This test only requires Key ON.
This is where Thermal Load comes into play...The fans are NOT designed to come ON just b/c you are using a/c...it is an AUX FAN and it is designed to only come on when the condenser pressures gets HIGH and the system is being TAXED, which is caused by a HIGH TERMAL LOAD..meaning, if one is using a/c on a cool day, don't look for a fan activation b/c one is simply not being called for..but if the ambient temps are high, along with humidity , you now present the system with a taxing condition. [ thermal Load]..the results are the sw closes , bringing on the fans , and that added air-flow across the condenser will keep the pressure in check... If one does have a fan activation on a hot day, then he does not have a low refrigerant condition..simply b/c a low refrigerant level will not have the capacity to pump the high side pressure up enough to trigger the sensor.
There is an eye-sight on your reciever/drier..if you see lots of foam, you are low.
If you want to test a low fan at low ambients , you can also cover the front of the car to help heat it up some...but the actual test would be to take a high side pressure reading to see if the system gets to sw cut-in spec when you have a hot day...if no, then possible refri level..if yes, then you know the sw is bad.
In your case you have verified low fan when it is hot out, so I don't suspect either condition, but watch for low fan on a hotter day/condition...

I would also do the blu sensor bridge, as the lower cut-in is a cool mod...they [ Benz] claim 107 is of no dnager to this car, but I don't like to see it and they were known for head gaskets to begin with, so anything above 100c is not to my likings............... [I think they made the claim b/c they has plentyof Complaints]...I use a cabin sw on all my Benz b/c I am a dash gauge driver and it allows me to get a jump on the system before the sesnors can act...it also allows me the preference of using the low fans , unlike the high fans that the bridge resistor mod activates. But that is choice........That cabin toggle simply jumper across that same high pressure/pigtail sw I mentioned earlier..just bring a two wire lead into the cabin from that sw and you are done...$2 well spent.
>>and lastly, addressing your condition..I wpuld split the rad and condenser a little and clear out the crap in between the two..you will be shocked when you see the debris in there and that is a common cause of your condition.
Aux Fan airflow means nothing to a plugged up condenser...it is the radiator that suffers the passive heat from the condenser and the results are the coolant temps getting to 107 trigger...107 trigger should be a rare occurance on that engine.

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A Dalton

< From pictures I have seen of this Bridged Resistor, it appears they have just lodged each end of the resistor lead into the plug? >

No.. you want to solder those ends...you have to simply take the connector apart [ it just snaps into the lock position..a little screwdriver blade takes it apart]..
You then solder the resistor ACROSS the two sensor terminals and then snap the connector back together...you have to leave the resistor showing outside the connector so that you can close the connector back up.
You want a metal film resistor ..as they are accurate/stable , holding R value regarless of temp changes ...Radio Shack...about a Buck for a 5 Pak.

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A Dalton