Theres a reason MB ditched the clutch fan and went all electric. Theres a few probs. I had the issue of overheat with AC on. Litereally every thing new, OEM parts. All behr stuff. Figured out that with AC on my fan clutch was clutching agressively at idle which causes condenser to cool enough to never kick the high side switch on. So AC is happy while your engine is cooking basically. Probably my brand new OEM switch is just junk and not kicking on at the right temp. So I wired a relay to turn on the low speed aux fan circuit whenever the clutch is engaged. This helped a lot to keep the system from running away, especially after getting off freeway. Interestingly enough if your system has low refrigerant this switch will not kick on and your car will run hotter as well, so make sure you got a proper charge if you use the AC. I also removed the plastic cover from the front of the condenser and cut bottom half off. This blocks a massive amount of airflow from the bumper when the vehicle is in motion, I noticed lower freeway temps after this.

All this helped but I'm kind of crazy so I fabbed up a bracket to mount an all brass ford heater core in the wheel well with a spal 5.5 inch fan on it. Now literally nothing will make it go above 100 with AC on. Still wish it would just sit at 80C like a newer mercedes but I guess that's the power of a 900 watt fan and feedback loop speed controller. I thought about it but theres very little room on my 300CE for a fan behind the radiator and I'd need to upgrade the alternator as well.

I think if I could just get a smaller diameter fan pulley Id be able to move more heat than all the mods Ive done so far but I can't seem to find anything even close that would work. That's the real ticket IMO, just move more air at idle from the clutch fan. ACM clutches are also "aggressive".

__________________
1981 300SD 512k OM603