Note: On the W123 (your car) the auxiliary electric fan will ONLY run when the A/C is on, and refrigerant pressure is high enough to trigger the switch on the receiver/dryer. There is no connection to the cooling system at all. Mercedes designed the main fan (and clutch) to be able to cool the car without the aux fan. The aux fan is there mostly for low-speed A/C performance, when there may not be enough air flowing over the condenser.
In later cars (W124, W126) the aux fans did have a coolant temp switch, on my car it turns the aux fan on at 105C.
Oh yeah, Tom's info was good, check all that stuff too.
And don't forget to clean the fins on the radiator AND condenser with compressed air or water spray, removing the radiator if necessary. An amazing amount of bugs & gunk gets in there!
Regards,