Even with the nutty MB fan controls, 100C is abnormal. I'll take all arguments on that. That should improve when you replace the clutch.

The low speed fan deploys based on pressure in the a/c system. I believe the trigger point is 300 bar, which isn't going to happen too often. But idling in traffic is a situation where it could. There are three components involved: the switch on the receiver dryer, the relay, and the fan resistor. The resistor is on the left inner fender, somewhere down behind the headlight. Make sure it has continuity. If not, replace it. I don't remember how the relay was set up for my '87 300d, but on my '87 190D it's a silver cube in the electrical center. The relay in the 190D was underdesigned @ 15A, if yours is the same you can and probably should upgrade to a 30A relay.