"Overheating" is a vague term. You have a temperature gage and should report what coolant temperatures you are seeing under various operating conditions.

I believe most models of the era were set up like my '88 190E 2.6.

The viscous fan clutch keys off radiator exit air temperature, and it will typically be fully tightened when coolant temperature reaches about 100C. You can tell by the "fan roar" you hear at about 2000 revs, and if you shut down the engine at about 100C and turn the mechanical fan by hand it should be much stiffer compared to when the engine is cold.

Even with a fully tightened fan clutch, idling in hot weather may not provide sufficient air flow at idle speed, and coolant temperature will continue to climb. At about 105C the auxiliary fan(s) should engage at high speed, and they can usually be heard. This should result in rapid cooling until they cut off at about 100C, and the cycle will repeat if the vehicle is stationary and the engine continues idlling. The above behavior is common in a hot weather traffic jam.

105C is about 221F and a 50/50 blend of ethylene glycol and water with a 15 psi cap boils at about 265F, so you still have plenty of boilover margin at 105C.

Independent of the above the auxiliary fan(s) will engage at low speed based on closing of the high pressure AC switch to provide adequate freon condensing. Often coolant temperature in hot weather, low speed driving will be lower with the AC on because of the high low speed duty cycle of the auxiliary fan(s) based on the freon condensing requirement. Coolant temperature many not reach the 105C requirement to engage the auxiliary fan(s) at high speed.

Search for a thread started by me a couple of years ago - auxiliary fan failure analysis.

Duke