Common problem on the m104 engined 124 cars. The electric fans have two speeds. Low speed is triggered by the a/c system, high speed is triggered when the engine temperature reaches 105C. The problem you have is low speed on the fans, as triggered by the a/c pressure, is not operating.

The reason low speed fails on these cars is the wire supplying power for low speed operating overheats and breaks. The break is at the resistor. It is located directly behind the drivers side headlight, next to the a/c receiver/dryer. Look for a cook wire no longer attached to the resistor.

The solution is to replace the wire with a larger (smaller guage) wire. Earlier years of the 124 chassis used a 2.5mm diameter wire, inexplicably MB reduced it to 1.5mm, with this problem the result. I spent about an hour running and soldering a larger diameter wire from the relay in the fusebox to the resistor. No problems since.

When my car was broken it behaved as you described - engine temperature oscillated up to 105C and back down. BTW, running the car like this is bad for the a/c system, with no low speed fans it sees excessive head pressure, which strains the compressor.

- JimY