For those finding this post in searching for solutions to the overheating, aux fan problems for the 300E W124, I thought I'd add my experience.
1988 300E at 260K miles was overheating etc. Had a shop (Ditmire -- recommended!) put in a 100K M103 engine with new head gasket etc plus new waterpump, radiator etc etc.After all that, still had an overheat problem and realized the aux fan (older W124 single fan system) was bad -- binding and blowing fuses. Finally replace that. I also add in Forgione's resistor mod to kick the fan in earlier.
First hot day in 2007 I think I'm all ready. 85 degrees, aux fan kicks in when it should but the temperature continues to proceed up to 120 C and the redline. Cursing, I am!
Turns out the new aux fan was spinning counter clockwise. The connector goes in one way, so I think the fan was wired backwards when it was manufactured. I reverse the wires and the car is finally now maintaining a nice temp
And a thanks to all those who posted information here over the years that was all very helpful. This is my contribution to the pile.
Lee Leitner