The cooling system on inline 6 w124s and w201s is marginal. The radiator and viscous fan is undersized and the aux fans kick in at much too high a temperature. There are several things you can do to improve the situation. Run a smaller fan pulley, increase the water to coolant ratio, run a colder aux fan switch or add a manual switch in the dash for the aux fans. As much as some people hate to hear it, this is a design flaw that only becomes apparent in hot climates. All the inline 6 w124s suffer from it. There isn't enough room for a properly sized radiator and viscous clutch fan.

__________________
CENSORED due to not family friendly words