I'm not sure myself, but I'm having overheating problems on my W124 too, and they seem to be related to the A/C...in other words, temps are normal when A/C not running, when A/C is on, and (I suspect) aux fans not doing their job, temps start climbing.

I can actually get my aux fans to fire up simply by disconnecting the two-prong lead from the thermostat housing. Did that last night as a test, and let them run for the entire trip home. Despite the 103 temps outside and A/C at max, engine temp remained between 85-87 all the way home, highway-city combo driving! So my guess is that they are not coming on early enough, or not at all!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car