Is your motor running too hot while driving or only at idle? The aux fans really have no effect if the car is moving at least 15 MPH. So, if it's the former, there is a coolant system problem (bad thermostat or water pump, plugged radiator, low coolant level or improper coolant mixture, bad cap on the expansion tank). If it's the latter, the problem may indeed be airflow (in addition to poor coolant circulation possibly). You know that the fan circuit is good. It may be that the 2-pole temp sensor that controls high aux fan speed is dying. I don't recall ever seeing a diagnostic test for that sensor, someone else may know. On the early W124s, the temp sensor was on the head, back about cylinder 4. I'm pretty sure it was relocated forward to the thermostat area on yours. You'll know you've found it if you pull off the 2-wire electrical connector and the fan comes on high (KOEO).

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

Does this test also apply for a 1995 S500?

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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa