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Old 07-08-2010, 07:01 PM
Jim Lee
Join Date: Feb 2009
Location: Durham, North Carolina
Posts: 61
Overheating due to aux fan flaky-ness

My 1995 E320 is overheating because the aux fans in front of the radiator seem to be turning off when they should be staying on. Most of the time they stay off and in this 100+ degree weather in Durham, North Carolina that is making me run way too hot. I have had the issue before where the blade fuse in the relay blows and they just stop working period. This time the fuse is good and I was very surprised to see the fans both turning on while I had it in the driveway idling. Then they turned off which it was pretty clear they should not of. Is there a thermostat or switch somewhere that I can test? When the fans are running they appear to be running fine and they spin freely when not running. Is there a way I can jump to the fan(s) to make sure that they are ok and start looking at switches or relays?

Thanks very much,
Jim Lee
1995 E320 wagon

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Old 07-09-2010, 07:32 AM
Cal Learner's Avatar
Registered User
Join Date: Aug 2006
Location: Marysville, CA
Posts: 781
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).
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
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Old 07-09-2010, 10:41 AM
Pili380SL's Avatar
Parts Geek
Join Date: Jan 2004
Location: Springfield, MO
Posts: 238
Originally Posted by Cal Learner View Post
You'll know you've found it if you pull off the 2-wire electrical connector and the fan comes on high (KOEO).
Does this test also apply for a 1995 S500?

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