In my 400E the Aux fans won't kick on by themselves?? If I pull the coolent sensor they will kick on hi and If I pu the AC on they will cycle on but they will not kick on by themselves when the car gets hot???

Is it actually overheating? MB's are one of a few cars where the temp gauge actually does fluctuate a little.

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1981 300TD
2005 C55
2004 E500
1999 ML430
1992 400E
1998 C43
2001 SL500
1988 300SEL
1986 190E 16V
1991 190E 2.6

Aux fans will come on if A/C compressor engages at any time as long as pressure is in the freon system (and the sensor is good) and will kick on when the temp on the motor reaches 105 degrees +/- 5 degress. If its not to the red its not overheating.

The 400E's (E420's) did have issues (melted wiring) with the ballast resistor for the aux fan, when operating the a/c the fans should run at 70% after 225 lbs or higher.

On most MB's after 1988 (up to 1996) the aux fans are triggered at 100% at approx 107C, if the a/c is off. There is a combi relay on that car for the different aux fan speeds.

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Sounds like the low aux fan circuit works when A/C pressure triggers the switch on the drier, but the coolant temp sensor may not trigger the high fans. If that's right, seems like either the engine temp doesn't actually reach the 105C cut-in point for those high fans, or the temp sensor is malfunctioning and doesn't produce the low resistance values that the circuit requires to trigger the fans. If you can you verify engine temp, you might be able to rule out a sensor malfunction. I don't recall ever seeing what the diagnostic test for that sensor is, but it must involve backprobing the sensor and watching the resistance fall to near zero as the motor heats up. Replacing the sensor with a known good one obviously works too.

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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

Here's some information that may be helpful.

http://www.k6jrf.com/MB_CTS.html

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Would this be true on a '93 190e 2.3 also? My sister's 190 will not turn on the aux fan, but it runs if you disconnect the cable from the sensor....

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'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

If the low-speed fans are not working, I would check the strip fuse first. Then, make sure your a/c system is charged. Low freon will result in the low-speed fans not activating. You can check the combi relay that MB Doc refers to by jumpering the pigtail wires going into the receiver/drier and listening for the relay behind the fuse box to click (engine off, ignition on). I am sure there are numerous threads that talk you through it.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Anyone know where I can get the coolent temperature sensor ? Everyone sells the four post one, but know one seems to sell the two post one???

got the 2 post, 1 post, coolent filler cap I hope one of these does the trick

FAN KICKS ON!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! SUCCSESSSSSSSSSSSSSS

My ballast resistor was fried so I ran a toggle switch in my glove box to operate the aux.fan manually. Seems to work o.k. Haven't tried the a/c yet but I'll keep an eye on my temp. gauge. It's on an 86300SL.