I have a 95 C280 and I am trying to figure out the aux fan operation. My ECT sensor, mounted on the t-stat housing, has 4 wires, and does not turn on the fan when it is unplugged.... should it? If the variable resistance decreases, pulling the plug is open for no fan. If I unplug the switch on the expansion bottle, the fans come on and stay on until the temp gauge shows about 87 degrees. If I let the engine idle with the air on max, the fans will come on for about 5-10 seconds and then go off - obviously not enough time to help. Given this scenario, this summer I will be in big trouble when the temps are high and the wife is driving around with the air running high. It seems like the cooling temperature circuit is not working, and the air cond pressure/temp circuit is not working very well.

I also think the viscous clutch is not working...seems to be at the same speed all the time, hot or cold. How do I check this fan?

It will either come on due to refrigerent pressure reaching a redetermined temp or for engine temp.

For engine temp, they will come on low around 105C and be on high around 115C I believe. I forget what the refrigerant pressure cut in points are. On my 97, they are infinitely variable so it setermines speed based on its logic.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record

Try the following link for info on the cooling fan operation of the 1995 model:

http://www.peachparts.com/shopforum/showthread.php?threadid=48571

Note: There is a lot of info in the archives of this site if you use the search feature.

I read that thread and lots of the others. I don't want to bother you guys without doing my homework. My main question is that my temp sensor (ECT) is apparently part of a 4 wire sensor instead of the typical single wire resistor to ground or 2 wire variable resistor with an in and an out. If the resistance is around 300 ohms to turn on the fans, I can jump with a small 200 or 300 ohm resistor to eliminate the sensor but I don't want to jump the wrong thing. I bought the CD for the 202 from MBZ USA but it does not feature much of the 104 engine - mostly the 111, 112, and 113, and the wiring diagrams, split up and poorly titled, are a nightmare to follow. I did find the B11/4 temp sensor and saw a picture of it, but the wiring diagram shows just 2 wires so the other half must be for something else. Oddly enough, if I unplug the pressure sensor off the expansion bottle for the air cond, the high speed fan comes on and stays on until the temp gauge get down to about 90 - that shows that the high speed relay, fuses, wiring, resistors, and fan motors are working. I am suspecting the sensor since occasionally the high speed fans come on if I run the air cond at idle long enough for the engine to heat up, but it takes a long time. Even then, the fans at high speed stay on only about 5 seconds.