When idling for a long time, and at least with the A/C on, I get the following behavior:
* The engine temperature will rise up to about 105 degrees, at which point the fan will go on, and the temperature will finally decrease.
* The temperature will decrease down to 95 degrees, at which point the fan will stop, and the cycle will start again.

This is surprising because the idle temperature on my M103 is pretty constant.

The other surprise is that since the fan seems then to be electrically rather than mechanically driven on the M104, it nevertheless doesn't stay on after switching off the engine, as is the case on other cars, and as I understood is one of the benefits of electric fans?

Interested in other experiences!

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1991 W126 300SE (M103)
1995 W124 E320 Wagon (M104)

I sold my M104-powered 300E several months ago, but while I had it, I was sitting in the drivethrough at Walgreens one night, waiting forever for a prescription to be filled, watching the temp guage do the same thing yours does...it would creep higher than I would like, then the fan would kick on, drop the temp, and the cycle continued. The odd thing was, it wasn't hot that evening, perhaps in the 50's.

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2002 Ford ZX2
2 x 2013 Honda Civics

Running the electric fan without coolant circulation is nearly useless. You are only cooling the water in the radiator, which will cool itself soon enough with no fan.

The advantage of all-electric fans is that there is no fan load on the engine at highway speeds.

Common problem on the m104 engined 124 cars. The electric fans have two speeds. Low speed is triggered by the a/c system, high speed is triggered when the engine temperature reaches 105C. The problem you have is low speed on the fans, as triggered by the a/c pressure, is not operating.

The reason low speed fails on these cars is the wire supplying power for low speed operating overheats and breaks. The break is at the resistor. It is located directly behind the drivers side headlight, next to the a/c receiver/dryer. Look for a cook wire no longer attached to the resistor.

The solution is to replace the wire with a larger (smaller guage) wire. Earlier years of the 124 chassis used a 2.5mm diameter wire, inexplicably MB reduced it to 1.5mm, with this problem the result. I spent about an hour running and soldering a larger diameter wire from the relay in the fusebox to the resistor. No problems since.

When my car was broken it behaved as you described - engine temperature oscillated up to 105C and back down. BTW, running the car like this is bad for the a/c system, with no low speed fans it sees excessive head pressure, which strains the compressor.

- JimY

anybody know if this was a problem in the c280 with the 104 engine?

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1994 C280
2009 VW Tiguan
1993 Toyota X-tra Cab SE-5


1973 220D ... Gone, but not forgotten
1991 Alfa Romeo 164L Gone, wife MADE me forget it

2006 Hyundai Tucson... just straight out FORGOTTEN!

Also, another common cause of no low fan is a slight refrigerant leak. This allows a/c operation, but not high enough head pressure to trip the high pressure sw for fan circuit.
Both this and the fan circuit can be verified by jumping the pressure sw at the reciever/drier and see if you have low fan.. if you do, then it is not the wiring/resistor circuit, but the refrigerabt level...The sw is the one with the pig-tail ends

Can someone identify the exact wire to look for.
I have exact same problem, fans work at high speed when temp gets hot but not at low speed at all when car is idling with air con on. I shorted the switch, (large round one with 2 wires coming out) where they join at the spade connectors and it made no difference. I noticed when attaching a 12v bulb to one of them it glowed up slowly. Air con takes ages to blow cold in traffic without these working unless I'm moving at high speed.

IIRC - The fuse on the low speed fan relay is also a common failure.

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE