Well, you asked some questions and everytime I answer, you have some guy or something you read that does not agree with the facts and circuitry of your chassis...
So, you can run your car without the coolant temp aux fan circuit and take that chance, but I sure would not recommend that to anyone, that's for sure.
Better keep that relay in the glovebox for when you have a temp gauge pushing the limits of head gasket time..you can just plug it back in real quick...

Here is the Aux fan circuits [ Both HIGH FAN and LOW FAN] for your chassis,just so you know from the Benz Service Manuals, not some guy..

You will note that the LOW SPEED fan is triggered by the a/c pressure switch and the LOW FAN comes on when that sw closes [ Switch S/32/1 in the schematic]. That is ONLY when the a/c is ON. Note that the power goes through Resistor R-15 in order to drop the voltage in order to achieve LOW SPEED FAN..

http://members.aol.com/ajdalton7/auxfan.jpg

Now, look at ENGINE COOLANT TEMP sensor [ B-10/8 in schematic]..
Note that when that sensor [ which is a thermistor, meaning it changes resistance values as the COOLANT TEMP changes] reaches the correct cut-in point of 105-107C. of Engine Coolant temp, it triggers relay C and a full 12 volts go to the fans and the results is HIGH FAN..meaning excessive engine coolant temp is checked by HIGH FAN.
..So, simply put , your car has a Low speed aux fan for a/c high pressure protectio/ aid ..and a High speed fan circuit for engine coolant overheat protection/aid. The low speed is for increased air flow through the condensor, and the high speed is for increased air flow through the radiator.
Two different design operations with completely different reasons for being.
Anyone who claims that aux fan on these chassis is just for a/c knows nothing about these circuits, period.

That's all.........have fun..........

Ok Ok so where is the coolant temp sensor? Should I check this, because it is the high fan thats running. The temp guage never exceeds the 80c. It always runs normal. Even after the car cools down it runs on high.

For position of coolant temperature see
W124 location of coolant temp. sensor.

Here is a pic of the relays M103.983 W124.050

__________________
1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE

I am trying to find this part ( aux fan switch ) on the net with no luck. Mine has two prongs on top. I can find a coolant temp switch, but this in not what I want. Is the aux fan switch called something else? Anyone have a website where I could buy one, and see what it looks like?

if fan is running all the time on high, something is triggering the relay. If the relay is turning on and off rapidly, the relay is probably working and something is shorting it out up stream.

If the relay is ok (the black one in slot 3 behind the fuse box, not the green one in slot 2 that controls the slow fan) then either the blue thermistor on the radiator return is defective (always on) or the wires are shorted out. These wires take alot of heat next to the radiator and the coating can deteriorate (as has been known to happen on the 300e wiring harness)

disconnect the connection from the blue thermistor and see if the fan continues to run. If so, the wires may be shorted near the connection.

Take a voltmeter and put the black to ground. Test one side of the connector to see if there is 12 volts. Then test the other side. If there is 12 volts on both sides then the wires are probably shorted. If so, it may be close to the connector and may be an easy fix. Look for deterioration of the coating of the wires.

If one side reads 12 volts, and the other side reads zero volts, then test the thermistor. The resistance is probably close to infinity when the temp is low. When it heats up to greater than 100 degrees c, the thermistor closes. so if it tests with zero resistance, it is probably working properly.

another even simpler test is to remove the connector from the blue thermistor. If the fan still runs (when the engine is not hot), the wires must be shorting out. if it stops, then the thermistor must be bad (always on). if the fan still runs when its disconnected from the blue, then try the voltage test above. 12 v on both sides suggests a short in the wires upstream leading to the relay.

I am currently troubleshooting mine and my wires look like they lost their insulation near the connector to the blue thermistor.