Failed clutch fan and the ridiculously high 105c trigger point for the aux fans are the death of these motors. Most 15+ year old Mercedes are driving around with a failed clutch fan. Most owners don't know any better.

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CENSORED due to not family friendly words

105C is still well within safe operating temperature. Is the engine making peak HP at this temp? Probably not. But there is absolutely no damage being done.

If the car is running at 105C often, there are big problems somewhere. I can count on one hand the amount of times my aux fans have kicked on in the last year. Only time it's acceptable to see 105C is idling in traffic, and the fans will kick on and bring it back below 100C very quickly. The fan clutch locks at 90C. So the engine should be running at just a tick above 80 during 99% of driving conditions.

The 95C aux fan mod is for those who either have other cooling issues elsewhere, or don't trust the millions of dollars Mercedes puts into engineering.

Consist of soldering in a resistor at the coolant temp switch on the cylinder head. If you do a search for "aux fan resistor" you will find some stuff.

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http://i193.photobucket.com/albums/z...-RESIZED-1.jpg
1991 300E - 212K and rising fast...

It does seem as though a 103 with a properly-operating and maintained coolant system does not overheat. If it routinely reaches 105, there's an underlying problem, probably in the coolant system, and not in the air flow system. Blaming the 105C sensor isn't the answer, although reducing that trigger point with a modified resistor might make sense if the vehicle is routinely used for towing. Otherwise, I would look elsewhere for the fix to an overheating problem.

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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

BAD300 reread tjts1 quote above.

Your engine is liquid cooled. This means fluid absorbs heat fron the combustion area of the head and block and is pumped into an air-cooled radiator. The fluid is cooled by air passing over the radiator and is re-circulated back to the hot sections again.

If the engine is overheating, it means that:

1. The fluid is not being pumped.
2. The fluid is not being cooled.
3. There is insufficient fluid.

There is a fan clutch on the inner part of the radiator fan that ensures that the fan only engages when the temperature is hot. If it fails...

To test, run the car at highway speeds (100 km/h plus). If the engine runs cool...great! Then run the car at traffic speeds (30 km/h or less). If the engine overheats...then you might strongly suspect the fan clutch.

The fan clutch is cheap to buy.

I changed mine from 105c to 90c with a 700ohm resistor in parallel but you can choose any temperature with different value resistor. 95c trigger point would require a ~766 ohm resistor. 750 ohm is an easy off the shelf solution. I also changed my thermostat from the stock 87c to 79c. Even without a clutch fan, the electric fans rarely trigger. I also added a backup switch in the dash if the temp sensor ever fails.
105c on a daily basis with a dead clutch fan is death to the headgasket.
I don't think mercedes takes it as a personal offense every time somebody modifies their 20 year old car. The fact of the matter is even if you keep your cooling fans in tip top condition, after 100k+ miles the radiator gets loaded up with all sorts of road debris between the fins. The radiator needs to be removed and thoroughly cleaned both inside and out to bring it back to its original cooling capacity. In my case I also had to straighten out a few hundred fins damaged by the brilliant 'mercedes specialist' who worked on it previously.
Its an NTC (negative temperature coefficient) temperature sensor, not a switch. Early cars (pre 87) have a temperature switch and adding a resistor to these won't work.
Keep in mind that using an 1100 ohm resistor as in the article ( http://pages.prodigy.net/jforgione/MB_CTS.html ) wont give the same results in our cars. The W140 uses the same temperature sensor but the fans trigger at 100c or 310 ohm. The M103/104 trigger at 105c or ~270ohm. You have to recalculate the resistor values for your specific target temperature.
http://www.sengpielaudio.com/calculator-paralresist.htm
The known data points for this sensor:
115c = 200 ohm
107c = 250 ohm
100c = 310 ohm
80c = 560 ohm

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CENSORED due to not family friendly words

Thanks tjts1

Thanks to all you guys for the info. This is why I appreciate & refer others in Aussie to this sight. Whilst there are some members here on another site that are helpful & know a thing or two. The ratio of Merc owners here to you well......speaks for itself. I hate getting a bum-stear (the runaround) when I need facts to build this car right & know how to maintain it as I will be keeping it now for the inevitable future. Even if it is just as a daily driver up ahead someday.

Thanks again.

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um.....why have men got nipples

The Bentley Publishers 'The 124 Bible' carries an detailed article on how to adjust the cut- in temperature of the fan clutch .and it works very well.

To bring down under hood temperature a Exhaust heat wrap on the manifold pipes works wonderfully.The engine and underhood stay much cooler .
mak
300se