Reading this thread I get the impression that the two electric fans run independently of each other,,, is this true ? And if so which is high and which is low ? I always though the two started and stopped at the same time. Perhaps this is why the 400E is running a bit hot these days.

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95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K

Your confusion is very common with these two different systems.

Fan Systems/Explaination :

The two fans are wired in parellel and ALWAYS work in unison.

There are TWO different circuits ,HIGH FAN and LOW FAN.

Each circuit is INDEPENDANT of each other , EXCEPT, they SHARE the same FAN MOTORS.

There is high fan relay, fuse , sensor circuit.

There is a low fan relay, fuse ,sensor , and dropping resistor [ Referred to as R15/ Pre-resistor.]

The a/c circuit can NEVER trigger a High fan, nor can the coolant temp circuit ever trigger a LOW fan..however , if the coolant circuit calls for high fan, that will over-ride a low fan demand from the a/c low fan circuit, resulting in high fan, regardless of a/c demand.

So, if one looks at a schematic, you will note that each of these independant circuits MEET at a COMMON terminal at/after the dropping resistor. [ the two wire side of the resistor]
That is the point in the circuit where either circuit feeds the FAN MOTORS. That is where they now share the fan motors, regarless of which circuit has a demand signal from it's circuits sensor [ Coolant temp sensor for high fan, a/c pressure sw/sensor for low fan.]
The low fan is accomplished by feeding 12v to the R15 resistor BEFORE this common terminal, thereby dropping the voltage to get a low fan RPM. Whereas , the High fan circuit by-passes thus R!15, allowing a full battery voltage to reach the fan motors, resulting in a HIGH fan RPM....
..so, two different circuits Sharing the same , parellel wired fan motors.
If you have a fan motor turning slower than the other , or not turning at all, the problem is NOT in either high/low fan circuits, it is a faulty FAN MOTOR .

I am not an expert, but from what I have read and seen with my car it seems that both fans run together. They can run at low speed or high speed, however, and they really go like crazy when they are on high speed. If they come on and your temperature is not over 100 degrees, then that is probably the low speed. If you ever get stuck in traffic on a hot day and your AC system is a bit low, so your fans don't come on at the low speed, then your temp may go up over 100 and then the fans will come on at high speed. I suppose they can also can come on at high speed without having low AC pressure, but I think it might take rather extreme conditions to get over 100 if your low speed fans are coming on with regularity due to the pressure in the AC system.

Others here are real experts, so they may chime in if I am getting too carried away with my newly acquired expertise on this matter.

Woops, I was too slow and a real expert answered while I was writing.

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I just couldn't give up on my 1995 E320.

I think it might be like always going back to that same bad relationship with an ex girlfriend.
You feel you love them too much, or you are just too stupid to know any better.



Flickr slideshow of my 1995 E320
http://www.flickr.com/photos/24145497@N06/sets/72157616572140057/

You are now as Expert as I....

It is all in reading this Great Forum. All the info is right here.

Thanks once again for the great info Arthur,,,,, I sure wish you lived in Western Pa.

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95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K

I did do a Gig once in Weastern Pa..at least I think that is W. Pa ??

York, Pa.
A Club right in the center of town, across from a Noodle Resturant..Very nice, but years ago, so my menory is cloudy on the exact Name of the Place [ prob not even there anymore.]
I do remember those long tunnels going through the mountains on the Pa Turnpike..What a road trip that was ...
Next stop from there was Erie , Pa...Completely different type town, if I remember correctly.

My wife was raised on a small dairy farm right outside of York Pa. She is still a great worker, thanks to that training on the dairy farm. She is a great cook too!

__________________
I just couldn't give up on my 1995 E320.

I think it might be like always going back to that same bad relationship with an ex girlfriend.
You feel you love them too much, or you are just too stupid to know any better.



Flickr slideshow of my 1995 E320
http://www.flickr.com/photos/24145497@N06/sets/72157616572140057/

York - East,,, Erie - West,,, about 100 miles north of Butler.

__________________
95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K

Another common failure is the resistor behind the driver side headlight. The resistor drops the voltage to create low fan speed. Have seen it queried more than a few times on the forum. As well, my low mileage/dealer maintained/Starmarked/owned by MB dealer management 124 came with a burned out resistor and no low speed fan operation. On this 1993 model the teltale clue was coolant temp rising to 105C, fans noisily engage, and coolant temp drops to ~95ish. Surprisingly, the A/C worked fine.

After replacement of the resistor and upgrading the wiring the low speed fans work great. The car sits on ~90C coolant temp and blows cold air all day long, no matter the outside temperature.

Since I know you're going to ask about the wiring upgrade. On the early 124 models a 2.5mm diameter wire supplied power to the resistor. For unknown reasons MB dropped this to 1.5mm on later models. The smaller wire tends to overheat and break near the resistor. Per a tip on this forum from Donnie, I replaced the entire wire from the low speed relay to the resistor and all has been well for several years.

- JimY