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.

__________________
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.

__________________
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

Yes , they dropped the wire size when they came out with the dual fans..but they under-estimated the current draw . They later changed the wire feed and also changed the fuse rating on low fan relay from 15 to 25 amp. [ the ones w/fuse on top of each relay]
Some guys in Tropical Climates take that same feed wire from the low relay to the R15 and use that to trigger another independent, higher amp circuit with another relay, which by-passes the R15 circuit altogether, allowing for High Fan for a/c condenser airflow. No need to change the wire from the 1.5 size b/c that now only feeds the coil side of the new relay. [ Low amps].
Never tried it, but I can see the advantage of a high fan for cond. airflow when one has the extreme thermal load conditions of Tropical type climates............................which lately, could be almost anywhere in the USA ...
Maybe we should just call that one the " Global Warming Modification TSB" ???
Here is the latest proto-type we are working on ..definate solution for
any Global Conditions , right into the next 10 years..

http://www.sl113.org/forums/uploaded/cees%20klumper/2005621181316_airco1.jpg