My Aux fans were not working and in my troubleshooting I thought I had isolated it to the fan motors. Well, I replaced the fan and still no joy. Int trying to troubleshoot more, I put the car in the fan test mode (hold Auto for more than 10 secs and the aux fan turns on). With no luck. However, being puzzled I spun the fan, while still in test mode and the fan started spinning.
Has anyone seen this? and could this be the control module?
I have a '98 E320 and had the exact same problem. With the fans in full test mode, they spun very slowly and at times didn't run at all until I pushed them with my finger. I found a fan control module on eBay for $70 and am awaiting its delivery. Once I install it, I will let you know whether it was the fan control module that was making the fans spin so slowly.
Also, my Mercedes dealer is EXTREMELY helpful every time that I encounter a DIY project on my MB. I asked them about this problem and they seemed to think that it was the fan control module, but of course, wouldn't fully commit to that. They quoted me $200 for a new one.
The fan control module on my '98 E320 is underneath the driver's side headlight. I was able to open the small access panel beneath the head light to see the module and get a part number from it. However, I think that the plastic cover will probably need to be removed to replace it.
Also, you may want to pay particular attention to the operating temperature of your car while the fans are non-operational. I replaced my condenser for my air conditioning last week (it's sandwiched between the radiator and the fans) because it had a bunch of little leaks in it and the air conditioning would not keep a charge. After I reassembled everything, I noticed that the fans were not running. I am sure the age of the condenser had something to do with its failure, but I can't keep from wondering if the leaks were exacerbated by the lack of fan operation. During highway operation, my engine is cooled fine by the ambient air, but when I get into slower driving, I notice that the engine temperature rises about 10 degrees C. In reading other threads, my understanding is that if the condenser is not being cooled properly, (i.e., by the fans) it may vent the refrigerant when it heats up, thereby affecting operation of your A/C.
Once I get my fan control module in (I anticipate sometime early next week), I will let you know whether my fans operate correctly. I would be happy to take pictures when I replace the fan control module, so let me know if you need them to help you along.
Well, I put the new fan control module in and it was NOT the problem. The problem is actually the fan motor. My dealer told me that a new fan was $200. Aftermarket fans were only about $50 cheaper, so I am just going through the dealer to get a new one. Hope this helps!
My operating temp doesnt go above 100C unless I am idling in traffic for an excessive amount of time. Given that I am in So. Florida, I am quite impressed that I have made it through and avg temp in July of roughly +90F temps, without any issues with the operating temperature.
I have noticed that the actual engine fan doesn't seem to be pulling very much air, so I am going to change the fan clutch to see if that helps too.
Can anyone actually tell me when the Aux fan is supposed to engage? I would assume it is based on engine temp. but I could be wrong.
Aux Fans and Fan Conrtol module 
Linear Mode
