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I hope this post can help others that experience the same issue - the engine cooling fan (a.k.a auxiliary fan) on my 2001 S500 would not stop running, even with the key out. I came out of my house after leaving the car sitting for 90 minutes and the fan was still running at full speed. The only way to stop it was to disconnect the battery (also could have pulled the fuse I guess; battery was easier to get to).
After much research on this excellent forum, and various other sources, it seemed to me that the fan control unit was at fault. WIS was useless in the case, I couldn't find any reference to the unit at all. Nor could I find a way to test the unit with SDS. No faults were shown (at least none that seemed to point to the fan control unit). However, there is indeed a fan control unit and on my 2001 it's located just above the horn and the airmatic compressor on the passenger's (right) side of the car. See pic for the unit in question. To get to it you'll need to whip off the lower right engine cover under the bumper; it's quite easy to then reach up under the car to remove the horn and get to the fan control unit. Note that later cars (2002 onwards) do not have the unit in this position; it's connected to the fan shroud (not sure if you can get it as a separate part). I know this because I looked at three 2002-onwards S430s at the junkyard today after luckily finding the unit I was looking for on a junked 2001. The junkyard charged me $9 for the part - came back home, fitted it, problem solved!
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
