I've had three 850s and they have all had this problem to some degree. I simply take the blade off the arm, slip a good sized box end wrench over the arm and slightly bend it several places along the length of the arm. If you want to remove the arm, you can do this in a vise also.

This may be a redneck solution to the problem but it will work. Just by-pass the relay and temp sensor altogether and have the fan run all the time. Just make sure you run the power through a switched fuse so that it only runs when the ignition is on. Radiator fans used to run all the time before th...