There is a resister mounted in the lower area of the fan shroud.It's the week link with that fan.That's probably your problem.I think it's about $30.00 from the dealer.Not the easiest to get at but not that bad either.Good luck. Have a 86 635csi, the aux fan works if hot wired, replaced both high & low temp switch today and still does not work (both fuses checked out o.k.)even though the temp gague shows the needle in the middle. Wondering if anybody out there has any suggestions to the problem. Thanks .