Hey guys...try this one for your fans.
A company called Micro Switch makes a switch usually used for processing eqipment and the like, it is a amall rectangular cast alum box with a tightly wound flexible wand poking out...the idea is that whatever the application the product, or a machine part can touch this wand...this unit when mounted under a chassis works really well, it has a normally open/normally closed set of contacts depending on how you wire it, it is sealed and can handle about 20 amps...not quite enough for a fan I think, but plenty to pull in a relay.
I have used this setup in conjunction with a relay manufactured by Square D which has a pnuematic delay, from 5 to about 45 seconds, totally adjustable in between, the whole setup is almost foolproof in the sense that these are industrial items and they really hold up well.
You can shorten the wand to get whatever speed cutoff you might need, most production cars shut the fan at about 40 to 45 mph through the ECM, the fan will actually restrict air if it is left on.
Good luck
