I'm currently planning on using this temp sensor because it's cheap, and it comes with adapters.
http://www.summitracing.com/parts/ATM-2385/Although I like how this one has a separate ground, and comes with the pigtail.
http://www.summitracing.com/parts/MSD-2310/I should be able to make the AC input work by either grounding or applying 12v to it
A delay on AC turn on/off is easy as well. I need to research the trinary switch a little more...
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If there is no temp signal when the key comes on you go to 90 percent?
Correct; I don't see any other way to do it. It would basically act as a failsafe; fan always on. I can add a short delay as well, so it doesn't come on as the key goes through ACC to start.
Feets has a fan on the way to me I'll test it with extensively, then I'll send a controller back to feets for him to test.
I'm still aiming for an end cost of ~$50ish, probably without the linked temp sensor.
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oh, and the led if nice, but I guarantee you can hear that fan when it comes on. you will know.
Heh, true 
It's an easy feature to add though 
