need a 12v switch on at 2volts or higher.

 I am looking for a way to switch a 12 volt power source (low amps to a fan relay) that will switch on at  2 volts or higher and turn off at 1.99 volts or lower. I need to read the air intake temperature sender on a superchargerd vehicle and turn on fans when the temp hits approx 112F(2 volts). I want to wire the fans this way instead of a manual switch or having them on at ignition. Any suggestions?

http://www.bcae1.com/opamp.htm   Midway of the page  Using an op amp as a comparator.

That is exactly what I am looking for. I found this kit which seems to be the same design as the one you suggested.

http://www.jaycar.com.au/productView.asp?ID=KC5377

I just have to figure out which resistors to use. If my input voltage is digital (0-5v) 2 wire, and I want to switch on at 2 volts, I guess my comparitive voltage would be zero?

I have not built a circuit since high school about 22 years ago, so please excuse my terminology.

No, the output flips when one input exceeds the other. There are dedicated Comparators as well - eg, LM311; or duals like LM319, LM393; or quads like LM324, LM339. Typically ~$2 from Jaycar, and they may be easier to use than OpAmps (eg the quad LM3900 etc). Not that I checked power supply options - you'd need a single supply device (eg, up to +15V etc) - not a dual supply (+15V & -15V & 0V). The actual circuit can vary - eg, using a voltage reference for ultimate accuracy, but I'd recommend a regulated supply - say 5V up to 8V (to handle supply dips to ~10V) with protection (ie, via a resistor and zenor to remove spikes). You then input your 2V (say via a resistor with a diode to each rail to limit spikes, and probably a cap for filtering) to whichever input (+ I think), and the - input goes to a resistive divider - eg, a trimpot so that the switching point can be adjusted. The trimpot could be a 10-turn job... Or an ordinary (trim-)pot with a resistor above (to the +V supply) and another to gnd (0V) to increase the sensitivity of the pot. EG - a 5V supply thru a 27k resistor thru the 10k pot thru 15k to GND means adjustment between ~1.4V & 2.4V. Ordinary tolerance resistors can be used - the pot adjustment compensates. I've forgotten how to add the hysteresis, but as I recall, that was simple. And I'd suggest an open-collector output comparator so that you can switch +12V relays etc (ie, you switch the relay ground) - and that output has to handle the relay coil current. (The output should have a protection diode for coil spikes) That should cost under $10, plus the labour. Maybe the "expensive" Jaycar kit isn't that expensive after all? Not that I checked its circuit, but it looks like what I just described... Use its relay to drive your fan relay if its relay isn't big enough.