After installing a twin cam radiator in my S2 (the price was right), I noticed 
the TC temperature set point for the fan to come on was higher than the S2. I am
in Houston, and want the water temperature to stay at or below 90C if possible 
(it would be even better if the summers here stayed that cool). 
Instead of trying to adapt the S2 otter switch (they have completely different 
mounting arrangements), I gutted the old TC otter switch and installed a 
thermistor as the sensor for this circuit.  

The schematic is fairly basic, and it allows you to independently set the temp 
at which the fan comes on, and off.  It works well, but I do notice the 
relay sometimes chatters a bit as it turns on, but it seems minor and I haven't 
worried about it.  If someone has any ideas why, please let me know.

It does not affect the manual operation of the fan, as the tranistor drives the
existing relay.  This would also work on a TC, but you will have to add a relay,
 or a higher current transistor.

Any questions, just email me.

Tom

1/03 Update:  Thanks to Neil Slavin, the little bit of relay chatter has been eliminated.
It is not shown in the schematic, but all you need is to add a 10K feedback resistor
fromthe cathode of the SCR to pins 3 and 5 of U1.  Neil also suggested adding an electrolytic
capacitor to the input for protection.