You can also buy a cheap variable resistor [ Radio Shack has what they call trimmer resistors.. with a screw that turns to change the R value.]
Get a 1K one and put it in series with a regualr Ik resitor. That give you RT of 2K to start with. [ you know that the mod can be anywhere between 1.1K and 1.8K , so you covering the entire Mod. range with this set-up..now you can tweak it to your likeing ]

So, if you bridge that 2K across the sensor circuit and get the car to the temp you decide you want high fan to trigger , back off [ drop R value] the variable R of your test rig until the fan cuts-in , that is your R factor.

Now , just take the R rig off and measure that Ohms value..that is the Resistance you want in the permanent resistor for the mod.and that value will trigger fan at the temp you have chosen. [ like I said before , don't make the mistake of going too low...many love to think they have done so well with there low cut-in w/o any regards to the consequences]

I am also an advocate of making sure that the cars temp gauge is Accurate, b/c you are using that for temp reference in all this stuff/experiments
That is the single wire sensor... and if you place a 38ohm resistor in the sensors place [ to ground], the guage should read exactly 100c.
That assures temp accuracy before the fact.............

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A Dalton