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You cannot adjust when the fan comes on, that is computer controlled. You should check the thermostat by putting it water and boiling the water. See at what temperature the thermostat opens up. The thermostat should open up at 180 degrees. If it does not open up at 180, you need to replace it.
If you want to adjust when the fan comes on you need to purchase an adjustable cooling fan controller as seen in this LINK