15. Then we fired up the car and let it come up to temp so we could adjust the potentiometers. car reaches 180 Since we have a 190-degree thermostat in the engine, we set the Fan Start On to 170 degrees and the Fan Full On to 210. When our car reaches 170, the fan will come on at 50 percent power, and if it reaches 210, the fan will spin at 100 percent power. Because the unit is adjustable, if these settings don’t keep our temp consistent we can just adjust the settings till it’s perfect.