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Answer to: 4 wired fan 5V 0.32A, which resistor to use for 3V?Sorry I do not know much about mac products but on a pc fans with 4 wires, one will be supply, one is a signal, one is ground and the other is what's called pulsed width modulation which is used to give you a wide range of fan speed as opposed to 3 wire which only gives you two settings. I am also going to assume you used DVOM to measure this and not an oscilloscope. Either way the green would be the PWM wire. The way PWM works is it counts the amount of +5v-0v in a given time period so unless your DVOM has a frequency counter you can not measure this accurately. Also you cant change PWM with a simple resistor so you can try the grey or brown wire but I wouldn't bet on it. How do you know the temperature sensor isn't working? Sorry this such a long answer for so little result and let me know if it works with the other wires.