Question iMac fan question

Dec 24, 2019
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OK so I have a windows PC that has a GTX1060 6gb Turbo graphics card which has a super annoying fan. After I ordered some replacement fans that had the same specs but different brands, the noise (continuous clicking/vibrating sound) improved slightly but not enough. So I thought I should try some fans used by Apple in hopes they're better quality (quieter). This would require some mod to the GPU but I don't mind that. I ordered three iMac fans from Aliexpress (two for the 21.5" iMac and one for the 27" model which has a larger fan). Since the fans wires are all black I didn't know the connector pinout but managed to see some letters on the fans PCB so I soldered a GPU compatible connector and tested the fans. The two smaller fans (for the 21.5 model) work properly but the large one is just going at max speed.

So my question is: could it be that the fan is faulty or it simply doesn't know how to communicate with the GPUs PWM circuit? Maybe the fans PWM circuit is damaged? I have a digital multi meter, is there any way I could test/measure between the fans pinouts if any sensor is ok or not? The wiring I've done is the same as with the smaller fans that work normally and is as follows: pins from left to right, back of the fan: "+, GND, tachometer(??), PWM" Thanks in advance to anyone taking time to help me out!
 

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