Corsair AF120 fan noticeably slower than the other one.

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Thrashtildeth92

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Mar 8, 2016
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I recently built a machine using an NZXT s340 case. This came with two stock fans wired into a molex connector that then connects to a molex connector on the power supply. I changed these out for two Corsair AF120 and since I built my machine, the top one appears to be running flat out, but the rear one is noticeably slower. They only had three pins each so I assumed they can't really be controlled or monitored from the motherboard headers, so I connected them to the power supply for a cleaner look like the originals. Anyone know what could be the problem could be? The slower one isn't making any bad noises or fluctuating with speed.
 
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Generally three pin fan connectors CAN be monitored for speeds and controlled via voltage, 4pin connector fans are PWM controlled and monitored for speeds (hence the extra wire) since that drives the PWM relay transistor. Probably the best way to monitor the speed and see if indeed they are running at different speeds is to properly hook them up to the motherboard headers and see what your BIOS reports the fan speed as.

That being said, there is some variation in fan speeds even between what would be 'identical' fans due to simple tolerances, minor misalignments resulting in a smidgen more friction in one fan than another, heck, even a longer wire can cause a voltage drop in a fan which isn't PWM controlled.

For a really cool article...
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Rookie_MIB

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Jul 27, 2013
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Generally three pin fan connectors CAN be monitored for speeds and controlled via voltage, 4pin connector fans are PWM controlled and monitored for speeds (hence the extra wire) since that drives the PWM relay transistor. Probably the best way to monitor the speed and see if indeed they are running at different speeds is to properly hook them up to the motherboard headers and see what your BIOS reports the fan speed as.

That being said, there is some variation in fan speeds even between what would be 'identical' fans due to simple tolerances, minor misalignments resulting in a smidgen more friction in one fan than another, heck, even a longer wire can cause a voltage drop in a fan which isn't PWM controlled.

For a really cool article on cooling fans:

http://www.analog.com/library/analogDialogue/archives/38-02/fan_speed.html
 
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