Odd Behavior of CPU Fan

John Hixson

Dec 13, 2010
I have a Core i7-930 being cooled by air with a Prolimatech Megahalems, RevB and two 120mm fans in push/pull configuration. The push fan is a three-wire fan connected to the CPU fan header on the motherboard. The pull fan is a four-wire pwm fan connected to an Opt fan header near the mb i/o plate. The odd behavior I'm seeing involves the pull fan. There are times when this fan is not running at all even though I don't believe there is any speed modulation connected with the Opt header--it should be running all the time. The push fan does run all the time. If I rap the pull fan housing lightly with a small metal tool or flashlight, the fan starts spinning and will remain that way until the machine is put to sleep or shutdown. What in the world could be causing this fan to require a light rap to get it going?? Anybody have any ideas on this? Thanks. John Hixson


Sep 7, 2010
i think that mobo header is not supplying enough voltage to the pull fan... could be that the bios fan controller is controlling the voltage in-order to control its speed... it could help if you can state your mobo model and your fan's model also...

IMHO the PWM fan should be connected to the cpu fan header so that the speed will vary according to the temp (provided your mobo have PWM control) and the 3 pin fan to any other fan header in your mobo... if you have two different fans then the ideal push/pull config is to have a slower fan push the air and a faster fan pull the air... it doesnt make sense to have a faster fan push air through the cooler and a slower fan blocking that air flow... So in your case connect the PWM fan to the cpu header and the other fan to another power source...

You can also control the speeds of your fan using speedfan... see this topic on how to use speedfan to control the speeds of your fans...
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