Question CPU Fan Not Working

Crag_Hack

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Dec 25, 2015
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Hi I have a situation a little more complicated than the title might suggest... I am fixing up an old computer. I noticed under stress the CPU temperature spiked to 85 C almost instantly. I opened up the computer and noticed the CPU fan was not spinning. I tried hooking up a 3 pin non-PWM fan to both fan headers on the mobo and the fan worked for both. The current 4-pin CPU fan however doesn't work on either when the computer boots and BIOS reports fan speed as N/A. I tried plugging the current CPU fan into another 4-pin PWM header on another mobo and it spun up right away. Does this mean the mobo's PWM functionality might be busted? I loaded BIOS defaults and toggled Smart Fan on and off with no results. I actually just found a spare 4-pin PWM fan I forgot about while writing this that I'll hook up later tonight or tomorrow and see what happens. Thanks!
 
I loaded BIOS defaults and toggled Smart Fan on and off with no results.
if there was an issue with the PWM control you should still be able to set a constant % with PWM control disabled.
if available, try changing it to DC mode and see if the original fan spins up at all.

also try setting a manual PWM fan curve starting ~50%.

and do the fan blades jump around at all when the system is powered on or just stay dead?
 

jasonf2

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Oct 11, 2015
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Header might be toast. If you can find a 3 pin replacement just ignore the CPU RPM in bios (to avoid post error) and match CPU curve on the fan on the other header and use it. It will be voltage control rather than PWM but it won't matter.
 

Crag_Hack

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Dec 25, 2015
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Thanks guys. I plugged in the spare 4-pin PWM fan and it worked in both mobo fan headers. Peculiar that the original CPU fan worked on the other mobo 4-pin header but not the original mobo 4-pin header. Should I bite the bullet and order a replacement fan? Any other tests I can do that would be prudent to do? Thanks again.
 
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Crag_Hack

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Dec 25, 2015
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It just occurred to me the power specs might be different. Turns out the original fan is 12 V .36 A and the spare is 12 V .14 A. Might this have an impact such that the spare fan works but the original doesn't?
 

Crag_Hack

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Dec 25, 2015
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Hi again, quick question - this CPU fan is one that goes on top of the heat sync. I just want the fan to blow air onto the heat sync instead of pulling air away from the heat sync right?
 

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