TechSpot

CPU Fan Speeds up and Slows Down

By digi123
Jul 6, 2005
  1. Hi All,

    I'm sort of new with all of this, but I'm having a problem with the fan on the MB. When I turned on the computer today, I could tell that the fan speed was really high, but then it went really low. Every 15 minutes or so, the fan would try to increase in speed, but go back to being really slow.

    I believe its not the fan on top of the cpu, but the fan next to it. I think its the chipset fan?

    I checked the bios settings for the temperature to be:

    Sys Temp: 32 C / 89 F
    CPU Temp: 60 C / 140 F
    PWM Temp: 44 C / 111 F
    CPU Fan: 0 RPM
    NB Fan: 0 RPM
    SYS Fan: 2900 RPM

    My CPU specs are:

    Abit IC7-G MAXII Socket 478 Intel 875P Intel MB
    Intel P4 3.0Ghz Prescott 800Mhz
    2 Gig DDR PC 3200 (4 512 DDR SDRAM)
    WD 240 Gb HD (2 120 SATA)

    From other posts within this forum, it seems that I need a new fan? I'm not sure which fan is causing the problem though. Does anyone have any suggestions on a fan if this is the problem?

    Thanks in advance,
    digi123
     
  2. Secondgunman

    Secondgunman TS Rookie Posts: 192

    Yes...

    Yes new computers will do that, as temps rise the rpm of the fans rise... But yours seems to be doing it often what motherboard do you have.
     
  3. Secondgunman

    Secondgunman TS Rookie Posts: 192

    Never mind...

    I now see what motherboard you have after I posted.
     
  4. mailpup

    mailpup TS Special Forces Posts: 8,452   +227

    It may not the the fan itself but the fan speed sensor in the motherboard. I notice that both the CPU and northbridge fans register 0 rpms. Well, clearly they are spinning since you can see and/or hear them. Perhaps the fans' power connectors have worked loose somehow so that they have power but the sensors' connection are not contacting.
     
  5. digi123

    digi123 TS Rookie Topic Starter

    Thanks for your response mailpup.

    That could explain why the bios is reading at 0 rpm. But at random intervals, I could see the fan speed up, then slow down again. I was looking at the bios to see if the cpu fan speed changes when the speed up occurs. It says the CPU fan is around 2000 RPM when I hear the fan increasing in speed (ex. sort of like reving up an engine), then when it slows down, the cpu fan speed is back to 0 RPM. But I can see the fan moving in the computer at a slower speed.

    Do you have any idea why this is happening?

    Thank You.
     
  6. mailpup

    mailpup TS Special Forces Posts: 8,452   +227

    It may be that when it registers 0 rpm, there is no power to the fan even though is it spinning - but from momentum only. 2,000 rpms is not that fast so it sounds like it's not getting up to full speed before it quits. It could be a power supply problem to the fans or motherboard, possibly the power supply unit itself. Can you check all the power connections to the motherboard and MB fans?
     
  7. digi123

    digi123 TS Rookie Topic Starter

    mailpup,

    I checked the connectors and everything seems to be ok. I also checked the bios again.. and I was mistaken in my original post. The NB Fan was the one fluctuating between 2000 and 0. The CPU fan stayed at 0 all the time. I'm not sure if this was b/c it doesn't have a sensor on the MB.

    I hear the computer trying to speed the fan up, then slows, and keeps trying again. When it slows down, it never comes to a complete stop. The slower speed seems constant, so I'm not sure if its b/c of momentum.

    I've also seen that the computer doesn't crash at all, just the fan sound is quite loud. The slow speed of the fan sounds almost like its vibrating. Not sure if that helps.

    Thanks.
     
  8. Secondgunman

    Secondgunman TS Rookie Posts: 192

    I would look at...

    I would look at a possible fan replacement, try to find some one with that fan and test it first then replace the fan if there is yar and or play that may mean you have a bent fin, shot bearing, bad bearing shoe, popped bearing, or all of the listed. if that CPU fan goes your system will make like a christmas tree in very little time after and then you are replacing the CPU heatsink as most CPUs now require you to buy the heat sink and of course the fan that died in the first place... the Fan maybe loose aswell so check that all screws and or mounting brackets are on properly it would be a shame to have a fan replaced when the old one worked well to start with it was just loose.
     
  9. mailpup

    mailpup TS Special Forces Posts: 8,452   +227

    It might be the fan. But aren't there two fans that are malfunctioning? Or is the CPU fan ok but just showing 0 rpms?
     
  10. Kaziglubey

    Kaziglubey TS Rookie

    I have the exact same setup as far as Mobo and cpu. My computer started beeping at me today because the cpu fan is only going at 2600 rpm while the chipset (also known as the Northbridge) fan was running at 6500 rpm. Replacing the Power supply didn't do the trick and it was doing the same thing after I took it out of the case and tested it outside of it. Took off the cpu fan and replced it again and it still beeped. The only thing I haven't tried is replacing the fan altoguether, which I'll be doing tomorrow. But by the looks of it, you may need to replace the cpu fan altoguether since it fluctuates and it should not be doing that at all. I'll post again after I replace my cpu fan. =)
     
  11. zephead

    zephead TechSpot Paladin Posts: 2,483

    in your bios, disable fan control and/or set the speed to high. see if that makes the (cpu fan at least) fans run properly.
     
     
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