Boots fine but CPU fan only stutters

[greenkoolaid]

Hi,

I just put a build together last night and I'm having a slight problem. If the graphics card is not plugged into the 6pin pcie cable the CPU fan starts fine, if however the 6pin pcie cable is plugged in, the CPU fan only produces quarter turn rotations every few seconds.

The MOBO is an ASRock H61M-HVS. The graphics card is a Gigabyte Radeon HD 6850. The PSU is a OCZ ModXStream Pro 600W. The CPU is a 2120 with stock cooling fan.

If this was the only behaviour I would assume a faulty PSU which is unable to provide adequate power to both devices, however if you start it once without the graphics card plugged in and power down, the next time you start it with the graphics card plugged in, both the CPU fan and graphics card will operate. This makes me think perhaps it is the motherboard since the PSU is capable of running both the CPU fan and graphics card as long as certain capacitors were charged. I should note this only works on the boot directly after, any subsequent boot with the graphics card plugged in results in the CPU fan stutter behaviour again.

Of course the ideal situation would be to test the system with a known PSU but I don't have one. I was wondering if anyone had a similar experience or a way I could determine the root cause.

Thanks.

 

[Rugger]

Try plugging a case fan into the CPU fan header and the CPU fan into one of the other headers. If you get the same behaviour then it's likely the CPU_FAN header on the mobo.

 

[greenkoolaid]

Try plugging a case fan into the CPU fan header and the CPU fan into one of the other headers. If you get the same behaviour then it's likely the CPU_FAN header on the mobo.

The fact that the CPU fan header works perfectly fine when the graphics card is not plugged in has led me to believe the problem stems from elsewhere.

When I switched the CPU fan onto a case fan header it still resulted in the stutter motion of the CPU fan, the case fan plugged into the CPU header worked fine. This just confuses me further.

EDIT: I should also note that with the switched headers when the graphics card is unplugged both fans will work.

 

[Rugger]

Do you have a molex-PCIe adapter? If yes, then try using that to power the GPU and see if you get the stutter on the CPU fan header. Sounds like some interaction between the PSU, GPU and mobo is causing the issue. If it works with the adapter, then it could be a faulty PCIe plug/rail on the PSU. It's hard to eliminate the x16 slot without another PCIex16 slot on the mobo to test.

 

[greenkoolaid]

Do you have a molex-PCIe adapter? If yes, then try using that to power the GPU and see if you get the stutter on the CPU fan header. Sounds like some interaction between the PSU, GPU and mobo is causing the issue. If it works with the adapter, then it could be a faulty PCIe plug/rail on the PSU. It's hard to eliminate the x16 slot without another PCIex16 slot on the mobo to test.

I've tried the molex adapter as well as another PCIe cable on both of the available outputs from the PSU.

Are you leaning towards it being a MOBO or PSU issue? It seems I simply have to replace one and see if it helps.

EDIT: It appears to be the stock intel fan was having issues, I overlooked this since the fan worked perfectly fine when no graphics card was installed. I'll need to switch it out but that appears to be the very simple solution.

 

[Rugger]

Wouldn't have thought the HSF fan was the issue since you got the same indications when a case fan was plugged in to the CPU header. If it is, then that's great!