Question Computer won't boot if case fans set below 60% pwm in bios

spenceweiss

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Nov 7, 2018
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Here are my specs:
MSI X570 Tomahawk Wifi
5800x3d
corsair vengeance ddr4 16gb
MSI 3080 Ti Gaming X Trio

I recently purchased a 5800x3d along with a noctua cooler and fans to go with it. I now have two nf-a15 chromax fans as intake (set to same pwm header). 1 nf-a12x25 exhaust fan, and a nh-d15s (with two nf-a15 chromax fans). However, when I set the intake fans to less than 60% pwm speed in the bios fan curve, the computer won't fully boot and gets stuck on the EZ Debug CPU light. When I reset CMOS by removing the battery everything works fine again, and boots no probem, but as soon as I set those fans below 60% in the bios, the CPU light comes on again and the same thing happens. Does anyone know what is happening.
 

kanewolf

Titan
Moderator
Here are my specs:
MSI X570 Tomahawk Wifi
5800x3d
corsair vengeance ddr4 16gb
MSI 3080 Ti Gaming X Trio

I recently purchased a 5800x3d along with a noctua cooler and fans to go with it. I now have two nf-a15 chromax fans as intake (set to same pwm header). 1 nf-a12x25 exhaust fan, and a nh-d15s (with two nf-a15 chromax fans). However, when I set the intake fans to less than 60% pwm speed in the bios fan curve, the computer won't fully boot and gets stuck on the EZ Debug CPU light. When I reset CMOS by removing the battery everything works fine again, and boots no probem, but as soon as I set those fans below 60% in the bios, the CPU light comes on again and the same thing happens. Does anyone know what is happening.
It is probably whatever fan(s) are connected to the CPU fan header. Motherboards are very sensitive to having good RPM values for the CPU fan.
 

spenceweiss

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Nov 7, 2018
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I have the two cpu fans on minimum of 40% (and higher for higher temps). I don't know why that would be the issue as I don't have a problem with cpu temps, but I'll try it. It might be because the max rpms on the nf-a15s is 1500rpm so it might read 40% as being low, though that doesn't explain why it still works when the case fans are set higher but not lower.
 

Karadjgne

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Are you positive on the fans? The A14/A15 do come in a 3pin DC version, and Chromax. In bios Auto is also sane as DC, regardless if a PWM fan is used. Resetting bios can/will change that PWM to Auto/DC.

If you set a PWM fan to 60% in DC mode, you supply it with @ 7v instead of the wanted 12v, and that becomes an issue, the safety mechanism in the cpu will see that as a cooling issue and refuse to boot. Many DC 3pin fans won't work at below 60% for the same reason, their range being 7v-12v.

Noctua pwm fans will generally drop to 20% duty cycle ±%
 

Karadjgne

Titan
Ambassador
Hmm. Cpu_fan header is always default PWM, for as long as I can remember, that's the only header which is, ever since vendors swapped to 4pin years ago. So those fans should not be an issue at anything from 20% plus.

But it's the intake fans, which are on Sys_fan header, which has zero bearing on the cpu at all. It's an unrelated header, doesn't require population at all.

I'd update the bios. That sys_fan header should have a different address to the cpu_fan header, if the bios is glitched its possible its using a single address for both, or reading something wrong, has the addresses backwards etc.

The intake fans are in a sys_fan header, not a pwr_fan or pump_fan or cpu_alt header?
 

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