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 Post subject: Motherboard not showing new fan's RPM
PostPosted: Mon Sep 19, 2016 2:20 pm 
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It seems my Scythe CPU fan (SY1225HB12SM-P) broke this evening, as I noticed it was very loud and when I checked it was running at full 1550RPM. I checked in the BIOS and the settings were as they should be but for some reason the fan was running at full speed regardless.

Luckily I have a few Arctic F12 PWM PST fans on hand which I bought a while ago, so I replaced the Scythe with one of them but my motherboard isn't able to detect the RPM, even when it's running at full speed, so I've had to disable the CPU Fan monitoring to stop it giving me an error each time I boot. It showed the correct RPM for the Scythe and there's only one way to plug in the fan connector, so I don't understand why it's not working as surely all PWM fans work on the same basis don't they?

I think the fan will still speed up as the temperature increases, as adjusting the fan profile in the BIOS changed the speed, so my CPU shouldn't overheat but I still find it disconcerting not being able to monitor the RPM.

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 Post subject: Re: Motherboard not showing new fan's RPM
PostPosted: Mon Sep 19, 2016 3:51 pm 
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Location: Guatemala
Have you tried the CPU_OPT? or another header? maybe the CPU_FAN failed?

Have you installed AI Suite / FanXpert2 to see if you still cant see the rpm or control it?

Asus mobos also have switchable headers, now a days all headers are switchable, but on yours, the CPU_FAN is switchable, enter the bios go into motoring, then go into the CPU_FAN, then look for Q_FAN, check if its AUTO (PWM) OR ADVANCED (DC).

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 Post subject: Re: Motherboard not showing new fan's RPM
PostPosted: Mon Sep 19, 2016 10:49 pm 
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Joined: Fri Aug 05, 2005 3:29 am
Posts: 1680
Location: UK
doveman wrote:
Luckily I have a few Arctic F12 PWM PST fans on hand which I bought a while ago, so I replaced the Scythe with one of them but my motherboard isn't able to detect the RPM, even when it's running at full speed, so I've had to disable the CPU Fan monitoring to stop it giving me an error each time I boot. It showed the correct RPM for the Scythe and there's only one way to plug in the fan connector, so I don't understand why it's not working as surely all PWM fans work on the same basis don't they?
The last time I used one of these, which was some time back, I found that the Arctic fan had a separate yellow cable for RPM sensing. This had to be plugged into the end of the PST fitting on the fan cable to feed a speed signal into the motherboard. From what I remember not doing this caused exactly the scenario you described. I don't think the newer ones are like this. It may depend on how old your version is.


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 Post subject: Re: Motherboard not showing new fan's RPM
PostPosted: Tue Sep 20, 2016 12:35 am 
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Posts: 864
Location: London
Abula wrote:
Have you tried the CPU_OPT? or another header? maybe the CPU_FAN failed?

Have you installed AI Suite / FanXpert2 to see if you still cant see the rpm or control it?

Asus mobos also have switchable headers, now a days all headers are switchable, but on yours, the CPU_FAN is switchable, enter the bios go into motoring, then go into the CPU_FAN, then look for Q_FAN, check if its AUTO (PWM) OR ADVANCED (DC).


I don't think the CPU_FAN could have failed. The RPM control and the RPM monitoring both work, it's just that the control but not the monitoring works with the Arctic fan and the monitoring but not the control works with the Scythe.

I run HwInfo64 and that doesn't show the CPU_FAN RPM (in fact it hides the CPU_FAN readout completely) with the Scythe.

I did try switching the CPU_FAN from PWM to DC when I was trying to fix the Scythe and found it ran slower/quieter on DC, which seems to confirm that the Scythe's PWM function became faulty somehow. It's definitely back on PWM now I've installed the Arctic though.

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 Post subject: Re: Motherboard not showing new fan's RPM
PostPosted: Tue Sep 20, 2016 12:52 am 
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lodestar wrote:
The last time I used one of these, which was some time back, I found that the Arctic fan had a separate yellow cable for RPM sensing. This had to be plugged into the end of the PST fitting on the fan cable to feed a speed signal into the motherboard. From what I remember not doing this caused exactly the scenario you described. I don't think the newer ones are like this. It may depend on how old your version is.


Thanks, that appears to be problem. I did buy them at least a year ago and maybe they were old stock even then. It does have an extra yellow three pin cable, which the leaflet refers to as "3-pin power plug" but looking at the connector more closely it only has a single wire connected, so it can only be for the RPM monitoring. The leaflet doesn't suggest plugging this into the extra male 4-pin (PST) header, which it describes as being for daisy-chaining more than one fan from a single motherboard header but instead shows as an "optional" step, plugging the three-pin plug into "the 3-pin AUX FAN scokets on the motherboard in order to read the fan speed" but it should work either way so I'll try plugging it into the PST header.

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 Post subject: Re: Motherboard not showing new fan's RPM
PostPosted: Tue Sep 20, 2016 1:48 am 
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Joined: Fri Aug 05, 2005 3:29 am
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Location: UK
It does need to be plugged into the PST connector, or the last PST connector in the chain if several are connected together. You should definitely not use one of the other motherboard fan headers. The advice to do the latter was incorrect. The motherboard CPU PWM fan header needs a speed signal as supplied in this instance by the separate speed sensing cable. In the absence of the fan speed signal it either generates a CPU fan error warning and/or continues to increase the PWM duty cycle until it reaches 100%. This feedback circuit was designed to deal with stalled or stopped fans. But no matter you have obviously got it working.


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 Post subject: Re: Motherboard not showing new fan's RPM
PostPosted: Tue Sep 20, 2016 4:20 am 
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Joined: Mon Jan 29, 2007 10:30 am
Posts: 864
Location: London
lodestar wrote:
It does need to be plugged into the PST connector, or the last PST connector in the chain if several are connected together. You should definitely not use one of the other motherboard fan headers. The advice to do the latter was incorrect. The motherboard CPU PWM fan header needs a speed signal as supplied in this instance by the separate speed sensing cable. In the absence of the fan speed signal it either generates a CPU fan error warning and/or continues to increase the PWM duty cycle until it reaches 100%. This feedback circuit was designed to deal with stalled or stopped fans. But no matter you have obviously got it working.


Yeah thanks, it's working fine after I plugged that lead in. It actually seemed to get faster afterwards (certainly one of the fans was spinning faster as it was a lot louder) before slowing down again after about a minute and settling around 1,030RPM. It still seems louder than without the RPM lead though, so I don't think it was running at 100% without that lead.

Anyway I just need to tweak my profile a bit, as I obviously set it up for the Scythe and the Arctic is a bit faster/louder than the Scythe at the same percentage/voltage. It'll probably be OK around 800RPM.

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