Fans turning off and on constantly

[Vekkao]

Okay so this problem just arose today when I left my computer on to download something, and I came back to it completely off. I thought it was weird, so I turned it back on, and I was looking at the fans, and they started turning off and on, constantly. Every single fan shut off completely, including the LED's in the fans. Then a couple seconds later, they'd turn back on. Then again and again this same thing. Though, when I go into my program I use to adjust my fans speeds and what not (AI Suite 3 for me) I set the fans to Standard speed (which they were already on anyways) and for some reason, it's then perfectly fine. I've been on it for at least a couple hours now after setting the fans like this, and nothing at all has happened, no computer turn offs, no fans messing up, though I'd like to know if this is a serious problem, as it happens every time I turn the computer off then back on.

 

[Shaun o]

Well as part of a test with some motherboards, depending on the brand, and model number of the motherboard.
When you first power the system on the bios tests the fans in two aspects.

1. To see if they all are in fact spinning, tested at the lowest rpm speed, no fan faults, not working.
2..Or when you first turn your system on the fans rev up then settle.

The second test is to find out the rpm rate capability of each fan in the system then it sets up a table in the bios so it can control the fans via PWM settings. Power With Modulation. some call it the power fan curve.

It`s calibrating the fans in short. the higher the rpm, the higher the voltage applied to the fans from the motherboard controlled by the bios.

You should have manual settings in the bios for cpu fan, and each chassis fan if all connected direct to the motherboard.
You may also have a fan alarm setting in the bios where if a fan is running bellow a set rpm it triggers an alarm posting of the system is stopped. and reset.

If the case lower the rpm setting in the fan error warning section of the bios to a lower rpm setting or disable the feature in the bios.
To avoid a constant reset. Or set each fan to a profile in the bios. choosing High, Medium, or low as the settings options.

 

[Vekkao]

So... This is normal then, is what you're saying? Because this hasn't happened before, I put the computer together a couple to a few months ago myself, so I'm just worried that I messed something up, though I don't think I did as everything besides a couple things I had to fix, it's ran perfectly. This is the first time this has happened though, but I restarted it more than once and it did the same thing. Though like I said, when I adjusted the fans myself, it worked just fine. I wasn't there when my computer turned off earlier, but it turned off all on its own (I don't have automatic updates on so it didn't restart for that reason or anything) which is what concerned me the most, probably because this started happening when I was away, though I can't be sure.

They're spinning just fine, LED's working as well, everything goes smoothly, then they turn off. Then on, then off. Never settling, it's just kind of a never ending cycle until I choose to manually set the fan speed myself. The RPM of the fans is pretty low on idle, though not too low, around 500-600 RPM. I have it so it sets automatically depending on what kind of resources my computer is using up.

 

[Shaun o]

Because if you look in the bios of the motherboard if it has fan monitoring and error detection.

And the alarm trigger is set to trigger at a fan spinning bellow say 600 rpm it resets the board in a constant loop because it thinks the fan in effect is broken.

You lower the value in the trigger. Or disable the fan error alarm.
Or set the profile of each fan in the system in the bios profiles to stop the looping caused buy a fan rpm spinning bellow the alarm trigger point set in the bios. sometimes with age fans begin to drop in rpm speed, due to dust build up ect.

But when the system freaks out each time you shut it down completely, then restart it.
The reason why is due to the fan error detection in the bios and one of the fans spinning slightly bellow the trigger point set.
so it performs a reset of the mobo, or on some boards halts the booting process to windows.

It depends on the mobo, but is normally the cause to looping or halting rpm fan error to low.

 

[Vekkao]

But this just started today, and the fans haven't given me a single problem until this, so why would it suddenly start from nowhere, when nothing has changed at all? Is there a way to actually prevent this from happening other than manually setting the fan speed myself every time on startup?