Not all PSU/Motherboards offer fan control as a device.
It could quite literally be a fail safe as the fan controller on the motherboard has died so they are always running at 100%. Do they ever slow down when the computer is at low load (when they are not needed at full speed as the computer uses less power)?
Originally Posted by andreasaspenberg View Post
the power supply needs cooling even when the pc is off or else it will overheat.
My old pc melted due to overheating when not turned on
Now I use a separate fan to cool my current pc when turned off
For both your interests... Yes your PSU can burn out when the computer is not running. However it should not require active cooling when not running as it will be switched into standby power mode which only consumes a few watts at most (usually less, passive cooling is more than sufficient).
So how did your PSU burn out? Well just because it is off does not mean it is not energized. So you can start the computer it must always keep the motherboard energized which means some mains power must flow (usually Watt or less in standby mode). To convert from the 240V 50/60 Hz mains to a low usable voltage it passes the current through large
electrolytic capacitors which serve to help make the device emission friendly (decouple bad digital noise from mains) and keep it running when under high power load (how your computer can keep running when the lights dim for a moment). The obvious problem, however, is the same as CRT TVs suffered from. Electrolytic capacitors suffer rather bad failures which can result in high temperatures, hazardous smoke, fires or even minor explosions. Although a standard PC PSU is not as bad as a CRT and there are safety measures such as the metal shielding case, it still does not stop it from happening from time to time. I have personally experienced at least 2 PSUs dying, one as recent as last year and each has various symptoms from an alarming smell to horrific noise. They usually fail when turning into active mode as that turns on all the high current circuitry.
However these failures are rare and mostly safe thanks to their design (especially in big cases that are well ventilated). Problems start when you use them outside their operating conditions. These include letting them overheat (dust or ambient temperature), incorrect operating voltage (voltage surges from lightning, imbalanced phases from poor power infrastructure, incorrect mains voltage (not supported by PSU)) or overload (hardware fault, short circuit, etc). Under these conditions the PSU might be more likely to fail.
Anyway back on topic. I do remember having a similar problem to the topic creator except inverted (the PC did not boot but all the fans started if I started the PC while still shutting off). I have not had this since I changed the PSU but I think you ruled this out by changing it already. Check the motherboard specifications and make sure the fan controllers are wired correctly.
