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Fix for PWM Fans Not Running at Full Speed on Legacy 3-pin Motherboards - Fix for PWM Fans Not Running at Full Speed on Legacy 3-pin Motherboards
Sun Jan 21, 2007 | 6:11P| PermaLink
If you're using heatsink with a 4-pin Pulse Width Modulation (PWM) fan on a motherboard that only supports 3-pin fans, there is a chance the PWM may default to its lowest possible speed. This can lead to the CPU overheating, as the BIOS is obviously not communicating with the fan and tell it to speed up as temperatures rise.

The solution to make the 4-pin PWM fan spin at its fastest speed is not too difficult...

Notice: While we have tested this tweak out, we cannot guarantee this method will work with all PWM fans. Furthermore, given the variety in construction and design of 12V DC fans, attempting this tweak may result in damage to circuitry and may invalidate the fan's warranty.

First find a separate length of wire. Use that piece of wire to bridge the yellow 12V+ (P2) and blue PWM (P4) wires coming from the PWM fan together. Once this is done the fan will always spin at full speed. Remove the wire and the fan will return to its default speed setting.

4-pin PWM fan power jacks pin diagram

  • P1 (Black) - Ground
  • P2 (Yellow) - 12V+
  • P3 (Green) - Tachometer signal
  • P4 (Blue) - PWM signal

For some reason, there is a complete lack of 4-pin PWM fan manual fan speed controllers in the marketplace right now, and that can create problems if you're using a PWM fan heatsink with a motherboard that only supports 3-pin CPU fans. It's been our experience that when most PWM fans are plugged into such a board, the PWM fan either runs constantly at full, minimum, or some medium speed. Unless it's thermally responsive, you end up being stuck with whatever default speed the fan decides to run at. If the PWM fan runs loudly, a 4-pin PWM fan manual fan speed controller would really come in handy to tame the noise....

In the FrostyTech labs we tweaked this Zalman FanMate 1 fan speed controller to accommodate 4-pin PWM fans, and rigged it with a hard switch to facilitate changing between the full speed and default speed mode of a PWM fan by bridging the +12V (yellow) and PWM signal (blue) connections. When the tweaked Zalman FanMate 1 is set to "full speed on", it has the added benefit of allowing us to dial in the fan speed manually, by varying the voltage to the fan as one normally would with this device (~6V to 11.5V).

Some PWM fans will vary their speed accordingly with the voltage change, others not as much. It largely depends on whether the PWM fan defaults to a minimum speed or not. Clearly, this is a very basic work around in the absence of true PWM-fan manual fan speed controllers, as the FanMate 1 contains no pulse width modulation circuitry. When the switch is left off, the PWM fan runs as if it was connected to any standard 3-pin fan header.


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