Can't adjust rear fan RPM

I just built a Mini-ITX computer, with an Arctic Freezer i32 cooler.
What's special with this cooler is that it turns off the fan unil the CPU reaches like 50C thanks to a PWM controller on the fan itself, which is handy. Another handy thing is that the cable for the fan on the cooler has a Y-splitter, so that you can connect another fan.
The motherboard (MSI B250I Gaming Pro AC) in this build only has 2 fan headers - one for the system fan(s) and one for the CPU fan(s). In this scenario I have 2 intake fans in the front sharing the system fan header, and a rear fan connected to the Y-splitter on the cooler.

Now, the problem is that this rear fan (also PWM) is going 100% all the time, and I can't adjust it since the PC only can see the CPU fan (a pin is missing on the Y-splitter), and as the CPU cooler is semi-passive it just says 0 RPM.

How can I overcome this, so that I can adjust the speed of the rear fan while keeping the semi-passive function on the cooler?
Reply to steffeeh
7 answers Last reply
More about adjust rear fan rpm
  1. With a "Ysplitter" connecting the rear fan + CPU fan, they should be able to control those as "one" - assuming the header on the board is PWM capable (it should be on B250).

    Assuming all aspects of the chain a 4pin PWN, you cannot control the rear fan and the CPU fan independently - as far as your motherboard is concerned, there's only 1 fan attached.

    Is the YSplitter only 3pin?

    Even if all aspects are 4 pin, the rear fan & CPU fan would need to be of (relatively) similar spec for any control to work.
    Reply to Barty1884
  2. I understand that I can't control them independently, but it feels like adjusting the fan curve on the "CPU Fan" in the PC should also affect the rear fan... however it don't. I guess it has something to do with the extra PWM controller on the cooler.

    I do however have another Y-splitter lying around, what if I switch around and let it show the rear fan RPM instead, perhaps the CPU cooler will still have its semi-passive function while letting me control the rear fan?
    Otherwise I don't think it would be a good idea to run 3 fans on the system fan header, might have to check with support beforehand so I don't burn the fan header by drawing too much current.
    Reply to steffeeh
  3. It's going to depend on the fans specifically.

    A very extreme example:
    Rear fan = max 5000rpm*
    CPU fan = max 1500rpm

    If those are running off the same Y Splitter & header - controlling the 5000rpm max fan to run at, say 1500rpm would necessitate a 3500rpm/70% reduction in speed.
    IIRC, it's actually the "speed" reduction, not the % reduction.

    So, in that example, the 1500rpm fan would be reduced by the same "3500rpm" and wouldn't run at all.

    *I know that's unrealistic and extreme, just making a point.
    Reply to Barty1884
  4. The problem might lie in rear fan. While your CPU fan can be set to 'zero' speed, that is not necessarily true for rear fan. The effect might be that while CPU fan is not spinning, the rear fan cannot go to so low RPM, and instead goes to full speed.
    Reply to DRagor
  5. Turns out the rear fan was after all regular DC. I've switched so the PC reads the rear fan and run them both in DC mode for the moment - can PWM fans be run in DC mode without any additional degradation or so over time? For now I lose the semi-passive functionality, but it's near silent at low RPM anyway. The two fans are pretty close to eachother in RPM range.
    Reply to steffeeh
  6. So, the rear fan is a 3-pin unit and cannot be controlled by the CPU_FAN header that uses PWM Mode. To answer your last question, yes, the PWM fan that came with the cooler CAN be controlled by using DC Mode, although that is not ideal.

    What would be ideal, though is to have the CPU cooler controlled by the CPU_FAN header using PWM Mode, and all three case ventilation fans controlled by the SYS_FAN header that can only use DC Mode. To do this you need either a different Splitter - one with three outputs instead of two - or two more two-output splitters. In the latter case, you plug two Splitters into the output connectors of the third one, making a "stack" that converts one SYS_FAN header into four output connectors. NOTE that you need to use SPLITTERS, and NOT a HUB. A Hub is a different device that has a third type or arm that must plug into a power output from the PSU. It works only with 4-pin fans and with 4-pin headers that use PWM Mode, and you do not have that for your application.

    BUT to do this you need to be sure that the total max current drawn by all three case fans does not exceed 1.0 amps load on the SYS_FAN header. For help on that, tell us exactly what those three fans are - maker and model number. I suspect they were supplied with your case - case makers often supply "free" 3-pin fans with their cases. So, if you cannot determine what those fans are, at least ell us what case they came in. Then we can look up the fan current specs. As a related note, most common fans pull less than 0.3 amps each so three on one header is OK. But fans that include LED's often pull more because of those LED's.
    Reply to Paperdoc
  7. The system fans are 1x Fractal Design Dynamic GP12 in the rear, and 2x Be-Quiet Pure Wings 2 140mm in the front

    I switched from PWM mode to DC mode in BIOS so it works currently controlling the rear fan but also affecting the CPU fan.
    I've set it so the CPU fan recieves just too little voltage to be spinning, mimicing the semi-passive function (the rear fan however still spins), and right at 50C it will start spinning
    Reply to steffeeh
Ask a new question Answer

Read More

Cooling Fan CPUs