Case fans connected to motherboard not working correctly.

I am building my first computer and I am having a problem with the case fans. Sometimes the fans (on High) will not start when the computer turns on, but I find that after a few minutes they will come on. The fans are very low when on "High" setting and stop spinning at all on the "Low" setting.

I have an Antec Three Hundred Two case with 2 included fans, and a Gigabyte GA-B85M-D3H motherboard. The fans that came with the case are 3-pin fans and can connect to my mobo's 4-pin sys_fan ports, and the fans come with a controller on the back of the case for High or Low. High should be around 1200 RPM, with Low being 600 RPM. However, in my BIOS I am getting speeds of ~450 RPM on "High", and the fans come to a stop on "Low".

In my BIOS the fans are set to "Normal", which according to my mobo manual says that this allows the fans to run at different speeds according to system temperatures.

Should I be getting an adapter to connect these fans to my PSU, and then they will be working properly? Or is there something in my settings I should be changing? Or do I have faulty fans.

Thanks for your help.
8 answers Last reply Best Answer
More about case fans connected motherboard working correctly
  1. Hi, First get 3 to 4 pin adaptors, that should solve it. If still the fans are spinning too slow,set in the BIOS the fan speed to high. Currently, they do get insufficient power from the mobo and therefore do not/slowly spin.
  2. I had changed the BIOS setting on the fans before, but nothing changed, but that was my fault because I didn't save and restart. I just changed the fans to "Full Speed" in the BIOS and after a save and restart the fans are now running at their advertised RPM and I can change them from Low to High with the controller on the back.

    My follow up question would be, since it seems like the fans are actually able to get enough power without a 4-pin adapter, was it just that the computer was not running very hot and so was telling the fans to run at only 400 RPM on high, but when switching to "Low" there was not enough power going to the fan to start it? And if my computer was only needing 400 RPM, is there a way I can overwork my computer in a sense, so that I can see if it is actually able to increase the speed of the fans when set to change the speed of the fans based on performance? Because I am unsure if 400 RPM was what my computer needed, or was it as fast as it could go for some reason when set to that option.
  3. There's no need for 3 to 4 pin adapters. A 3-pin connector plugs into a 4-pin header perfectly fine.

    I believe that you have too many things controlling your fans.

    The fan controller on the back of the computer is probably a simple set of resistors that slow down the fans. No auto control. Just plug the fans into a constant 12V connector and get a choice of two speeds.

    You have a second, much better, control system. The motherboard is actually adjusting the voltage to your fan connector, slowing down or speeding up the fans depending on the heat of the CPU. So it's quiet when you don't need a lot of cooling, but the fans can automatically speed up to full blast when things get hot.

    I think you've got BOTH systems working. The auto system reduces the voltage to the fans and the resistors in the fan controller knock it down even more. Not even enough to get the fans started and limiting them to very slow speeds.

    Since you've got the better system on your motherboard, the best solution would probably be to get rid of the fan controller. Just plug the fans directly into the mother board. Or, if it has a "full speed" option, set it to that. Let the motherboard control the fans.
  4. Thanks hwc1954, only problem is the fans are not able to be unplugged from their built in controllers in the case. The fans are already (and have always been) plugged into the motherboard. The fans have a wire going to their 3-pin connector, and another wire going directly into a switch in the back of the case. Each fan has it's own switch. So I'm not sure how to make it not connected to that.

    And is there a way to make my system work really hard for a bit, so I can see if the fan speeds increase from the motherboard? I ask because every time I check the fan speeds when the motherboard is controlling them, they are around 450 RPM, and I just want to make sure that the motherboard is actually able to get them to a higher speed.

  5. From looking up a review of the fan, it looks like the HIGH SPEED setting of the switch bypasses the speed resistor, so that setting should be fine for motherboard control. The test I saw showed the fan starts turning at around 33% of max power (MB control). That means that it may, occasionally, not start right away. But, it'll start as soon as there is any heat being generated. That's fine. in the test, the fan was running at 445 rpm at the motherboard's typical lowest idle speed (40% voltage). I wouldn't worry about it. Set your MB to control the chassis fan headers (voltage control, not PWM, if there is an option).

    As far as making the fans ramp up in speed, anything that involves ripping/processing video will generate a lot of heat. Ripping a CD in iTunes will also do it.

    Or, a surefire way is to download the Prime 95 app and run one of its torture tests:

    It loads the CPU at 100% and sends the temps soaring within seconds. The fans will speed up quickly when you start the torture test and slow down steadily when you end it.
  6. Best answer
    hwc1954 has the right ideas. You have been using two different control systems that combine to cause problems sometimes.

    These fans are connected through a High / Low switch on the back of the case. The Low position just puts a resistor into the fan power supply lead, reducing the supply voltage.

    The mobo's SYS_FAN ports are 4-pin, but the manual is missing some info (most do not make this point clear). When you plug a 3-pin fan into a true 4-pin fan port the fan always runs at full speed. This is because a true 4-pin port supplies a full +12 VDC on Pin #2 at all times, and a PWM signal on Pin #4 for the fan to use to reduce its speed. But a 3-pin fan cannot use that PWM signal, so it never reduces speed. To get around this "problem", SOME mobos allow you to change the 4-pin port to behave like a true 3-pin port. On that port type, the +VDC supply lines' voltage is varied by the mobo's control system to set fan speed where it is needed. A few mobos even automate the process of deciding which fan type has been plugged in, and will automatically control fan speed (if you let it) for EITHER type of fan. In your case, OP, the manual does not tell you that you can change between port modes, and it does not tell you the ports are self-configuring, either. It DOES tell you that there is a fan control and optimizing application called Easy Tune included on your mobo's CD that you can install on your C: drive and run under Windows. It will show you the temperatures that are being monitored (case temp and CPU temp), and the fan speeds that is is measuring. It also gives you the opportunity to fine-tune how your fans' automatic control system will work, or to manually set some fixed speed. This app MIGHT give you the option to set port mode, I don't know.

    So, I recommend you proceed this way.
    1. Set the rear switch to High and set the BIOS SYS_FAN port option to "Normal". This allows the mobo to control the case fan speed according to actual temperatures measured by a sensor on the mobo. IF the mobo can do this (that is, if it uses true 3-pin fan automatic speed control), your fans will start up at full speed for a few seconds when you turn on, then will slow down a lot. As you do more work and heat starts to build up, the fans will speed up and slow down in response to temperature changes.
    2. If you find, instead, that the fans are always running at full speed, you should change the port option to "Disabled", which will give the fans full voltage at all times and not try to control. THEN you use the rear switch to set the fan speeds to Low if you judge that they don't need to run full speed.

    I can't be sure how your mobo is doing things. A simple 4-pin fan port should not be able to run your fans at less than full speed. However, you description says that they DO run relatively slow when the rear switch is set to HIGH, and they simply don't start when the rear switch is set to Low. That sounds like the mobo ports are not sending full +12 VDC to the fans on Pin #2, just as a 3-pin fan port would do. That is why I said try my Step 1 first and see if the fan speeds really do change in response to case temperature. And don't forget to check out the Easy Tune app from your CD.
  7. So I did some testing over the weekend. I did what you suggested in option #1 and set my fans to "Normal" in the BIOS and the rear switch to High. I then loaded a few HD videos and got my CPU usage up to 100%. My CPU fan increased up from 1150 RPM to about 1450+ RPM, but both case fans stayed at about 500 RPM and did not move from there even after leaving it like that for about 10 minutes. During this time my temperatures did rise quite a bit.

    I used SpeedFan to view my temperatures and RPMs because I do not have a CD drive to load the Gigabyte EasyTune, and it seems a lot of people have trouble with getting it to work online and many did not recommend it. I was only using SpeedFan to view information and never set anything with it. I had something labeled "Temp 1", "Temp 3", "Temp 1", "Temp 2", two "HD" and "Core 0" and "Core 1". The HD's stayed pretty much that same at 25 and 30 C, the 2nd instance of Temp 1 and 2 never changed from 28 and 30 C. The first instance of Temp 1 and 3 when idle were 26 and 22 C, and when stressed Temp 1 would not change but Temp 3 would go up to 40-44 C. Core 0 and Core 1 were idle at around 25-28C, and slowly kept increasing when stressed to 50-55C, which is where I stopped doing anything.

    When I had the fans set to Manual in the BIOS, and on the Low setting on the back of the case, the fan speeds were at 850 and 750 RPM for the two fans. The temperatures remained the same at idle, and also under stress. The main movements being Temp 3 at 43 C, and Core 0 and 1 at 50-53 C. The CPU fan would get to about 1500 RPM.

    With the High setting on the back of the case, the fan speeds were at 1300 and 1200 RPM for the two fans. The temperature reading were pretty much exactly the same, with Temp 3 going to 44C as a max, and Core 0 and 1 maxing at 54C and usually between 49-52 C.

    So it looks like not only do my fans not increase when set to Normal in the BIOS, but even when set to Manual and put to High, they don't help cool my system at all. So, any suggestions or input? Are those temperatures normal, is it safe to be maxing my system out with those as sustained temperatures? Do I need to improve my fan set up?

    As far as the fans, they are the 120mm rear exhaust and the 140mm top exhaust that came with the case. I was thinking about getting a 120mm intake fan for the front even before any of this, mainly because I wanted to make sure cool air was coming in, and because I wanted to try to make some positive pressure so that air would escape out of all the tiny holes in my case without filters and help cut down on dust (since my front intake has a dust filter and is able to support 2 120mm fans, although I was only thinking of getting 1).
  8. OK, so it appears this is what is happening. You have two fans that came with your Antec case, each with a 3-pin connector. They are also connected somehow to a rear switch that has two choices: Low or High. Currently they are both connected to (separate, I think) mobo 4-pin fan ports. Although internal case temperatures change significantly, these two fans' speeds do not change. Hence, we can conclude that the 4-pin mobo ports to which they are connected are behaving exactly as a 4-pin port is designed - they are delivering a fixed voltage to the 3-pin fans plugged into them. You could have the same result if they were connected to a fixed supply voltage such as that available from a larger 4-pin Molex output connector from the PSU. This is the connector more commonly used as the power supply for an older IDE HDD - it is about ¾" wide with four round holes in a straight line. An adapter cable like this:

    would do the job, although it's more than you need. One end of it plugs into the output from the PSU to give you the Ground and +12 VDC supplies. It then has three outputs of the 4-pin fan type, but you can plug your 3-pin fan connectors into two of them, leaving one unused. That gives them both their full voltage and they will run at speeds set by your rear switch but not controlled by the mobo, which is what you have now. The female connector of this adapter is designed to do two jobs - feed one of the fan's speeds back to a mobo 4-pin fan port for monitoring, and feed that port's PWM control signal to the 4-pin fans connected to the adapter. But you don't have 4-pin fans, and I suspect you don't need to know the speed on one of the case's exhaust fans since you only change that by a manual switch. So do NOT connect this female plug to anything. Just use this adapter to create two (plus a spare) power connections for 3-pin fans from one PSU 4-pin Molex output connector.

    Now to your concerns about additional cooling. In terms of case ventilation you have three exhausts - the two Antec fans - one rear, one top - plus the effect of the PSU cooling fan. You have no intakes. I agree with you that it is preferable to create a slight positive pressure inside the case to blow air out of small openings. Since you have space for them, I suggest you buy and mount in your case front two fans as big as will fit there (120 mm or bigger?) where there are dust filters supplied. Buy 4-pin fans for this and plug them into the (now freed up) mobo SYS_FAN ports. Your choice: you can connect each to its own mobo port, OR you can buy a splitter adapter cable for 4-pin fans like this one:

    and connect both fans to one mobo port. The port can supply two fans at once. Note that this splitter has one output connector with no Pin #3. This is good - it ensures that the speed signal from only ONE of the two fans is fed back to the mobo port for monitoring.Doing this will increase air flow through the case and bring the intake / exhaust ratio into better balance.

    Whether a fan is an intake or exhaust unit is totally determined by which way you mount it in the case. Look at the fan carefully. It will have two arrows on it. One shows you the direction of rotation of the fan when its voltage supply is connected properly. The other shows you the direction of air flow through the fan.
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