4 Pin Fan Header Problem

Hi, :hello:

I have a HP dc5700 which I am modding to a little quieter as it is my file/media server. I have already put in an Antec Phantom 500 and am following it up by replacing the two 92mm fans with Noctua NF-B9's.

The problem I have is with the Fan Headers on the Motherboard. The dc5700 has one Fan Header for the CPU and two Fan Headers for Chassis Fans. The system only comes with one Chassis Fan so I am sticking with one CPU and one Chassis Fan.

All Fan headers on the Motherboard are 4 Pin but only the CPU Header allows a 3 Pin Fan Connector to be used. I will attempt to illustrate the difference.





As you can hopefully see, the CPU Fan Headers plastic edge is shorter than the length of the 4 pins to allow a 3 Pin Fan Connector to be used.

So the Noctua fans as I'm sure you have already gathered use 3 Pin Fan Connectors which meant I had to connect the Chassis Fan using the Molex adapter provided in the Fans box. This would all have been lovely except the dc5700 BIOS displays a POST error if no Chassis Fan is detected forcing the user to press F1 to continue booting. Doh! :o

I can see three ways around this problem.

Trick the Motherboard into thinking there is a fan connected to one of the Chassis Fan Headers and leave the Chassis Fan connected via the Molex adapter. I couldn't find any such information on whether this is even possible yet alone if a product exists.

Make use of an adapter to allow a 3 Pin Fan Connector to connect to a 4 Pin Fan Header whose alignment edge is longer than you would normally find on a CPU Fan Header. Searches for such a product proved fruitless as I kept on finding Molex adapters.

Borrow the 4 Pin Fan Connector from the removed fan and connect it to the Noctua Chassis Fan. I'm sure this is possible but I wouldn't want to connect the wrong wires and either spin the fan the wrong way or "blow" the circuits.

I am hoping all that makes sense but hoping more someone has a solution!

Thanks in advance for any assistance :love:

5 answers Last reply
More about header problem
  1. Can't you just plug the CPU fan in the chassis fan header, or am I not getting the question right (I've never seen a 4pin header that didn't allow a 3pin connector to be plugged in.)
  2. Hi Gulli,
    Thanks for replying.

    I could plug the original CPU Fan into the Chassis Fan Header but I am replacing the existing fans and the Noctua Fans I am using have 3 Pin Connectors.

    I had never seen a 4 Pin Fan Header before today and it took a bit of Googling to find out that they normally allow 3 Pin Fan Connectors as well. It seems that HP, in their infinite wisdom, thought it wise to only fit 3+4 Pin headers for the CPU and not for the Chassis Fans.
  3. I guess you could just buy another fan that has a 4pin connector.

    BTW: normal motherboards only give a warning when there's no CPU fan plugged in, they don't care about chassis fans, it seems HP didn't want anyone to mod their systems one bit.
  4. Well the Noctua's weren't cheap and after hearing them run with their Ultra Low Noise Adapter while still giving a good static pressure, I am inclined to keep them now.

    I will need to investigate how to wire a 4 pin connector onto a 3 wire fan. Think this is the best option.

    Thanks for your replies.
  5. Well, I sorted the problem and thought it best to post the solution so someone else can benefit.

    The cable assignment for a 3 pin fan header is as follows ...

    Black = GND
    Red = +12V
    Yellow = Sensor

    The cable assignment for a 4 pin fan header is ...

    Black = GND
    Yellow = +12V
    Green = Sensor
    Blue = Control

    Sensor is where the fan sends back pulses for RPM monitoring
    Control is the PWM option for adjusting fan speed (more info here on page 17 http://www.formfactors.org/developer%5Cspecs%5CREV1_2_Public.pdf)

    So I took the 4 Pin connector of the existing HP Fan and put it onto my new Noctua Chassis fan and all was good. Just made sure to align the cables as per their use as opposed to colour.
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