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Four case fans, who to wire?

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  • Homebuilt
  • Cases
  • Fan
  • Hard Drives
  • Systems
Last response: in Systems
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June 5, 2009 1:15:05 PM

I am unshackling myself from dell, bought a pc case and a motherboard and a hard drive and a good cpu cooler, and took the dual core from old dell and hard drives and vga card and stuff and am putting into my first home made build!!!

Im trying to understand the fan wiring and how to wire.
The case comes with 4 fans: Front, top, back, and side small hard drive fan.

From the front comes a 4 pin with only 2 wires, a green and white; and two small white 3-pin connectors that i belive are sysfan 1 and sysfan 2 that plug into motherboard (rivited front stuff prevents me from tracing the front wires to the origin, thats part of the problem),

For the top and back fans, there are 4 pin with male and female each.

So, where do i plug the 4 pin from front with 2 wires (green & white)? I suspect it should be connected to maybe the top fan input, then the top fan output connects to the back fan input 4 pin connector?

Which 3 pin would be sysfan 1? i think one is for the small hard drive fan and the other controls the case fans.

Info greatly appreciated.

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June 5, 2009 5:26:44 PM

Front fans are the puzzle. Some fans come with one 4-pin Molex male (connects to PSU Molex female same as used for ata drive power, etc.) plus one 3-pin to connect to a mobo SYS_FANx. Do NOT plug both in - you either do the Molex for constant full-speed fan, or use the mobo connector and allow the BIOS to control the fan speed according to case internal temps. But not both at the same time. However, you have a third 3-pin for ... what? Just maybe there are two fans in the front - one MUST be connected to a mobo SYS_FANx and the second is optional; that is, the second should be to the other SYS_FANx if you have it, or to a Molex if you have to. But that's just my speculation - see if your case documentation clears this up.

You appear to believe that one of the "front fans" really is more of a HDD fan than a general case ventilation fan. In that case I suspect the real case vent fan should be connected to SYS_FAN1. But again, consult the case and mobo manuals. If the BIOS has separate controllers for its two fan outlets, likely one is based on measured temperature somewhere in the case or mobo itself, and this should be controlling the true case vent fan. If the other fan truly is a HDD cooler, it is unlikely the BIOS has any way to monitor HDD temperatures (but it is possible, I think), so that fan's speed should NOT be controlled by the BIOS. If you plug that one into the mobo connector, just set the BIOS to run at full speed all the time. However, that still can be very useful because the third wire in the 3-pin is used to send to the mobo the SPEED of this fan. Connecting the fan this way allows the mobo to monitor that speed, make it available for you to see in the BIOS Setup screens, or make it available to separate monitoring software. It also means it can show you an alarm if it fails. Or,if you connect it instead to a Molex connector, that already makes it independent of BIOS controllers. In that case, though, you also should tell the BIOS not to pay any attention to the speed of the fan on SYS_FAN2 because there is none connected!

As you say the other two are fitted only with Molex connections including a second parallel connector so you can "daisy chain" them off one PSU output. If you choose not to use the second mobo outlet for the 3-pin connector for the second front fan (maybe actually the HDD fan?), then you could plug this also into the daisy chain. But that means you must identify clearly which front fan this is, and NOT connect the 3-pin from it.
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