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Case fans not working when molex cable connected to mobo

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January 3, 2012 4:57:07 PM

I am a first time computer builder and working on my first build. I was able to boot up with the cpu and ram hooked up to the mobo outside of the case. I then installed the components inside the case and attempted to hook up the case fans. My case has 4 fans, and I plugged them all into one of the molex cable and then into the mobo. I was unable to boot up. I then unplugged the molex cable from the chassis fan header on the mobo, and the mobo booted just fine.

The peripheral cables and fan connections are 4 pin, and the chassis fan header on the mobo is 3 pin. My case has knobs and adjustments that I can control the fan speed from the outside of the case so I imagine that i will be able to regulate the speed manually. I'm guessing that a disadvantage to not connecting the cable to the mobo, is that I can't monitor their RPM's? Do I have to connect the end of the molex cable to the chassis fan connection on the mobo for the fans to function properly or can I just give them power and leave the end of the cable unplugged?

Motherboard: ASRock z68 Extreme 3 Gen3
CPU: i5-2500k Intel 1155
Corsair 750TX 750W
Case: Antec Nine Hundred Two V3
January 3, 2012 5:47:53 PM

Bump! Almost 50 views and no reply. I need to know if I can leave the end of the molex cables unplugged from the mobo. The fans are powered and functional, even though the end of the cable is not connected to the fan header on the mobo. Is this ok?
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January 3, 2012 6:16:00 PM

there was an article about the cooling including fan connection and placements on toms recently.

for me, the only reason to connect fan to mobo is for fan speed control (to reduce speed when not need to become quieter and to speed fan up when casing is heat up)

if the pc is working correctly with no tmep issue and also no noice issue, it's ok for u.

the reason for the fan not working when connect to mobo is may be the fan is not designed for mobo connection
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January 3, 2012 7:27:43 PM

trispir said:
I am a first time computer builder and working on my first build. I was able to boot up with the cpu and ram hooked up to the mobo outside of the case. I then installed the components inside the case and attempted to hook up the case fans. My case has 4 fans, and I plugged them all into one of the molex cable and then into the mobo. I was unable to boot up. I then unplugged the molex cable from the chassis fan header on the mobo, and the mobo booted just fine.

The peripheral cables and fan connections are 4 pin, and the chassis fan header on the mobo is 3 pin. My case has knobs and adjustments that I can control the fan speed from the outside of the case so I imagine that i will be able to regulate the speed manually. I'm guessing that a disadvantage to not connecting the cable to the mobo, is that I can't monitor their RPM's? Do I have to connect the end of the molex cable to the chassis fan connection on the mobo for the fans to function properly or can I just give them power and leave the end of the cable unplugged?

Motherboard: ASRock z68 Extreme 3 Gen3
CPU: i5-2500k Intel 1155
Corsair 750TX 750W
Case: Antec Nine Hundred Two V3

Wow
Your fans can get thier power in different ways
A) connected to the psu, direct to Molex ( 4 pin HDD connector )
B) connected to the motherboard fan header , powered by the 24 pin ATX connector
C) connected to a fan controller, usually powered by a Molex connector

You were essentially trying to power to a power port.
Never plug any power connector from the psu into your board other than the 20/24 and 4/8 pin connectors .

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January 3, 2012 8:22:36 PM

delluser1 said:
Wow
Your fans can get thier power in different ways
A) connected to the psu, direct to Molex ( 4 pin HDD connector )
B) connected to the motherboard fan header , powered by the 24 pin ATX connector
C) connected to a fan controller, usually powered by a Molex connector

You were essentially trying to power to a power port.
Never plug any power connector from the psu into your board other than the 20/24 and 4/8 pin connectors .


Thanks for your response. I understand much better now how powering the fans and mobo works.

Last night when I was working on this issue I experienced a very quick amount of smoke when trying to plug the end of the molex to the fan header. The smoke came from the HDA region of the board. The mobo stayed on and is able to start up just fine. The USB and PS/2 Port are working and the BIOS posts.

Do you think I was able to escape without any issues, or could something turn up later today when I install the optical drive and SSD?
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