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Chassis Fan Works, But Not Monitored


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#1 singjai

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Posted 19 November 2011 - 12:56 AM

My CPU fan and my chassis fan are each connected separately to the motherboard.
The CPU fan has a 3-pin connector that plugs directly into the motherboard.
The chassis fan has a 4-pin molex connector plugged into the motherboard by way of a 3 pin adapter wire.

When I enter the BIOS or run Asus PC Probe II the speed and temperature numbers for the CPU are both displayed. But there is nothing displayed for the chassis except the legend "NA" where temperature and speed numbers should be. The chassis fan is running at the highest setting, but there is no measurement of temperature nor of fan speed. This generates no error message, nor does it prevent my computer from operating.

This isn't a problem as such, but it's something that genuinely puzzles me. It happened on my other computer as well and I "solved" the problem by replacing the chassis fan, which (like this one) had a 4-pin molex connector connected to a 3-pin adapter wire, with its brand mate (same model number, different suffix), which had 3-pin connector I could plug directly into the motherboard. That was the only significant difference between the two fans, but the latter was detected where the former was not. All I could say in response was, "Oh, really?"

Is this all I really need to do again? Could someone please tell me why the 4-to-3 makes it not work, if indeed that's the case? Thanks very much.

Motherboard: Asus P5KPL-CM
CPU: Intel Core 2 Duo E8400
CPU Fan: Arctic Freezer 7 Pro
Memory: 4GB Corsair DDR2-6400
Power Supply: Antec True Power 450W
Case: Antec Dark Fleet DF-30
OS: XP Media Center SP 3

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#2 Nicholas Basso

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Posted 19 November 2011 - 03:29 PM

4 pin large (aux 12v such as is used for IDE hard drives/optical drives) Molex does not carry the tachometer signal. It is only + and - voltage to the fan. Thus the third Tachometer pin on the 3 pin is left open, thus the system will not read any speed.

4 pin small molex (looks similar to the 3 pin but with 4 pins) carries tachometer AND a PWM line, and should adapt properly.

I'm guessing your 4pin molex is the large kind, mostly intended for direct connection to the power supply, and without any speed control.

For more information, see:
http://en.wikipedia.org/wiki/Computer_fan#Fan_connector

EDIT:
For an image, see:
http://en.wikipedia.org/wiki/Molex_connector#Disk_drive_connector_.28Molex_8981_Series_Power_Connector.29

Edited by Nicholas Basso, 19 November 2011 - 03:31 PM.


#3 rotor123

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Posted 19 November 2011 - 05:51 PM

the 4 pin Molex does not carry speed information from the fan. the two center wires are grounds and the red and yellow carry 5 volt and 12 volt. You need to change to a three pin fan if you want to monitor its speed.

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#4 singjai

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Posted 22 November 2011 - 02:08 AM

Thanks. I thought it might be something that simple. The stock chassis fans are 3-speed Antec TriCool 120mm Blue LED with large molex connectors.

#5 Nicholas Basso

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Posted 22 November 2011 - 10:20 AM

Thanks. I thought it might be something that simple. The stock chassis fans are 3-speed Antec TriCool 120mm Blue LED with large molex connectors.


Those fans are probably sufficient to be left in as manually controlled fans. If you have a spare vent for another fan, install a 3-pin fan and use that for the CHAFAN header. Otherwise, leave it out, it won't really hurt things that much. Usually the change in noise will be enough to alert a wary user that a fan has failed, which is all the monitoring is really good for anyhow.




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