questionshow do i add a case fan to my pc if there is not…

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Use one of the 5V molex connectors (for drives) will work for most case fans. They come with adapters.

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You can buy fans that have a 4-pin connection and plug directly into the power supply. If there isn't a free 4-pin molex connector, never fear. They are made to plug inline (think christmas lights) so you can plug your device that's using the connection into the back of it.

If you already have a 2 pin or 3 pin fan, you can get a 4-pin molex adapter that essentially does the same thing for little to nothing on Newegg or Amazon

Here's a 4-pin Molex to 3-pin fan connection adapter

Here's a splitter for a 4-pin Molex

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@capguncowboy: The fan is a 4 pin (PWM?) connector, so would I just need one of these instead? http://www.amazon.com/Splitter-Molex-headers-CONNECT-MULTIPLE/dp/B009D3HV1Q/ref=sr_1_3?s=electronics&ie=UTF8&qid=1357695825&sr=1-3&keywords=pwm+to+molex

Also, the power supply on this computer has no open spots that I can see are accessible. This is definitely not a computer built to be modified.

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@jeffrjohn: Wait -- are there ANY free fan connectors on the motherboard? What's the model Number and I'll look it up online to see where they're located.

I just read your question in its entirety (didn't do it before -- sorry about that). Despite what they're named (sometimes they'll be called CPU01 and CPU02), you can connect any fan to those. The CPU badge just means that when your CPU isn't hot (you're not using it), it will turn the fan down. You can disable this setting in the BIOS if you'd rather it run full blast all the time, or use a program like SpeedFan to control the fan manually.

EDIT: I looked up the motherboard (ASUS A8M2N-LA) but from the pictures, I only see the one fan connector at the upper right of the board (above the memory slots).

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@capguncowboy: Yeah, that's the only slot I see as well. So, would your method work with the splitter I linked to? Thanks.

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@jeffrjohn: I can't seem to find a 4-pin PWM adapter that plugs in as a pass through connector. I am almost certain that you can use an adapter like this one: http://www.amazon.com/HDE%C2%AE-4-Pin-Molex-3-Pin-Cable/dp/B0081K3ZS0/ref=sr_1_5?ie=UTF8&qid=1357697721&sr=8-5&keywords=molex+fan

You'll need to plug it into the back of your CD/DVD drive and then plug the connector from the power supply into the back of the adapter. Then you can hook your fan into the 3-pin connector.

All the fans on my case are 3-pin and they plug into 4 pin slots on the motherboard. Sense would say the 4th pin isn't necessary to run the fan. It more than likely produces an RPM feedback for fan controllers.

Just be sure that when you plug it in, you line it up accordingly (there should be rails that align to a block on the plug head.

EDIT: If you're in a hurry to get the item, there are probably more options available online or at a local PC shop, but you will more than likely pay more than a few bucks

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Are you sure that there's even space for another case fan? Prebuilt computers are not built with modifications in mind, so often all of the slots or power connectors are filled.

And yes, in the 4-pin fan headers, the fourth pin is used for RPM control.

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How 'bout this one? Has the 4-pin molex pass-through power connector, fits in any available expansion slot?

http://www.newegg.com/Product/Product.aspx?Item=N82E16835106104

Under $10, free shipping.

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I'll just chime in and say if it's a PWM connector or 3-pin, then the fan will run at full speed all the time if plugged directly into an adapter to a molex connector. This might be louder than you'd expect, because the fan is designed to run slower and quieter most of the time.

Fans like @75grandville posted often have speed adjustment dials or levers (though the linked one does not).

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Looks like @capguncowboy has this thread under control...
Newegg, Tigerdirect, Amazon, Monoprice -- any number of places sell them.
Last time I ordered from Amazon because I was in a hurry (used Prime).