[SOLVED] Case fans wont spin

Apr 20, 2020
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I have 2 Rosewill LED case fans that need to be powered by my PSU by molex as i dont have an available header for them on the motherboard.

I tested both fans by connecting them to the one (currently in use) chasis fan on my motherboard and they spin at full rpm. I also tested every molex connection that my psu molex cable has and the only result is the LEDs light up, but no fan spin unless given a manual spin. In that case, they only spin at a low rpm.

Components are:
-EVGA Supernova 850W PSU
- 2 Rosewill LED Fans (dont know specific size or type) w/ both molex and motherboard pin connections

Does this sound like the molex cable from my PSU is faulty? Pease help
 

Paperdoc

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OP, you are right - the whole design of 4-pin Molex connectors is to prevent connecting them "backwards". That is enfoced by the bevelled corners of the connectors. But Karadjgne has a really good point. What you describe certainly sounds like your fans are getting only the 5 VDC supply, not the 12 VDC the need. That might happen if the fan connectors were made (wired) wrong by the maker.

So look at the connectors carefully. In the female from the PSU, the YELLOW on one end is +12 VDC, and the adjacent BLACK is Ground. Now look at the male connector on the end of your fan wires. It MAY have only 2 pins in it, but I suspect it has all four because that male-and-female pair is intended to pass along to the next device the power from the PSU - it "replaces" the PSU output you have just "used" to power your fan. BUT many such systems have those two Molex connectors joined by short wires, and then TWO smaller wires coming out of one of those Molex's that go to the fan motor. Can you tell which pins of the male Molex are connected to the two small wires to the fan motor? If you can, then examine which holes of the PSU's output Molex those two male pins connect to. Do they go the the Yellow / Black pair on one end of the female Molex, or to the Red / Black pair on the other? They SHOULD be to the Yellow / Black pair of the female from the PSU. In fact, if the two small wires to the motor have colours, they normally would be RED for the +12 VDC wire (should mate to a Yellow in the PSU output) and Black for Ground (should mate to Black).
 

Karadjgne

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Sounds entirely like the wires on the molex connector of the fan line up with the red lead on the molex from the psu, not the yellow. The red is 5v, so probably below the starting threshold of the fan. The yellow is 12v. (R-b-b-Y). You need the pins on black/yellow, not black/red
 
Apr 20, 2020
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Sounds possible because my molex cables from the psu are all black as opposed to the previous psu that had coloured cables. I don’t quite know how to manipulate the cables to get black to yellow. Sorry, im not familliar with molex cables at all.
 
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Karadjgne

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You misunderstand. The molex from the psu is R-b-b-Y. The plug on the end of the fan should match up to the other end, the b-Y, not the R-b. It's like you put the plug and molex - backwards.

But here is my question, are you 100% positive the those are in fact both fan power? Is it possible they are supposed to be factory replacements for a case and the connector isn't a psu molex, but the power from a case fan switch. It's been years since any fan came with a molex-psu connector except for an odd few.

LED are either 12v (if powered by the fan) or 5v if externally powered.

How many wires are in that fan-molex? 2 or 3 or 4?
 
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Apr 20, 2020
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Im not sure I understand how it could be backwards, the molex cord from the psu has female connections, and the cables from the fan have male and female, but theres only one way to connect the psu and the fan together ( female psu to male fan) I cant flip the cables in any way to get a different connection. Sorry if i sound stubborn but I am just very confused as to how I can get that proper connection if the two cables only go together one way.
 

Paperdoc

Glorious
Ambassador
OP, you are right - the whole design of 4-pin Molex connectors is to prevent connecting them "backwards". That is enfoced by the bevelled corners of the connectors. But Karadjgne has a really good point. What you describe certainly sounds like your fans are getting only the 5 VDC supply, not the 12 VDC the need. That might happen if the fan connectors were made (wired) wrong by the maker.

So look at the connectors carefully. In the female from the PSU, the YELLOW on one end is +12 VDC, and the adjacent BLACK is Ground. Now look at the male connector on the end of your fan wires. It MAY have only 2 pins in it, but I suspect it has all four because that male-and-female pair is intended to pass along to the next device the power from the PSU - it "replaces" the PSU output you have just "used" to power your fan. BUT many such systems have those two Molex connectors joined by short wires, and then TWO smaller wires coming out of one of those Molex's that go to the fan motor. Can you tell which pins of the male Molex are connected to the two small wires to the fan motor? If you can, then examine which holes of the PSU's output Molex those two male pins connect to. Do they go the the Yellow / Black pair on one end of the female Molex, or to the Red / Black pair on the other? They SHOULD be to the Yellow / Black pair of the female from the PSU. In fact, if the two small wires to the motor have colours, they normally would be RED for the +12 VDC wire (should mate to a Yellow in the PSU output) and Black for Ground (should mate to Black).
 

Karadjgne

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Mostly by 'backwards' I mean literally that. The molex is as Paperdoc says and is 'keyed' by means of square pegs and D shaped pegs. So supposedly it only fits in a single direction. But many molex are not Molex brand which has a very high quality control, but a cheaper substitute where the keys are not quite molded right or made from a softer and more flexible plastic. Basically 'Cheap crap'.

Unfortunately this can also mean its not that hard to insert the molex backwards, with your 5v wire on the fan (missing) aligning with the 12v of the psu, and your 12v wire from the fan aligning with the 5v from the psu. Some might call it one side upside-down.

Like putting your right hand in a left-hand glove, pinky finger ends up in thumb and your thumb ends up in the gloves pinky. Flip the glove upside down and it fits right to the fingers.
 
Apr 20, 2020
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Thank you guys for all the help and info abt the cables, I found that the best option I had was to buy a fan hub connected to my motherboard. All fans are now plugged in and working great!
 
May 18, 2020
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Your motherboard could also be the problem; if the circuit supplying the power to the fan is fried/short, then your fan will not be able to spin. There are several fans in a computer, including a PSU (power supply unit) fan, CPU (central processing unit) fan, case/chasis fan, and the GPU (graphics processing unit) fan.
 

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