Question CPU fan only runs if ground connected to sensor pin??

[SaffaDude]

Rebuilding an old PC using a new second hand motherboard, DQ67OW. The CPU fan, Coolermaster X dream i117 cooler, does not run. I get no errors on startup, PC runs. I removed the fan and measured the fan header, 12.3V between pin 1-2.

I connected the fan to a molex to fan socket adaptor to run it directly from PSU, no joy. Testing it externally on a 12V power supply the fan does nothing. However, when I connect 12V to pin 2 and 0V (ground) to pin 3 (sensor / tach) the fan runs. Ground / 0V on pin 1 does nothing. This doesn't make sense to me. It is a odd size fan so not so easy to replace, would prefer not having to buy a whole new CPU cooler.

Am I missing something??

EDIT: It is a 3-pin fan.

 

[Lutfij]

Can you pass on a picture of how you're hooking that fan's connector to the fan header on the motherboard? Can you hook up a PWM fan to the CPU_FAN header and see if that behaves the same way? I'm curious to learn what processor you're trying to cool since the processor should come with a stock cooler, which is PWM driven, mind you.

Does the 3pin cooler's fan operate without issue on the other header?

 

[faalin]

Have you tried a different fan on the header to see if maybe the cooler itself has an issue.

If the molex to fan adaptor doesnt work then i would say there is an issue with the fan on the cooler then.

 

[Paperdoc]

Take a close look at the female connector on the end of the fan wires. Hold it so you are looking into the open holes on the end. The connector has two ridges running down one side, and these two fit around a plastic "tongue" sticking up beside the three pins of the male mobo fan header. Turn the connector so those two ridges are on TOP of the connector while you look into the holes. Now the hole on your LEFT is for Pin #1, and it is the BLACK wire. The Middle hole (RED wire) is Pin #2, and Pin #3 on the RIGHT has a Yellow wire.

Is that how you were labelling the connections when you say you could get it to work only when you connected power to pins 2 and 3? If not, then you had the labels wrong.

Note also that the (+) 12 VDC wire from the power supply must go to the MIDDLE (RED wire) hole, and the (-) side to the #1 hole - the LEFT end when held as I said.

The connector system design intends to make it almost impossible to plug the fan into the mobo header backwards, by virtue of the ridges and tongue. BUT if the "tongue" on the mobo header is missing, then you CAN plug it in "backwards" and get the connections wrong. Did that happen?

As a further check, are the WIRE colours the way I just said above, or are the Black and Yellow reversed? On the other hand, the only photo I found for that cooler shows the cable from fan to connector is a 3-wire all-black one, so there may be NO colour coding to see.

Another item to check. On the only photo I could find for that mobo it shows the CPU_FAN header is a 4-pin one. So the "tongue" beside its pins is next to Pins 1 to 3 of the header, and Pin #4 is beyond the tongue. When you plug in a 3-pin fan, this means that Pin #4 will NOT be in use - it will be outside the connection and visible.

By the way, on that mobo the CPU_FAN header may NOT be able to control the speed of a 3-pin fan - it MAY be designed to work only in the newer 4-pin PWM Mode. That means the 3-pin fan on that cooler would operate only at full speed all the time once it works properly. But this factor should NOT be involved in the problem you are trying to solve.

Here's a MAYBE. If you are sure that the way the connector contacts are labelled is correct (as above) and plugged in properly, it is possible that the connector on the end of its all-black wires was mis-connected with wires 1 and 3 reversed. If that is the conclusion, then re-wiring those two may be your solution.