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My nVidia GeForce 8800GT had a serious problem with its stock cooling fan: it started to make strange noises and I later discovered that it was hitting the heat sink, because of an abnormal rotation that was causing the blades to go up and down. I couldn't find any way to fix it.

I bought a replacement cooling set (fan and heat sink) which I managed to mount on the card with no hassle at all. But I suddenly discovered that the power supply plug of the fan had two pins whereas the connector on the card had four pins. The fan plugs on the motherboard, instead, have 3 pins! I felt discouraged and started complaining about myself... :(

Fortunately (or not?) I discovered that on the MB there was an unused male plug to install an IEEE 1394 connector. It had 9 pins, and two of them were coupled and marked +12V and GND respectively. I distorted the pins in order to force the fan female plug to make contact with these two pins.

I started the system, and the fan worked. The GPU temperature is fine.

I'm not here advertising my bravery, but merely asking: how much is my machine at risk? Is this tricky workaround dangerous, or not?

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3 Answers 3

up vote 3 down vote accepted

Unless you disable FireWire (I assume the main board will disable the connector when you do so) it should keep running fine. Also make sure that Windows doesn't have permission to turn of the connector/FireWire interface in order to safe on power, this might cause the main board to drop power on the connector.

However, do mind that most cooling sets have fans which are regulated based on chip set/core temperature. The way your fan is now connected will keep the fan spinning at all times.

I would try to look for a proper cooling fan. This workaround should hold but of course it's not a preferred solution.

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Plugging a 4 pin fan plug to a 3 pin motherboard is perfectly fine.

Fan has a 4-pin power connector; desktop board has a 3-pin fan header: enter image description here

Pin  4-Wire     Pin  3-Wire Support 
1    Ground     3    Ground 
2    +12 V      2    FAN_POWER 
3    FAN_TACH   1    FAN_TACH 
4    FAN_CONTROL     N/A 

As you can see, the 4th pin is used for fan control, but if your motherboard supports fan control using PWM, then you should still be able to control it's speed the 3-pin way using with SpeedFan based on the GPU's temperature.

Edit: As for the 2 pins, we can safely guess it also lacks the tachometer - simply means your Power Supply does not have any use for the speed reading of the fan.

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+1 for the diagram –  Bigbio2002 Feb 6 '12 at 19:40
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The 2-pin, 3-pin, and 4-pin connectors are usually compatible. The only differences are whether the fan speed is reported to the device and whether the device can control the fan speed.

With your setup, the graphics card thinks the fan isn't spinning, since it's not getting a signal from it. The graphics card might reduce its performance as a result. (Or it might not, since it knows it's not overheating.)

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