Question My Newly AMD ryzen 5 3400g cpu fan is not working?

Mar 3, 2021
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Hey There I just bought amd ryzen 3400g. I'm having trouble on how to fix the cpu fan it runs only for only 1 secs.
 

Karadjgne

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3 pins is a voltage controlled fan. 4 pins is a voltage constant fan. The pins are 12v, ground, speed sensor and pwm.

So a 3pin will use the 12v, ground and speed sensor and the motherboard knows to change the voltage from 5v-12v to change the speed of the fan. The connector itself is keyed, has little tabs that line up with the motherboard socket, so it only fits in 1 way and the pins line up correctly.
 

Paperdoc

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Tell us your mobo maker and exact model number, so we can look up its manual and advise using its features.

You CAN connect a 3-pin fan to a 4-pin header, am discussed above. Just to verify that this has been done correctly, look at the mobo male (with pins) header. The plastic tongue sticking up beside it is beside Pins 1 to 3, and Pin #4 is beyond the tongue. Pin #4 is the one that should NOT be used by the 3-pin fan connector.

Most mobos today use only 4-pin headers, BUT allow the user to change HOW the electrical signals for that header are sent out, through option settings in BIOS Setup for that header. Here we are dealing with the CPU_FAN header. The options often include: Voltage Control Mode (aka DC Mode), PWM Mode, and Automatic. Many are shipped set to PWM Mode or Auto by default. Auto is supposed to test the connected fan at start-up and choose the correct option based on that. But if there is no AUTO option this cannot be done and you need to set the option yourself. Further, some mobos do not even have an option for this header - they use only the PWM Mode and offer no choice.

Now, if you connect a 3-pin fan properly to a header that is using the new PWM Mode (whether by option or by the no-choice-available way), that fan will always run full speed. You report the fan starts, but quickly stops, so your header must NOT be using PWM Mode. Either that, or you have plugged it in wrong. So, MAYBE your CPU_FAN header already is set to use DC Mode (or Auto, but it sets itself to DC Mode). What else could be involved?

Three configuration options for that header might be involved. Some headers have an option for the MINIMUM speed of the fan that you can specify. IF that is set too low for your fan, once it goes into the full control by the header and is told to drop to minimum speed for the cold system, it will stall. The second related item is a choice in the fan PROFILE setting. There the default automatic settings establish the "curve" of what speed to run the fan for what measured CPU internal temperature. Alternatively, you may have the option to set your own custom "curve" using a Profile called Manual or some such. In either of those two cases, you may have the lowest speed (at lowest temperature) set too low and that might need adjustment. And another: the Profile items also include an option for Quiet operations you might have chosen. This is just a setting of a fixed low speed, and that MIGHT be too low.

As I said, If we knew your mobo, we could check its manual and provide more precise advice.
 

Karadjgne

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well yeah what does it have to do with if it goona work or no in 3 pin slot?
Everything. 3pin was the original motherboard solution for fan headers, prior everything was molex, and later the 3pin was upgraded to 4pin for pwm use. But in a stroke of actual genius, motherboard manufacturers actually thought it out and included 3pin and 4pin on the same header, so either could be used. This solved multiple issues of compatibility as many cpu coolers are 4pin pwm, but not all are and many case fans are 3pin, but not all are.

So a fan is s fan. Doesn't matter if it's 3pin or 4pin, they both work. BUT they are keyed to fit in only one way to prevent you from sticking the power and ground in the wrong orientation and frying the fan.

Yes a 3 pin works in a 4pin header, the 4th pin on the header is simply bare.

In bios, the fan header is default set to auto. Some headers this is tantamount to default pwm, some default DC. So if the 3pin runs at full speed constantly, it simply means a trip into bios, find the fan header settings and change that header specifically from (Auto) to (DC/Voltage).
 
Feb 16, 2021
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Ok but why u type all of this? He said he used 3pin and connected it to 4 pin, i said its fine, then u came in and said its fine but on A4 statement.....
 

Karadjgne

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Because Op is not the only reader. There are many ghosts who just look up an answer and never actually touch anything. Those ppl often do not know to ask for a little more detail or explanation that answers further questions they weren't thinking of at the time.

It's like the question "can I put 16" rims on a car that has 15" original" and someone simply answers 'Yes'. That's often not going to work out well without further explaining that you'd also need to check tire hight, width, rim backspacing and offset, just to make sure it clears the fender wells.

Cpu_fan header is almost always 4pin dedicated pwm. You put a 3pin fan there, it's going to spin at max, always Unless you can change the header to DC/Voltage setting in bios.

So 'Yes' it'd work, but 'No' it may not work as originally intended, creating frustration and further digging into answers to find a solution.
 
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Paperdoc

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I agree fully with Karadjgne, including the ideas that OTHERS may need more info to understand. Further, though, OP's query was that the fan STARTS but runs only for a second or two, then stops. So the fan functions. BUT such behaviour is NOT because the 3-pin fan was connected to a 4-pin header configured to use PWM Mode - that would have caused continuous high-speed operation. That was the focus of my earlier post. There I outlined three configuration options that MIGHT be involved in causing a 3-pin fan connected to a header that IS using the proper DC Mode to stall.
 

Karadjgne

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My Asus motherboard gives the option to Optimize fans. Many 3pin today are using much better and more efficient motors so will start at 5v for low speed, and reach 12v for max speed with little current use. But, there are more than a few older designs, especially the cheaper fans, that are using older, cheaper motors that not only require slightly more current, but will not start/stay running at less than 7v.

During POST process, there are 0 controls over anything, everything is given its maximum rated power for self test. So the fans spin. Then Bios starts up and rudimentary controls are established for voltages, current, clock speeds etc. That includes the bios set fan curves, min/max voltages etc. If the default for the bios is 5v-6v for the cpu_fan header, and you have the fan curve set for silent or standard (common practice) and you have a cheap 7v fan, it's going to stop. It simply doesn't have the necessary voltage to run.

In my case, the simple fix is to allow the Bios to Optimize the fans (it checks for min/max voltages, min/max speeds) and set new default fan curves according to what the fan can actually do.

Or, in bios you can set it for performance mode, manually set minimums for 8v etc.

You have options, depending on what behavior you want and what the bios let's you have.

Rule of thumb: 5v = 40%. 7v = 60%. So if bios wants to run the fan at 40% minimum, jack that upto 60% minimums gets you a 7v output from the header.
 

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