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CPU_FAN connection not properly working on motherboard - what is best option?

A PC I'm working on was reporting a CPU_FAN error. The fan would spin fine on start up but was very erratic afterwards- resulting in the PC ultimately shutting down. At one point I got it working for a few days, but the second I tried to put on the side panel to the PC (I had left it open), it shut down the PC immediately (I guess it short circuited or something?)

Since then, I connected the CPU fan to another fan connection on the motherboard and the fan appears to spin fine, so I'm fairly certain it's the CPU_FAN connection. Now though, my other fan at the back of the PC isn't plugged in (the cable seems too short to reach the CPU_FAN connection.

What are my best options at this point? Will my PC function if the CPU fan is running and not the back fan? Short of replacing the motherboard, what else can I try?

The BIOS settings seem limited (HP Pro 3500), but I believe some BIOS settings may allow me some additional options, esp. since the fan itself does not seem to be bad.
4 Solutions
Usually if the CPU fan works, and the one in the PSU, that is good enough. I only use additional case fans for very special situations.
Gary CaseRetiredCommented:
You can power the rear fan from one of the molex connectors on the power supply.   Just get a simple adapter cable:  http://www.amazon.com/Molex-Computer-Connector-Y-Splitter-Adapter/dp/B00DU8ZZ0O
Alan HendersonRetired marine engineerCommented:
Be aware that if you're not using the dedicated CPU fan connector, then a failure of the CPU fan would not shut down the PC. It's possible that you could cook the CPU.

Check the CPU temperature warnings and shutdown settings in your BIOS, and maybe disable the CPU fan warnings/shutdown controls.
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web_trackerComputer Service TechnicianCommented:
If it generates too much heat with the back fan not plugged in your computer will shutdown to prevent damage to the cpu. If that happens then you know that your system is not cooling properly and will need to plug in the back fan some how, follow gary's idea of buying a y splitter adapter. Note if you use this type of connector to connect it directly through the power supply the fan will run at full speed all the time. When you connect it to the mobo you get fan speed control based on the temperature inside the case.
most cpu fans have a sensor - and thats controlled on the cpu fan connector; not on the other one
so i think  maybe the sensor is bad, or the control circuit
easiest is to test with another cpu fan on that connector
ruhkusAuthor Commented:
OK, I tried testing with another cpu fan and that one worked fine. However, it was only 3-pin, so technically I'm not sure if  it's the fan or the mobo sensor that stopped working.

However, I also was able to rotate the case fan so the wire was closer to the CPU_FAN connection. So basically I switched motherboard connections for the CPU fan and the case fan. Both are running fine right now. Can I probably keep things this way (both fans running at full speed now), and if a fan fails, still have some assurances of the PC shutting off?

Should I encourage the user of this PC to shut down his computer nightly (or at least every few days) now, as opposed to leaving his PC on 24/7?
you really should test with the correct 4 pin one to be sure imo
the fact that both run fine now only tells me that they have 12 V -  and no mechanical problems
i always shut down my pc 3 x per day
Gary CaseRetiredCommented:
It'll run fine with a 3-pin fan.   All modern CPU's have thermal throttling => if the CPU gets too hot, it will simply STOP (This won't shut down the PC gracefully, but it will keep the CPU from "frying" itself).

No reason to use the PC any differently than normal -- if you leave them on 24/7 and have it set to "sleep" when not in use, just keep doing that.
i agree, Gary, i'm only telling him that the only sure way to check if the CPU FAN control on the mobo works is testing it with the proper fan = 4 pin
Reece DoddsCommented:
Is it that the sensor is faulty to the point it's not motivating the fan with enough power to spin fast enough (or at all) or is it that it's reporting incorrectly and hitting thermal shutdown prematurely?

You could likely go into the mainboard BIOS and try a few settings with the thermal control... eg. many AMD system use "cool 'n' quiet".  Disable this.  The fan will run at graduated speeds rather than with dynamic monitoring.  Or look for other fan speed/failure settings that may be incorrect.  If there's a setting to just run the CPU fan on 'full' rather than auto - choose this.

You could also check/monitor to see what the temperatures actually are at when you put the case side back on and the mainboard reaches thermal shutdown point.  Most mainboards will allow you to adjust it from, say 60deg to 90deg (celsius).  It'd be handy to know the CPU model though because there are recommended operating temperatures.

If the sensor really is faulty, you can bypass the sensor completely and run the fan on full speed by modifying the fan connector so that the yellow wire (for 3-pin connectors) is disconnected.  This will supply the fan with a constant +12VDC via the (usually fluctuating voltage) red (middle) wire because the pulse  wire (usually yellow) is disconnected and the fan speed will report as 0 RPM.  The mainboard will compensate by supplying full voltage.
If the fan is a 4-pin, do the same with the wire that is two across from the black ground wire (this should line up with inside edge of the three-pin compatibility the key (notch) that the connector slides onto.  That way the fan speed will report zero and the power fluctuation is controlled ONLY by the PWM wire (the 'other' wire on the 4-pin).  If the PWM cannot regulate correctly, disconnect the wire for it from the connecter also.  OR, find a 3-pin fan and connect it to the 4-pin connection sans pulse wire (the yellow one).

NOTE:  certain brands like to use different colours than standard, so always remember that black is GND, +VDC is next to it, then PULSE, then PWM (if present)

The molex adapter Gary suggested is also a good idea.  Depends on what's available to you really.

But before you do any of this, try a BIOS update on your mainboard.  Sometimes the vendors release fixes for thermal issues similar to the one you describe.
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