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A scientific experiment - John C. Kirk

Mar. 17th, 2010

01:39 am - A scientific experiment

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I recently upgraded my desktop PC - this involved cannibalising bits from a dead one, and buying a few new components. Unfortunately, something's not quite working: the CPU fan gets power from the motherboard, but the BIOS isn't getting any signal back, so it thinks that the fan is running at 0 rpm and the CPU temperature is 0ºC. So, either there's a problem with the fan/cable, or the socket on the motherboard is faulty.

I don't have a spare CPU cooler that will fit into my new motherboard. The one from my old motherboard only has 3 pins on the end of the cable (rather than 4), and the one from the dead motherboard has the screws on the heatsink spaced too far apart to fit the holes (which is bizarre, since they're both socket 775).

So, here's my plan: leave the existing heat sink and fan in place, but don't plug it in. Meanwhile, plug the giant heatsink/fan into the new motherboard, and go into the BIOS to see what it says. If it displays a speed then I know that the motherboard is fine, and the new cooler is broken. If I don't see a speed, I need to send the motherboard back. The only slight drawback is that the CPU will immediately start to heat up. I've done some digging on the web, and various random people suggest that it will be ok for a few seconds, and things will only start melting after a few minutes. So, if I'm quick enough, I should be fine.

I realise that this may not be the most prudent course of action, so I'll sleep on it before I implement it. If you see a tower of flame coming from my flat tomorrow evening, that probably means that things didn't go according to plan.

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From:shuripentu
Date:March 17th, 2010 10:11 am (UTC)
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There do exist CPU fans that run at sufficiently low RPM that some sensors won't detect any rotation at all; I had one. The recorded CPU temp is eyebrow-raising, though, and suggests either dead or intentionally missing sensors - it may be worth checking the motherboard manual.

If you CBA to send the motherboard back, there is usually a setting in the BIOS where you can turn off the auto-off feature (which is what I had to do with the previously mentioned fan). And no, your computer is not going to go up in flames if it runs for a little while with no fan. :)
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From:johnckirk
Date:March 17th, 2010 06:56 pm (UTC)
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When I first turned the machine on, it said "CPU fan error - press F1 to continue". That's when I checked the BIOS settings, and I've changed them to "Ignore" so that I can actually boot up the machine. However, that's not a brilliant long term solution, since I won't notice if the fan fails later (once the case is sealed up). So, if I'm going to risk a meltdown, I'd rather do it under controlled conditions - that way I can chuck the thing out of the window if I see flames :)

On a positive note, the CPU temperature is displayed as "0ºC (32ºF)". So, that implies that they really are using Celsius as their default temperature scale, which impresses me. (Unlike the Voyager episode where someone gives vital signs in Fahrenheit...)
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From:shuripentu
Date:March 17th, 2010 07:35 pm (UTC)
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I don't think I've ever seen CPU temps referred to in anything other than Celsius by default. :)
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From:karne_k
Date:March 17th, 2010 11:05 am (UTC)
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Modern CPUs all have temperature sensors on chip and will shut off before they burn out. Still not a good idea without a heatsink mind you since they'll still get very hot. With a heatsink and the case open, I doubt it'll over heat at all.

I'd suggest booting up and installing something like Speedfan - that'll give you the real fan and temperature values from the onboard sensors and the cpu sensor.

The 3/4 pin issues suggests to me that the mobo is simply not getting the TAC signal from the fan (check the yellow wire is connected to the right pin). You can get adaptors for that or just modify the wiring yourself.

No temp from the mobo sensor is more strange - if Speedfan still gives no values, I'd suggest a BIOS update.


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From:johnckirk
Date:March 18th, 2010 12:42 am (UTC)
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Thanks for the tip - I've now upgraded the BIOS, but unfortunately it didn't help. I can see the temperature of the motherboard and the speed of the chassis fan, but I can't see the CPU temperature or the speed of the CPU fan.

Just to clarify the pin issue, there are three motherboards and three CPU fans:

* Old desktop - 3 pin socket, 3 hole fan.
* New desktop - 4 pin socket, 4 hole fan.
* Scaleo - 4 pin socket, 4 hole fan.

I wasn't planning to buy a new fan until I realised that my old one had the wrong connector and the Scaleo one was too big :( Anyway, I connected the Scaleo fan to the connector, leaving the new fan/heatsink in situ, and I got the same results: the Scaleo fan whizzed around, but I still couldn't see the temperature/speed in the BIOS setup. The fan connector will only fit on one way round, so I assume that the relevant pin is broken.

I've contacted the seller, and hopefully they can send me a new one. However, I got the last one from Amazon, and SpecialTech don't seem to sell motherboards through their own website. If necessary, I'll buy a new mobo elsewhere and get a refund on this one, so it's just a bit of a faff.
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