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I was testing my PSU with the paperclip method and checking the voltage of every wire just to be sure it is 100% functioning.

What surprised me was that every wire was working just fine but one, the blue wire in the 24-pin connector that connects to the motherboard. It was reading -37 V instead of -12 V (as far as I know it should read -12 V). Anyone have any idea? Also my PC won't turn on; is there any chance that this fried my motherboard?

Initially this was the problem: Asus anti-surge protection fried (?) motherboard and PSU

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    All ASUS (as far as I know) mobo has voltage sensors on every voltage. It is highly likely that the mobo detects this unusually condition and will not turn on as a safety measure.
    – cybernard
    Commented Dec 26, 2013 at 1:37
  • That's pretty enlightening because it didn't work on my brother's computer too! Maybe there is a chance that my MB is not fried (: Thank you!
    – Yuri Padilha
    Commented Dec 26, 2013 at 2:10
  • ASUS has a software package called AI Suite which, since you have an ASUS board, you can install and you can view voltages in real time. There is even a Sensor Recorder(part of AI Suite) which will record them for you over time. Since you can't turn it on this is not entirely helpful.
    – cybernard
    Commented Dec 26, 2013 at 2:14
  • Hum, yes, right now I can't turn it on. I guess the only thing I can do now is to wait for coursair support's answer. Most probably my PSU fried my mobo because I also tried to use my brother's PSU in my PC and I had no success, it just won't turn on.
    – Yuri Padilha
    Commented Dec 26, 2013 at 2:22
  • How many watts is your brother PSU? If it is too few the ASUS board will know.
    – cybernard
    Commented Dec 26, 2013 at 2:31

2 Answers 2

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Are you testing under load or not? Power supplies are not always designed to be run without a load (and at the same time produce the correct output), especially switching supplies which they pretty much all are today. Also sometimes the load on one circuit makes another part of the supply work correctly.

(I have repaired as many as 100 or more switching power supplies, not all alike either. It was my job once.)

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  • I tested it only connected with a fan. Actually it reads -37v without the fan and -38v with it. All the other wires remain on their perfect voltage. 5v, 3.3v and 12v. What can I use to load it?
    – Yuri Padilha
    Commented Dec 26, 2013 at 3:15
  • What I've found is the most likely component to fail in a system is the one that handles the most power, and your power supply fits in that category. It is very rare for a power supply to fry a motherboard. But it is quite common for a power supply to fail. You say you replaced it. Did you replace it with another brand new one made to fit your box? If you have not done that then do that. I think it's very likely that your system will come back to life at that point. Also unplug other things like drives just as a safety precaution until you get the bootloader alive.
    – Elliptical view
    Commented Dec 26, 2013 at 3:23
  • Thank you for your help Eliptical! Now I guess that the best thing to do is to wait for the coursair suport to send me a reply ( I told them the problem ) and get a new PSU. Also meanwhile I'm gonna search for a PSU to test my mobo!
    – Yuri Padilha
    Commented Dec 26, 2013 at 3:49
  • You're welcome. Hope it works out for you.
    – Elliptical view
    Commented Dec 26, 2013 at 4:11
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Negative DC voltages (-5 V and -12 V) are rarely used by most modern motherboards, specifically, -5 V which is not present on modern PSUs. The -12 V line, although available, is not normally used in the motherboard's logic circuitry, but some board designs use this for serial and LAN circuits.

So in my opinion, this is not the main concern and maybe not the main reason for the "dead" motherboard. Since the positive DC voltages of the PSU tested OK, try to test the motherboard with only the CPU and fan and see if the on-board LED lit up and the system powers up (use/short out momentarily the power switch connector on the motherboard, just to eliminate a broken power switch). Before doing this, try to reset/clear CMOS memory first. If the system powers up, connect the other peripherals one by one, in between power ups and hopefully finds the culprit.

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  • Hey Miggy! Thank you for your reply! Well, I think I did it already. Anyway I just did it all over again and had no success. Funny I measured all the wires of the PSU again and there's only one negative voltage, which is the blue wire that now reads -39.9v!! I had only a simple fan that lights up a neon connected to it when I tested. To test the PSU in the mobo I connected the CPU power ( 6 pin 12v ), connected the CPU fan in the mobo and connected the 24 pin into the mobo. Am I missing something? I tried to turn it on touching with a screwdriver into the +/- power switch contact. Hope I didOk
    – Yuri Padilha
    Commented Dec 26, 2013 at 14:32
  • Just curious, using your brother's psu or any other known good psu, does the negative dc (blue wire) gives the same or significantly the same voltage?
    – miggy
    Commented Dec 26, 2013 at 17:43
  • I didn't measure the voltage of his PSU's wires, I guess I can tell him to do this for me today. Most probably it's gonna read -12v because his PSU has working just fine on his computer. When I measured the voltage of the wires I was using the paperclip method and a simple fan with neon only.
    – Yuri Padilha
    Commented Dec 26, 2013 at 19:36

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