View Full Version : OT: computer turns off by itself.

Mike W
07-03-2007, 01:29 PM
My 4 year old PC with XP started turning off by itself. Sometimes it would not restart. When it turns off, it closes any open windows and then shuts down. I tried to boot up in cmos setup and once it shut down after 1/2 hour. Today it turned itself on and has been running for 3 hours.

I have tried a new power supply and I do not have a virus. The cpu is not getting hot. I have never seen it above 31 deg. Do you think the motherboard is flaky?

When it is running, I don't see any problems going on. It just decides to shut down by itself. Today is the first time it has ever turned itself on. Thanks for any ideas.

07-03-2007, 01:42 PM
What about a flaky on/off switch. In that it's doing a soft shutdown, it's almost has to be receiving the logic signal on the switch pins on the board. IIRC, the switch on a PC just puts a logic signal across two pins on the power supply telling it to be on or off. I think it's 5 volts but I don't know which is which. The last PC I had apart had a jumper style plug that connected between the switch and these two pins.

I'd try plugging a jumper in in place of the switch and seeing if it does any better. If so, then replace the switch.


Mike W
07-03-2007, 02:06 PM
I thought about the switch and will give that a try. It is 5 volts. One time when it would not turn, I forced the power supply to turn on by connecting the power on connection to ground on the power cable to the mother board. The power supply came on but the hd would not start when I did that.

07-03-2007, 02:23 PM
Interestingly enough, I was just working on a coworker's machine today. I'd replaced a failed power supply.

Unfortunately, the machine freezes on the "HP Invent" spash screen, so it has other problems. Reseating cards and memory sticks didn't get it back.

The relevant part was that I tried disconnecting the hard drive ribbon and power cables. Turn the machine on, and it turns itself off after about 2 seconds. Put the hard drive back, and it powers up and stays up. (but still freezes)

The new power supply is 250W, versus the original 150W. My guess is that, without the hard drive, the computer doesn't draw enough from the supply for it to regulate properly, and it shuts down. Switching supplies are often like that, but I've never seen this happen with a computer.


07-03-2007, 02:33 PM
Lots of possibilities for that bahavior. Bad switch is actually one of the less likely. Power supply, board overheating, or various hardware defects could also do it.

And not drawing enough power shouldn't be a problem for a PS. My current computer has a Antec 550W powersupply and by far and large I don't need a fraction of that. And I recently put a 400W into my wife's simply because it was on sale and the cheapest route when her's failed. She's got a very low end system with everything integrated on the mobo and very low power needs used for nothing much but internet access. If you think the PS is the issue, pin-out the voltages from the mobo connector. They should be within 0.1V regardless of the load.

07-03-2007, 02:38 PM
I thought about the switch and will give that a try. It is 5 volts. One time when it would not turn, I forced the power supply to turn on by connecting the power on connection to ground on the power cable to the mother board. The power supply came on but the hd would not start when I did that.
ATX uses an indirect PS control via the motherboard. I don't know specifically how it works, but bringing up the PS directly probably won't do it any favors. Shooting from the hip with a WAG, I would say you possibly have a mobo failure. But impossible to say remotely. First try reseating everything (if you haven't already), being careful to discharge and balance potential before starting. If that does not fix it, remove all unnecessary cards such as sound cards and NICs. Next, PIN out all voltages when it’s in failure mode (won’t boot) to see if you lost tap o the PS. If that does not fix it, you are into component swap diagnostic land. At least that’s how I usually approach an otherwise unknown problem like this…

07-03-2007, 05:49 PM
I used to have this very problem some time ago.I lived with it,until the power supply went belly-up.After replacing that,I had no more problems.
That's not too say you are suffering the exact same problem,but if you have a spare psu,it's worth trying to substitute that.At 4 years old it's probably due for replacement anyway.

07-04-2007, 12:26 AM
Four years old puts it at the tail end of the bad bypass capacitor debacle. Check the various bypass caps for bulging tops. If any show even the slightest bulge they are part of the 1/3rd of all capacitors made in the 2000 to 2003 time frame that are defective. The fix is a new motherboard.

A defective start switch will cause shutting down. Pressing and holding the start switch for more than 5 seconds causes a running computer to power off unconditionally. It will also cause the symptom of the computer starting when it is pressed and shutting off 5 seconds later. If the switch is shorted the computer will not turn on until the switch has opened momentarily.

Mike W
07-04-2007, 12:52 AM
I just got home and was informed that it turned itself on and shut down at least a few times. Today is the first time that it has ever turned itself on. I turned it on ok. I will try a few things tommorrow. I can't see replacing this old motherboard. I will check on a upgrade one way or another. Thanks.

07-04-2007, 12:59 AM
If windows is properly shutting down (closing open applications, etc.) I'm hard pressed to believe that it's a hardware problem. Any of the hardware problems that I've had result in very abrupt shutdowns.

You mentioned that there are no viruses, but you did not describe how you checked for them. A couple years ago, there was a series of viruses that caused windows computers to randomly shut down (following a proper shut-down procedure). If memory serves correctly, many of them successfully avoided detection by antivirus programs because they actually took control of the program. There are a few available tools that search for, and remove, these types of programs. One that comes to mind is Stinger, made by MacAfee. Stinger is free, and once downloaded, will scan for numerous viruses that can attack antivirus programs.

Edit: I started writing this before you posted to say that the computer turned itself ON. That's pretty strange. I'm not sure what would cause that. I guess a hardware glitch is the best answer.

Mike W
07-04-2007, 01:45 AM
It is kind of strange. When it shuts down, xp closes any open windows and then logs off and shuts down. Not like you would expect if the power just failed. I just got a flash drive with Linux on it that I can boot it up with. I was going to try that just for fun.

07-04-2007, 03:34 AM
Oh, well that's another animal entirely. You can program a "soft off" for the power switch such that a momentary press tells windows to shut down. So a contact in the switch momentarily shorting could trigger that. Likewise, when off, a momentary short could trigger the power on. It's not really an on/off switch, but rather just a trigger that is software latched in the mobo. Starting to sound more like the switch...

07-04-2007, 10:31 AM
I tend to agree with BadDog on this one. I had forgotten about the soft shutdown that can be triggered by the power switch. His diagnosis makes a lot of sense.

If you change the action that happens when you push the power button (it's a setting under the "Power Options" control pannel), see if that action happens instead of the computer shutting down automatically. I recommend the "Ask me what to do" option. If that window pops up randomly, instead of the computer shutting down, then you've really isolated the problem.

07-04-2007, 11:59 AM
If you have a reset button just move that wire to the power on connector position and try it for a while.

Mike W
07-04-2007, 09:06 PM
It ran fine for a couple of hours last night and several today. After this morning I swapped the reset switch for the power switch and will see what it does.