System turns on by itself.

Posted on 2014-01-23
Last Modified: 2014-01-29

I’m curious as to whether anyone can provide some insight as to why a motherboard would draw power 25 minutes after the power switch was pressed. Please read on for details.

I was brought a Compaq desktop PC to test after the customer said it was blue screening. He did not document the errors so I have no idea of the cause.

I connected the system to a surge protected AC outlet and pressed the on button and absolutely nothing happened. I checked the cables and ensured that the green power LED on the PSU was on. It was and everything was connected correctly.

I swapped the PSU but the result was the same, no power. I swapped out the RAM, cleared the CMOS and replaced the CMOS battery. Same result.

I removed the mainboard, placed it on my test bench and connected all the necessary cables. Still no joy. I removed the power jumper from the front panel header and made myself a coffee. I then called the customer to give him the bad news.

After about 15 minutes into the telephone conversation the system powered on. But I hadn’t touched the thing for 25 minutes and the power jumper was not connected! How can this happen?

I have never seen this happen before and I have no idea why or how it happened so if anyone has any ideas I would love to hear (read) them.  

Kind regards.

The Funster.
Question by:The-Funster
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LVL 19

Assisted Solution

Patricksr1972 earned 67 total points
ID: 39804413

Sounds like a bad contact, i would guess in the area of the CPU, the machine has recieved a queue to boot before removing the on connector so it struggles to get its power on and finally through some kind of electrical interference finds it way to the next connection and boots...

I would try to take out the cpu and clean its connectors to start with.
LVL 48

Assisted Solution

dbrunton earned 66 total points
ID: 39804446
>>  I removed the power jumper from the front panel header and made myself a coffee.

Not understanding the power jumper bit ... did you just disconnect the on/off button here?

I'd check the motherboard for bad capacitors - see - for starters and I'd also try starting the motherboard by shorting the power on the pins on the motherboard with a screwdriver, ie at the pin block there.

Author Comment

ID: 39804623
Hi. Thanks for the quick replies.

Re: your reply dbrunton. The test bench (Cooler Master Labs) has a reset and power cable that connect to the pins on the front panel header so the system can be powered on or reset using the power and reset buttons on the test bench. These negate the need to use a screwdriver. They were disconnected when the system powered on.

I'll be out now for the rest of the evening but I will check the CPU and capacitors tomorrow.

The Funster.
LVL 48

Expert Comment

ID: 39804761
Ah, mind not fully engaged.  Thought you were talking about the front panel header on the box rather than the motherboard.


Hmm.  You've done BIOS and memory.  

Possibly remove CPU and reseat (sometimes not fastened down correctly).  Remove fan connection and reseat.  Was CPU fan spinning while not powered up?

If it starts up again I'd get into the BIOS and check startup options and temperatures and do resets within that but I'd expect your clearing of BIOS to have taken them back to default anyway.
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LVL 91

Expert Comment

ID: 39805875
what motherboard is this?  some are famous for bad capacitors.
when you push the power buuton -  do you see any light on the mobo?
is 5V and 12 V there?  measure on the spare PS connectors , - fans running = 12 V ok

here a troublesooting chart  for the PS  : supply 0

Author Comment

ID: 39813244

After a very busy few days I'm back on track with this thing.

I've checked for bad caps and removed and cleaned the CPU and also checked to see if I can see any obvious solder faults. The odd thing is that since the delayed system start that brought me to this situation, the darned thing has powered on every time without issue.

I have to say though that the various tests I have carried out since have shown that there is definitely still an issue. The CPU, RAM and mainboard tests I have carried out intermittently fail to complete, freezing during testing. Oddly though, when they do complete they show no errors.

I've swapped out the RAM and PSU but the results are consistent. I don't have an alternative AM2+ mainboard or CPU to try without investing money, only to probably confirm my suspicions that the mainboard is screwed.  

In response to nobus, the mainboard is an Asus M2N68-LA in a Compaq desktop. I've tried 3 PSUs in total and all have been successfully tested with a standard PSU tester.

My customer has decided to go for a replacement system rather than have the faulty system repaired but I'll wait another day to see if anyone else here has anything to add.


The Funster
LVL 91

Accepted Solution

nobus earned 67 total points
ID: 39814274
Funster, please not htat bad capacitors can also reside in the PS (i know you tried 3)
also note that a PSU tester is not a valid test, it only test the PS with a small load, not under normal conditions, so you must consider that as a first indication only - not conclusive.
you can test the PS on other boards, to be sure.

also, look for minidumps - since it was bluescreening, it may have more info
find them in windows \minidumps and post the latest here
you can also look in event viewer

Author Comment

ID: 39818671

I'm going to have to pull the plug on this one because my customer called this morning requesting his faulty unit back in order to pass it on to a local PC recycling company. As a result I'm not in a position to carry out any further testing.

I understand his reasons. He's replaced his system and wants to keep my bill to a minimum. (I would of continued the testing for free just to get to the bottom of things but it's his PC so I have no say in the matter.)  

In response to your last reply nobus, I'm obviously no longer able to check for minidumps. As for the PSUs, the 2 units I used from my own equipment have been used regularly on other systems with no issues. The PSU in the customer's Compaq system is of the type used regulary by HP. These units have a status LED on them. If the PSU developes a fault, the LED flashes.

Because of my situation I realize that this issue is going to remain unsolved. I think the fairest thing to do is split the points between all respondents.

Thank you all for the advice you offered.

The Funster.
LVL 91

Expert Comment

ID: 39820109
thanks for feedback, funster

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