# Desktop turns on and off by itself BEFORE posting!



## Justarius (Sep 25, 2008)

Hi there, I'm new to the forums and I'd like to say thanks in advance for reading. I've been working with PCs for years and I know my way around so feel free to be as technical as you'd like when responding.

The problem: Dell desktop (Dimension 5150) turns itself on (without pressing button) boots for a few seconds but before posting turns off again. Won't respond to power button unless you unplug the power cable and plug it back in at which point the cycle repeats.

Background:

My friend went away for the weekend and came back to find his computer off (he leaves it on). He turned it back on and it started to power up (lights came on, fans and hard drives start to spin up, etc.) but before it could post, it shuts back off (so after 3 or 4 seconds). He couldn't get it to turn back on again until he physically disconnected the power. 

At this point he turned it over to me and I did the following:
- Tested power supply with my PSU tester. It passed.
- Double-checked by using a spare power supply. Same problem. Plus the green LED on the motherboard is lit as long as power is connected. I'm pretty much ruling out the power supply. It was replaced not that long ago anyway with a decent Antec PSU (the Dell OEM ones are junk)
- Unhooked the cables from all non-essential components (DVD, HDD, Floppy) and removed LAN, TV Tuner and Video card

Still no change. Except, now I have a new symptom! The PC will now TURN ITSELF ON as well as failing to post. i.e. As soon as I plug power back in it starts booting (but still fails and shuts off again and then has to be unplugged).

Finally I removed the Ram so now only the power supply, CPU and the cable that connects the case buttons (which on the Dell is an all in one cable for everything. No separate cables for power, reset, HDD led, etc.) are connected to the motherboard. 

With the Ram out it gives a long error beep but still fails and shuts off before posting. I also tried re-seating the cable that connects the case buttons/LEDs.

I'm stumped! I figure it's one of the following in order from most to least likely:

1. His motherboard is pooched
2. The electronic on/off switch is screwed up. If it was being constantly activated that wold explain why the PC turns on when power is connected and why it shuts off again shortly thereafter (as I'm sure you know, if you hold down a power switch for a few seconds it prompts a hard shut down). However, it doesn't explain why the PC is then dead unless you physically disconnect and reconnect the power.
3. Some problem with the CPU? If it was instantly overheating maybe but we're talking like 3 or 4 seconds after turning it on...

He's out of warranty so I thought I would check with you guys before calling Dell. Any thoughts?

Thanks for your time!


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## TyDeacon (Aug 11, 2007)

It does sound like the motherboard to me.


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## grimx133 (Jan 15, 2008)

Have you tried pulling the front panel connecter off the header and then shorting the pins for the power switch, momentarily? It's probably marked PWR_SW on the motherboard, though I've never seen that board so can't be sure. Would also be interesting to see if it tries to start spontaneously as soon as it's plugged in without the switch being connected.


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## Doby (Jan 17, 2006)

Check and make sure the heatsink is tight if not that would explain the sudden shutdown.

As grimx said it would be interesting to see if the power switch is somehow to blame but the insperions I worked on are not labled on the motherboard but you can trace the wires back to the switch and see what pins they connect too


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## Justarius (Sep 25, 2008)

Hey guys, thanks for the replies.

grimx133 - I had thought of this but it's a Dell so all the front panel connectors are merged into a single cable that plugs in similar to if you were plugging in an IDE cable. There's no labeling of the which pins do what. 

That being said, as Doby points out I could in theory figure that out by trial and error or dissecting the front panel and trying to trace. Not looking forward to that though!  

Doby - I'll also try the heatsink. I have some Artic Silver paste so I can reapply. It's pretty much the only thing left to try but I was stalling in hopes there was something simple I had overlooked.

Ugh, so trace pins, scrape off thermal paste, clean and reapply or call Dell's support and suffer through an endless list of scripted questions they won't let me answer in advance... hmmm, I guess it's like ripping off a bandaid. I'm rooting for the pins! 

Thanks for the advice, I'll post what happens if anyone is interested.


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## Justarius (Sep 25, 2008)

Hmm, I think I'm out of luck on tracing the pins. The ribbon cable used for front panel connectors goes to a small daughter card that is soldiered to the front of the case (by the front USB ports). Looks like it handles the front USB, headphone/mic jacks as well as panel on/off/reset/LEDs. So no way to tell what is what. I guess I could randomly try shorting 2 pins but since this connector also handles USB and sound... Plus there seems to be less pins than all of that PLUS front panel controls...

Damn Dell proprietary cases... <grumble>


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## Doby (Jan 17, 2006)

Don't call dell if its out of warranty they want like 147 bucks and most likely tell you a blown motherboard.

How did you make out with the heatsink? I had dells before where the plastic retainer that the heatsink latches to crack and the heatsink was loose causing it to shutdown.

Randomly cross the pins you don't have much to loose at this point but my bet is the cpu is overheating because somthing is messed up with the heatsink or thermal paste. The motherboard giving off post beeps is a good sign.


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## Justarius (Sep 25, 2008)

Justarius said:


> 2. The electronic on/off switch is screwed up. If it was being constantly activated that wold explain why the PC turns on when power is connected and why it shuts off again shortly thereafter (as I'm sure you know, if you hold down a power switch for a few seconds it prompts a hard shut down).


Hey guys, thanks for the replies. It turns out my second hypothesis was correct. I tried reseating the heatsink just to be sure and then finally started randomly shorting pins where the Dell proprietary front panel ribbon cable plugs in until I could get it to turn on and bingo bango, it worked! 

So my new question is how the heck do we fix it? That case is totally proprietary. There is zero chance of moving the motherboard to a new case. So I guess I can call Dell and ask how much for a new case but that will be painful. Other options, maybe it's the daughter card at the front of the case that seems to be where the front panel connections are made. Maybe Dell will replace just that? Or maybe I can rip the panel off (looks difficult but doable) and replace just the button somehow? I'm doubtful because of the daughter card... One would think a cable of some sort must run from the button to it though and maybe that can be replaced or re-soldered.

If none of that is workable I have 2 other thoughts...

1. Spray some contact cleaner on the button and hope for the best
2. Forget the rest of the front panel (he doesn't need the USBs and LEDs/Reset switch are optional) and jury rig in a NEW power switch that just connects to the correct pins.

What do you guys think?

Thanks for reading!


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## Doby (Jan 17, 2006)

Contact dell spare parts and see if you can replace daughter card, if thats out of the question the only other option would be to rig a switch, seeing as the machine will start by touching its only a monetary switch


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