Is my PC's power supply possessed?

February 23, 2008 9:46 AM

When my parents' PC is turned off, the fans on the motherboard (case and cpu) spin. What's the deal? Setup and more info inside...

When my parents' PC is turned off, the fans (and lights) on the motherboard (case and cpu) spin. The computer is NOT turned on, but when it is turned on, it works fine (POST, HD spin up, OS boot). What's the deal?

System Setup:
asus a7n8x-e deluxe motherboard and a generic power supply of unknown wattage, plugged in to an APC PowerChute Personal UPS.

This behavior happens whether or not the pc power supply is plugged into the UPS on a battery-backed up outlet, or just the surge protected ones on the UPS. It also happens when the pc power supply is plugged directly into a wall.

This PC is getting a bit long in the tooth, but it hasn't exhibited this behavior before, and has been sitting in the same place in their house for the last couple of years. If it's as simple as picking up a new ATX power supply my parents would appreciate not having to buy an new PC or motherboard or what have you.

I've been building PCs for years, but haven't seen any weirdness like this before. Any ideas?

posted by zackola to technology (8 comments total)

Does it keep spinning forever or just for a little while? My first thought was maybe it's trying to do some additional cool down before cutting the fans.
posted by jclovebrew at 10:18 AM on February 23, 2008

Sounds like a heat-protection circuit kicking in to prevent overheating after shut-down. Cars do this, too.

Makes perfect sense, though it would drive costs up somewhat. If you run some motherboard-monitoring software (seen it used; sorry, can't name it), you can monitor the board temps, and determine if it's running hot these days (which can change over the life of the board, which might explain why it didn't happen before).

All of this assumes it only runs for a minute or so, of course.
posted by IAmBroom at 10:49 AM on February 23, 2008

It keeps spinning forever :-/
posted by zackola at 11:08 AM on February 23, 2008

I don't think it's a heat protection circuit because it happens pre-startup too, not just post-shutdown.
posted by zackola at 11:45 AM on February 23, 2008

ATX PC's are 'live' all the time unless you physically unplug it or use the master switch on the psu; it has to be, as there's no mechanical link between the switch on the front and the power supply, only electronic controls from the motherboard bios.

The board fan headers are normally turned off via the bios, sometimes with a 'spindown' time when the front switch is first turned off.

It could be pretty much any component of the PC that's causing the system to fail to idle properly, though since it boots normally when turned 'on' it's almost certainly a motherboard fault, either in the bios itself or the power regulator circuits. It's not particularly likely to be the PSU itself, they're pretty simple beasties, though it can't be ruled out. Best way to confirm that is check the voltages in the BIOS; if any of them are seriously out of whack, then it may well be PSU.

I'd do a few things, just to check - reset the bios to defaults, try changing the q-fan settings, blow out any dust (no vacuum cleaner!) and reseat as many parts (especially RAM) as you feel confident of. If you're feeling brave, you could flash the bios with a newer version.

It wouldn't hurt to run memtest86 and a cpu burnin test like prime95, just to make sure the RAM and CPU aren't failing when booted.

Without being able to run other tests, like doing component swaps, my best money would be on a motherboard fault I'm afraid. It's definitely getting long in the tooth for repairs, though it is just about possible to still find new socket-A motherboards.

BTW - hope they have good backups. Hard-drives of that age are definitely at increased risk of failure, and the fans bearings might well be going.
posted by ArkhanJG at 1:58 PM on February 23, 2008

Sorry, I missed that you've been building your own PC's for a while, I'd imagine you're pretty comfortable with everything I suggested! I've seen this fault most often when there was a problem with CPU or RAM stopping the PC going through pre-POST properly, so just sits on a black screen with the fans running full beam as soon it's turned on at the mains, though that's because it can't complete and get to POST, which isn't the case here.

I've also seen your exact problem once before, that did turn out to be a motherboard fault (blown capacitor on the PWM IIRC).

One last thought that occurred to me is check the reset and power switch plugs on the motherboard - it's just about possible it's some odd mechanical or short problem with the on/off switch or reset switch itself.
posted by ArkhanJG at 2:18 PM on February 23, 2008

Thanks ArkhanJG. Once I get back in front of it I'll check out all these suggestions.
posted by zackola at 7:11 PM on February 23, 2008

Well, finally got around to having a closer look this weekend. Turned out to be the PSU after all! Slapped an old 350W ATX in there and everything is looking good. Very very weird.
posted by zackola at 7:10 AM on April 21, 2008