Computer won't boot, no POST codes, cpu fan not working

jbclem · Joined: Apr 16, 2008 Posts: 14

I've taken out all the cards, installed a pci DMAN POST diagnostic card. It tests for 3 voltages on the motherboard (+5v, -12v, +12v) and they are ok. The RESET signal is inactive.

I seem to be getting voltage, I've also checked with a voltmeter at the motherboard plug. I also checked at the pins the cpu fan plugs into, there is 5v at one of the pins, nothing at the other two. The cpu fan blades turn freely but the fan doesn't power up. Another clue is that the hard drive light (on front of case) is on solid, doesn't flicker. The hard drives seem to spin up but never make the boot up sounds (reading/writing). They seem to quiet down about 30 seconds after I try to start the computer.

Those are the clues I have. This has been an occasional problem that goes away if I keep trying to start up. This time it won't start.

Can someone tell me if a bad cpu fan will create this problem, keep the computer from even starting the POST. My DMAN POST card does not show any numbers, and there are no beep tones heard.

I'd like to look for the motherboard grounding out, other than a real obvious place, not sure where this can happen...can the hold down screws short out?

Any suggestions for me?

John

jbclem · Joined: Apr 16, 2008 Posts: 14

I just plugged a different cpu fan into the motherboard, it doesn't run either. Since there is 5v at one of the three pins, that must mean that there is a signal that turns on the fan(completed ground?) and it's missing.

Forgot to mention what kind of computer this is. lt's an older Asus P2B motherboard, with an adapter that holds a P3 1000mHz cpu. 512mb Ram.
3 hard drives.

John

drwho07 · Joined: Nov 29, 2007 Posts: 1134 Location: Central FL, USA

My first guess would be that you've lost the -5 VDC from the power supply.
That's the one voltage your tester doesn't check.
You need to use one of the PSU testers that plugs right onto the big power plug that goes to the mobo.

I'd jack in a new PSU to test that theory. Without the -5vdc line active, the PC won't boot.

I probably have a dozen bad PSU's in my junk box and they are all missing the -5vdc.

Are any of the filter capacitors on the motherboard bulging or leaking?
Those are the little cans with plastic wrapping and shiny tops.

Cheers Mate!
The Doctor

jbclem · Joined: Apr 16, 2008 Posts: 14

Hi Doctor,

I don't have a PSU tester, but can you tell me what the sequence is on the large white plug that carries the voltage. I can look for the -5v on the plug if I know where to find it. Actually, I can look anyways but I don't know if there is only one -5v or more than one.

I'll also take a close look at the filter capacitors. And I may have another power supply I can try.

Thanks for the help.

John

drwho07 · Joined: Nov 29, 2007 Posts: 1134 Location: Central FL, USA

Unfortunately,,,,, I speak as an experienced computer tech with a whole shop full of stuff, including PSU testers and of course extra PSU's, to swap with when needed.

I recently took in an eMachine for repair...... with a bad PSU.
The -5 volt line was dead.
Sorry, I don't know what pin that's on.

After putting in a new PSU, I found that the mobo was dead too. I could only suspect that something on the mobo shorted out and took out the -5v line.

Schmidt happens! (even to the nicest people)

Good Luck,
The Doctor

jbclem · Joined: Apr 16, 2008 Posts: 14

I checked all the voltages at the motherboard power plug, there was -5v, +5v (several), -11.70v, + 11.89v. I then tried using a brand new power supply. Same problem, the motherboard wouldn't even start the POST, and the cpu fan still wouldn't spin even though it had 5v on one of it's pins. I also removed the 3 pieces of RAM and tried them out one at a time. No difference.

Also looked at the many filter capacitors, didn't see any signs of leakage or bulging.

Can't think of anything else to do besides try another motherboard. Have I missed something? My gut feeling says I have.

John

Werebo

It's rather remote, but have you checked the power plug connectors, when it's hooked into the mobo?? The thing to look for is if any connectors are being pushed up, as the plastic moulding is pushed down.

I had this happen with a HD power connector, and it took ages to find - as I was inserting the molex connector, one of the little tubes that slide over the pin was too tight. The result was that the metal connector tube pushed out of the molex plug, not enough to fall out and give the game away but just enough to not make that vital connection.

As to your original question about the mobo mounting-screws shorting out - o,not usually. It's worth checking to make sure they're screwed down tight, but not over-tightened A cracked mobo is just as dead as a sort-circuited one Wink

However, it's always possible a dropped screw (or one from elsewhere) has landed somewhere vital - even rolled under the mobo.

Failing that lot, I'd start to suspect the mobo has suffered a total-existence failure.

goretsky

Hello,

Was the motherboard working properly before with this processor and memory, or are all the components untested? If the latter, are the processor multiplier and FSB jumpers set properly for the CPU?

Regards,

Aryeh Goretsky

Weejie · Joined: Apr 20, 2008 Posts: 1 Location: Sconnie Botland

jbclem · Joined: Apr 16, 2008 Posts: 14

Thanks everyone for the help. I tried a new power supply and the problem was the same, so I've reluctantly concluded it's probably the motherboard. I have another motherboard I was going to use for my second computer, but I'll install it temporarily in this one. The only question I have is do I take a chance on using the old power supply, even though it seems ok. If the output voltages are correct does that quarantee that it will do no harm. Even if I don't use it here, I'd like to keep it as a spare and know that it's good.

John

Werebo

If the voltages are correct, there's little chance of damage on the alternative mobo. The only likely chance of damage, would be if the mobo was defective - in which case, it's best to know rather than later. Wink