Old MOBO, PS and CPU-fan running without start button.

GIGABYTE - Motherboard - Socket 775 - GA-945P-S3 (rev. 1.0)
I have everything disconnected apart the two power connecters.
When I turned on/connect the AC, the Power-Supply turns-on and the
CPU-fan runs at full speed.

Has anybody got an idea why this is happening?

Patrick wrote:
GIGABYTE - Motherboard - Socket 775 - GA-945P-S3 (rev. 1.0)
I have everything disconnected apart the two power connecters.
When I turned on/connect the AC, the Power-Supply turns-on and the
CPU-fan runs at full speed.

Has anybody got an idea why this is happening?

Look in the "Power Management" section of the BIOS.

I have a Gigabyte manual from a 945P motherboard,
but not the same one as yours, and the last item in
that BIOS section is:

AC Back Function

Soft-Off When AC-power back to the system,
the system will be in "Off" state. [Default value]

Full-On When AC-power back to the system,
the system always in "On" state.

Memory When AC-power back to the system, the
system will return to the Last state
before AC-power off.

I had a BIOS NVRAM setting change the other day,
*without* tripping the checksum. And resetting
my settings and re-programming the custom ones
brought it all back to correct operation.

That shouldn't be necessary in your case. Just
set it back to the default choice they showed in
the manual, the Soft-Off, and retest and make
sure it works. Shut down. Shut off AC power at
the back. Wait several minutes for the inrush
limiter to cool off. Switch on. And if the
corrected setting worked... the machine
should quietly wait for you without doing
a thing.

*******

The second possibility, is you've had a hardware
failure, or you dropped a tool inside the
computer and broke the pullup on the PS_ON#
signal and now it's pulling to ground and
won't let go. Normally, when PS_ON# control is
lost, it's the motherboard end, the transistor
is no longer strong enough to pull the signal
low and turn on the PSU. But in your case, it
would be the opposite scenario. The signal
never returns to the de-asserted state (
where +5V appears on PS_ON#). Either a pullup
resistor is missing, or the transistor has failed
in the ON state.

These aren't too likely, unless
there's something pretty abnormal going on
(like, a leaking electrolytic capacitor spilled
orange/brown corrosive liquid on the traces and
ruined something). I've only had PSUs do that,
no mobos so far. My failures were in my
collection of Antec Channelwell ones.
(The Antec Delta might well be fine.)

Paul

On 23/12/2020 20:44, Paul wrote:
Patrick wrote:
GIGABYTE - Motherboard - Socket 775 - GA-945P-S3 (rev. 1.0)
I have everything disconnected apart the two power connecters.
When I turned on/connect the AC, the Power-Supply turns-on and the
CPU-fan runs at full speed.

Has anybody got an idea why this is happening?

Look in the "Power Management" section of the BIOS.

I have a Gigabyte manual from a 945P motherboard,
but not the same one as yours, and the last item in
that BIOS section is:

AC Back Function

* Soft-Off** When AC-power back to the system,
************ the system will be in "Off" state.* [Default value]

* Full-On*** When AC-power back to the system,
************ the system always in "On" state.

* Memory**** When AC-power back to the system, the
************ system will return to the Last state
************ before AC-power off.

I had a BIOS NVRAM setting change the other day,
*without* tripping the checksum. And resetting
my settings and re-programming the custom ones
brought it all back to correct operation.

That shouldn't be necessary in your case. Just
set it back to the default choice they showed in
the manual, the Soft-Off, and retest and make
sure it works. Shut down. Shut off AC power at
the back. Wait several minutes for the inrush
limiter to cool off. Switch on. And if the
corrected setting worked... the machine
should quietly wait for you without doing
a thing.

*******

The second possibility, is you've had a hardware
failure, or you dropped a tool inside the
computer and broke the pullup on the PS_ON#
signal and now it's pulling to ground and
won't let go. Normally, when PS_ON# control is
lost, it's the motherboard end, the transistor
is no longer strong enough to pull the signal
low and turn on the PSU. But in your case, it
would be the opposite scenario. The signal
never returns to the de-asserted state (
where +5V appears on PS_ON#). Either a pullup
resistor is missing, or the transistor has failed
in the ON state.

These aren't too likely, unless
there's something pretty abnormal going on
(like, a leaking electrolytic capacitor spilled
orange/brown corrosive liquid on the traces and
ruined something). I've only had PSUs do that,
no mobos so far. My failures were in my
collection of Antec Channelwell ones.
(The Antec Delta might well be fine.)

** Paul

Thanks Paul for your reply .

I have machine torn apart at the moment (secondary PC). I connected the
PS to AC (without connection to MOBO) and the PS fan was turning. Took
the PS apart, noticed a few bulged capacitors, cleaned some dust-bunnies
out and put the PS together again. I will (when I get a 'round-tuit')
check the voltages coming from the PS, etc.

Thanks again Paul for your consideration.

On 23/12/2020 12:53, Patrick wrote:
GIGABYTE - Motherboard - Socket 775 - GA-945P-S3 (rev. 1.0)
I have everything disconnected apart the two power connecters.
When I turned on/connect the AC, the Power-Supply turns-on and the
CPU-fan runs at full speed.

Has anybody got an idea why this is happening?

I replaced the PS (was due a new one on my primary machine), the MoBo is
now working, no HD/SSD connected yet but have access to the BIOS and
have several 'Macrium' backups.

Thanks Paul (and anyone else) for your interest.