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PMU Reset failed; do again?

I have reset the PMU on my Power Mac G4 before with success. This time I was hurrying and forgot to disconnect the usb cord to the UPS, and the ethernet, and the speaker connection. (All of those items were off and not connected to the house power at all.) I did disconnect all other peripherals, the G4 power, and the G4 monitor cord. Now the computer is still in the same state: the light comes on behind the power button but immediately turns off and there is no startup at all. Should I push the PMU button again? I have read that that can kill the PMU. (I did let at least 10 seconds go by before I reconnected things.)

PowerMac G4 MDD Dual 1.25, Mac OS X (10.4.11), 2G ram; original Superdrive; Apple Computers since 1988

Posted on Aug 15, 2011 3:44 PM

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Posted on Aug 15, 2011 5:18 PM

Pull everything, including the PRAM Battery, let sit an hour or two & try the PMU once more.

11 replies

Aug 15, 2011 10:26 PM in response to BDAqua

Thanks for the suggestion. I hadn't thought of the battery. (it's the original...late 2002.) I followed your advice but the computer is still not able to start up. Do you think it would hurt the PMU if I replace the battery and reset it again? Also am I supposed to press the PMU button more than a second or two?

Aug 15, 2011 11:20 PM in response to Picky

I'd replace the Battery, or try a new one, MDDs are really finicky, not sure I could recommend the PMU Reset again, but I think it takes 10-20 seconds iirc.


Might be time to replace the PRAM Battery, 4 years is close to their lifespan, far less if ever without AC power, & can cause strange startup problems...


http://eshop.macsales.com/item/Newer%20Technology/BAA36VPRAM/


Or Radio Shack for around $20.

Aug 16, 2011 2:37 PM in response to BDAqua

After leaving the MDD unplugged and battery-less overnight, I installed a new battery. I did not reset the PMU, but plugged in and had no luck starting up. So I unplugged, pressed the PMU for about 3 seconds. I plugged the power and monitor cords in again, maybe too soon (!) and when I pressed the power (this time using the monitor built-in "button") I heard the faintest hint of a fan trying to start, but then silence and nothing more. Now I have the battery pulled again and all unplugged. Next time, should I reset the PMU before or after I reinstall the battery? Should I also let the computer sit plugged in for half an hour or so before I unplug and press the PMU again?

Aug 16, 2011 3:33 PM in response to Picky

Picky wrote:


Next time, should I reset the PMU before or after I reinstall the battery? Should I also let the computer sit plugged in for half an hour or so before I unplug and press the PMU again?


OK, pressing the PMU reset without the battery is pointless. You want the PMU to reset, so it has to be running and it needs the battery to do that. Now, the other issue is that you want it to complete its reinitialization. If you remove the mains power and the PRAM battery, let the machine sit for an hour or two to let any remaining charge in the logic board electrolytic capacitors drain off, then put the battery back in, press the PMU reset once, then wait at least a minute (much longer than actually necessary) before connecting mains power to the computer, you have done all that you can do with a PMU reset. If that doesn't cause more to happen, then you either have a power-supply or logic board problem to deal with.


BTW, while it might not be completely necessary to press the PMU button after replacing the battery, it is the better strategy because voltage glitches associated with inserting the battery may foul the power-on initialization of the PMU.

Aug 16, 2011 8:17 PM in response to BDAqua

WELL this is WACKY! I did some chores, and after an hour or two, I decided to push that power button one more time just for the heck of it. (I'd left it plugged in.) It Booted up! I'm crossing my fingers now. Making all kinds of extra backups.Thank you so much BDAqua and old comm guy! Sorry there isn't a way to change your answers status from Helpful to Correct.

Aug 16, 2011 9:11 PM in response to Picky


Picky wrote:


WELL this is WACKY! I did some chores, and after an hour or two, I decided to push that power button one more time just for the heck of it. (I'd left it plugged in.) It Booted up! I'm crossing my fingers now. Making all kinds of extra backups.Thank you so much BDAqua and old comm guy! Sorry there isn't a way to change your answers status from Helpful to Correct.


This is neither unusual or much susceptible to explanation. I had experienced some of the same issues with the G5 last year, including the "leave down for a while and then boot successfully" events. What the ultimate cure was is not exactly known, except that a power supply replacement certainly eliminated the recalcitrant behavior.


What I can say from long* experience is that eventually certain components will age, most notably the aforementioned electrolytic capacitors, which have a tendency to "dry out" over time and give up some of their µF, which can eventually cause some decidedly bizarre behavior.


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*I decline to put the actual and unseemly large number to that particular adjective, thank you.

PMU Reset failed; do again?

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