On button power failure -- causes intermittent start

hello, I went through the incredibly anoying acount creation process here to help you guys out. I am a tech with many decades of experiance . At any rate the bottom line is that in my opinion it is likely to be a case of a power supply being loaded down either by too many devices using too much power or a faluty device within the omputer that is suckign up too much on one of the voltage line. a power supply powers up and the volatages are monitored as the power up is in progress if a voltage is not present or if one is not there within a certain time frame an error will occur resulting in a power down..one solution is a power fupply with more power output in case a voltage is being over loaded or to verify an overloading condiont you may try unplugging all but the absoulte esential devices and see if the problem goes away indicating an overload condition. the problem can also be verified by a tech by monitoring with a scope or logic analizer all the volatate lines and triggering on the power fault signal looking at what previously happend which should reveal which voltage line is not comming up or comming up too slow. I know this is technical but if the right tech reads it hopeuflly he wil lunderstand what I am suggesting he shoudl troubleshoot.

easyest fix is try a Power supply with more wattage output. if tha tfixes it then no need to go further if not then ou may have to troubleshot by the process Ive discribed. GOod luck hope this helps.

Also check for devices heating up excessivly.

my 11/2009 macpro has done the same thing for a couple of months, would start with the smc unplug method for a while then this week wouldn't start at all. took it to the genius tonight and it started right up and tested fine with their power cord, tried my power cord there - wouldn't start. gave me a loaner power cord and so far works fine at home. if it keeps working fine, it was just my power cord going bad. hope so.

power cord didn't work. for a while the first day it did, then back to intermittent no start, seems to be worse after it's shut down for a while. i've been able to start it after unplugging and plugging back in, back to the genius bar. looks like a power supply issue from what i've read here.

I have the same problem with my Mac pro Desktop recently. I can't figure it out if it's power supply or memory module...

Diagnostic LED on the motherboard indicates everything is fine, power supply, graphic card..

This is what I've done so far but no improvements. The computer still shuts down unexpectedly.

-I replaced the front fan module.

-I replaced the PSU.

-I replaced the motherboard.

-I replaced the battery on motherboard.

-I replaced the CPU on motherboard.

I would appreciate if you could provide some tip that you can think about. Thanks.

So you replaced the PSU? With a brand-new one, I assume? My assumption after doing extensive testing today (stripping everything out except PSU and back board) was that it must be the PSU, despite the fact that the back board's 5V Standby LED lights when the diagnostic button is pressed. Sometimes she boots, sometimes she don't wanna boot. I jumped the pads to power on and also used front button—it is not the power switch on the front board, as its the same when jumping it. When you start it she goes "click", and that's all you get. Then, interestingly, you have to hold the power button down (or jump it—same thing) for ten seconds to give it a hard power-down. Then, generally, after doing this cycle from 1-6 times or so, it will boot up. Is this the same for you? That indicates to me that a component either needs to be heated to a temperature where it will work, or it maybe is a capacitor in the PSU that isn't holding charge correctly and needs cycling to store it up (doesn't make a lot of sense), or it could be a defective relay. The caps in the PSU all passed at least a visual inspection—not saying much because a visual failure (ie, swelling or leaking) would almost certainly result in a total lack of power; although I have seen instances in automotive computers where a leaking cap just resulted in a weird behavior like one cylinder missing in the engine, which destroyed the engine... That cap wasn't in the power-supply side however, like most of the ones in the PSU.

Anyway, I am curious if you have this exact same issue, as it does strongly suggest the power supply. And, also, did you replace with a brand-new one or a used one? The only other parts to suspect would be the back board or the connection from the PSU to the back board... I did see some not-so-awesome-looking pads on the backboard, possibly indicating that they may have had some manufacturing defects.

Please let me know! Thank you. Otherwise I'll be tearing the PSU down again and going through it with a fine-tooth comb, which is a pain in the D when you just want a machine that works! Still, better than throwing money at a new PSU just to find that it doesn't work, either, and the problem lies on the back board somewhere.