My Mac G4(MDD) Baffles Me!To turn on or not turn on?

[visvaldis]

It’s a Mac, G4, dual processor, MDD. For about three months I’ve had this problem: press the power button, the light flashes briefly, goes off, Mac does not turn on. I press the button several times per hour. Finally, after about 90 minutes the Mac turns on. I reset the time and date and everything works fine.
The only problem is when I click (in the Apple menu) sleep, the Mac shuts down instead. So, next day I have to play the stupid “press the power button game” again.
Some weeks ago I called for help. A guy from Geeks-Online came by to have a look (for $140). He said it can’t be the power supply, because the Mac runs perfect once it is on. He said it must be the power button. New parts for this Mac are not availlable. From Usedmac.com I found a power button and ordered it.
Today I replaced the original power button with the one from Usedmac. While the Mac was open I took out the battery and reset the PMU (pressed down for at least 5 seconds, reinstalled battery). I plugged the Mac in, no change. Same old problem.
I’m stumped. My hard drive has many various projects so I’d like to keep the Mac for a good while yet.
I'm not a Mac expert, but will appreciate all suggestions. Thanks!

 

[ThunderSnake]

A guy from Geeks-Online came by to have a look (for $140).

Yikes! You could have picked up an identical machine for $140. (Seriously, if you want to keep this machine going for a while longer, the best way to spend money would be on an identical parts machine.)

He said it can’t be the power supply, because the Mac runs perfect once it is on.

That's not exactly true. A failing PSU can cause intermittent problems that get worse over time or fail in one way, but not others. Do you have a volt meter? When the machine is turned off, see if you get a reading between the purple wire and black wire on the left corners of the MB power connector (while it's attached to the MB). I'm defining "left" as you're looking down from the open side of the computer, or the side of the connector opposite the retention clip. The reading here should be 5 volts.

 

[SimonUK5]

PRAM battery maybe?

 

[cocacolakid]

I had a similar problem with a 1.42 dual MDD a couple of years ago. It would sometimes turn on and sometimes not. Finally I had to hold the power switch down firmly for several seconds to get it to turn on at all. Once it was running it was fine.

I changed the PRAM battery, reset the PMU, etc., with no change.

After reading a ton of threads here I learned it was the PSU. I got a new (old) one off eBay and swapped it in and no longer have the problem at all.