What if PMU Reset is held too long?

Question

I've been working with a dual 867 MHz MDD G4 PowerMac. Back in the Fall (I think October or November) it suddenly would power on, but not boot up. The Power light comes on and stays on, no chimes (although there is one noise that sounds like the audio supplying power to the internal speaker), the fans run, the Hard Drive & Optical Drive are getting power, the red light on the motherboard stays lit, the video is dead, the USB keyboard & mouse are getting power, the battery reads a little over 3.6 volts. At the time I tried the suggested fixes (unseating / re-seating the video card, removing / installing the memory modules, resetting the PMU, leaving it unplugged, etc.) none of which had any apparent effect. But one day it just started working again, and worked up until about 3 weeks ago when its' behavior became pretty much identical to what it was doing in the Fall.
This time I have focused on trying to reset the PMU, yesterday I re-seated the CPU card. At times I know I have held the PMU reset button down longer than the suggested firm but quick press, likely as long as 10 seconds. This was done with the power cord removed, and the battery out. Does holding the PMU reset too long actually put the PMU in a permanent state of confusion? What are my choices to make sure everything is starting over from scratch?
From what I've read, this is likely either a CPU, or logic board problem at this point and either one will be pretty expensive to try and isolate.
I have a MDD FW800 that I could potentially try substituting the CPU from, but the bus speeds are different (dual 867 is 133Mhz, FW800 is 167MHz), and I'm not sure if the CPUs would work in the other logic boards.
Is it time to declare this dead, or is there any low cost things I can try?
Thanks in advance for any help.

Answer 1

Hi-
As long as the PMU reset was done with one push, even a long push, there shouldn't be any problems. If the push were actually two or three, then the PMU could quit responding. Anyhow, a push is a push, so one can assume the nvram is reset (unless the PMU is compromised....)
I assume that the machine doesn't respond to keyboard commands? Have you tried booting to OS install disc? How about a verbose mode boot (command+V)? If it will boot to verbose, you can see where the machine is stalling, possibly allowing you to determine the cause of your problems.
If you have a CD with the Apple Hardware Test on it, you could attempt to run that on the QS. Get a copy here, burn to disc, and give it a try....
You could test the power supply to see if that's the problem. There isn't a proper guide for the QS that I can refer you to, but if you use these instructions (generally), and apply it to the pinout for the QS on this page, you should be able to test the voltage of the PSU.