Dead G5 Power Mac: This is what I've tried so far

5 replies [Last post]
Joined: Jun 2 2011
Posts: 4

My G5 Power Mac, dual 2.0 (early 2005) just won't start. It's completely dead. I read that the power supply was the most likely suspect so I got another one cheap from a guy on eBay. Swapped that one in and still nothing. Checked the battery and switched the PMU (according to Apple's instructions, no change. Now I think I've pressed the PMU more than once, which Apple warns not to do.

The power supply from eBay could be bad too. I need to learn to test it. I found pinout diagrams, but I'm wondering if I need to jumper any of the connections to switch the power supply on for testing. Any thoughts on my situation?

pinout diagrams:

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.
cwsmith's picture
Joined: Oct 13 2005
Posts: 699
Re: Dead G5 Power Mac: This is what I've tried so far

I have an identical G5 to yours, and I've done similar troubleshooting on it and similar machines.

When you test the PSU (at least under Apple's instructions in the service manual), you're only testing for 24V trickle charge. This voltage is present anytime the PSU is plugged into the wall, to provide power to the PMU and SMC circuitry. If you have no trickle charge, the PSU is dead. If you have trickle charge, the problem is elsewhere.

I presume you've plugged it into a working wall socket? (You may think that's a stupid question, but ...)

Was the charge on the battery under 3V? If so, try without the battery, and/or replace it.

Does the front LED power light come on at all? Does it blink in a particular pattern? (Three flashes, pause, two flashes? Two long, one short? One if by land, two if by sea?)

Check that RAM is paired properly. Memory should be installed in matched pairs. Beginning with the centermost slots, install one stick from each matched pair. Add additional sticks in matched pairs, working outward in a mirror-image from the center pair.

To clarify, if you have eight slots ABCD EFGH, where A is at the top and H is at the bottom of the logic board, the first matched pair should be in D and E, the second in C and F, the third in B and G, and the fourth in A and H, sort of like playing "Chopsticks" on the piano. There should be no unmatched pairs, or gaps between sticks.

If you have only four slots AB CD, you should match B and C, then A and D.

Have you tried disconnecting unnecessary components one at a time (FireWire devices, USB devices, secondary HD, primary HD, optical drive [note: disconnect both power and data cables from drives], AirPort Extreme card, Bluetooth card, modem card, all but one pair of [preferably Apple-branded] RAM sticks, PCI and AGP cards) to see if any one of them is interfering with the power-on sequence?

Have you tried replacing the front panel board (the one that contains the power switch, USB, FW and headphone ports)?

Beyond this point, you're down to troubleshooting the CPUs and the logic board. You'll need to make sure you have the proper diagnostic utility for this machine in order to run the necessary thermal calibration if you remove or change position on the CPUs. Without thermal calibration, all fans will run at full bore all the time: you'll think you've moved next door to a DustBuster factory. Try all the other troubleshooting steps *first*, and report back.


Dozens of Macs; a handful of PCs; Apple ][e, ][c and ][gs; a few iPods; lots of parts
17 guitars, 2 baritone guitars, alto guitar, 6 bass guitars, bajo sexto, 3 ukuleles, mandolin, banjo, sitar
3/4 scale upright bass, 4pc drum kit, congas, travel congas, bongos, cajon, cabasa, 12 harmonicas

Joined: Jun 2 2011
Posts: 4
Re: Dead G5 Power Mac: This is what I've tried so far

Household current is good. Can hear power supply click when connecting power cord. I have not tested power supply yet. Need some guidance on that (see question at end).

Battery at 3.6v when installed in holder.

No led lights.

RAM in matched pairs and symmetrically configured.

Tried to start with HD's, video card disconnected and no peripherals.

Have not tried replacing front panel board.

Do I need to remove the power supply to test it? Or can I pull off the P1 connector (the big one in the middle of the logic board) and then power up the unit and test it while it is installed? If it fails to test out I would think I could pretty much rule out other problems.

I'm using pinout diagrams found in this thread:


Hawaii Cruiser's picture
Joined: Jan 20 2005
Posts: 1434
Re: Dead G5 Power Mac: This is what I've tried so far

You've double checked all connections to the motherboard including the front panel connection? With new PSU installed, plug the G5 into the wall and leave it plugged in overnight and then try it in the morning--has worked for me with older towers. It also could be the video card. It's a stock video card? A bad video card kept at least one G4 tower I had from any sign of life at all. Replaced the card and it booted right up. My approach to checking would be, battery and PMU first, ram seating second, video card third, and then PSU. Gotta have another video card to check, of course. I've got a flashed PC Radeon 9800 Pro in my G5 and in a couple of G4's. They're the most easily flashed and compatible PC cards for AGP Macs. They work great. I guess you don't have extra comps to switch out and test parts with the G5? Usually, you really have to stress out a video card or a PSU to make them go bad. RAM, on the other hand, goes bad sometimes for no apparent reason at all which always makes it the primary suspect for startup problems, but the comp will usually turn on, although often not boot, with bad RAM. Going straight to the PSU and investing money in that right away is jumping the gun from my experience. Like I say, if you've got another video card handy, that might be the first and easiest to check off the list. Try booting without a video card installed. I don't remember whether or not you'd get at least a panel light blink without a video card, but any blink would be a real sign of hope.

Joined: Jun 2 2011
Posts: 4
Re: Dead G5 Power Mac: This is what I've tried so far

I got some really good help on the Apple Community forums here:

Using that and the pinout diagrams I linked to earlier I determined that the original power supply is bad. The new (used) power supply from eBay checks out good. The failure seems to be at the switch or in the path between the switch and the power supply. I'm betting on the switch and crossing my fingers. Kits for rebuilding the switch are available.

Now I'm looking for some guidance on removing and rebuilding the switch.

Below are links to images of a switch kit and the installed switch - i/o housing

Joined: Jun 2 2011
Posts: 4
Re: Dead G5 Power Mac: This is what I've tried so far

On the other hand, the PMU function of the logic board may be fried. I'm trying to find a wiring diagram for the switch-i/o board to help me test the switch.

BTW -- The G5 service manual says ya gotta pull out the logic board and just about everything else to get the switch out Crying