7 Replies Latest reply: Jul 16, 2012 6:35 PM by rccharles
DebWillard Level 1 (0 points)

My IMac PowerPC G5 is rebooting itself.  I have tried resetting PRAM, to no avail.  What should I try next?

PowerMac G5, Mac OS X (10.4.11)
  • Kenichi Watanabe Level 7 (32,842 points)

    You'll need to clarify.  The iMac just randomly reboots for no reason?  Or are you saying that when you shut down (to power off), the iMac reboots instead of shutting down?  Or is it something else, like the iMac never finishes the startup sequence, because it reboots continously.


    If it's the second one (reboots instead of shutting down), it may be a USB device.  I had a USB device that, when connected directly, would make my iMac restart whenever I tried to shut down.  Try disconnecting ALL devices, except for standard keyboard and mouse, and shutting down.


    In any case, (since you reset PRAM already) you may want to try resetting the SMU (power management)




    Be sure to follow the steps precisely.

  • DebWillard Level 1 (0 points)

    Thanks for your quick response.


    Here's what happens:  I press the power-on button.....it goes through the usual start-up sequence of screens.....the fan comes on.....then it restarts itself.....same sequence......same restart-itself......loop-de-loop....


    I tried resetting the SMU; the only problem was pressing the power button and plugging in at the same time due to distance between the two, so I got a clamp for the power button, which I think I had positioned properly. At any rate, didn't work.


    I guess I need to try to next option, i.e.removing the back cover, to see if my IMac G5 has the little button on the main logic board.  I don't know which model I have. I acquired my IMac G5 after the original owner died. Haven't removed the back cover before, so I'll have to find instructions for that. Or find a friend who knows what to do.

  • Kenichi Watanabe Level 7 (32,842 points)

    If that iMac G5 is the type with a flat back panel, it is an earlier model that was easy to work on internally.  Apple even published "DIY" instructions




    If the back is more curved, then it is not as easy to open up.  These are later models.


    I don't know what would cause this problem.  If the iMac is the type that is easy to open, you could troubleshoot by disconnecting the hard drive and optical drive, in turn, to see of the continous rebooting will stop.


    Since this model is fairly old at this point, it may have a dead PRAM battery (especially if it has been disconnected from power for a while).  However, I have not heard of it causing the problem you describe.


    If you press and hold the Option key during startup, do you get to Startup Manager before it does the reboot thing?  Startup Manager is a screen that shows a list of available startup disks.  If you get there, are there any startup disk choices shown?

  • rccharles Level 5 (7,574 points)

    What are the screens you are seeing on the mac during boot?  What is the last one before the reboot?


    Try a safe boot.

        Shutdown your machine.  Hold down the shift key.  Poweron.  Wait awhile Wait awhile while you harddrive

          is being checked.


    Do you have the orginal dvd/s for the machine?  Should boot one of them & run disk utility.

    You should run disk utility.  It under on of the pulldown after you enter the language.

         a) verify the disk

         b) update your permissions.



    Try a startup manager boot.  Should give you an idea if the hardware is working.

    Try holding down the option key then power on. This brings up the startup manager. Click on your hd. Click on right arrow key.


    Shut down the Mac, then power it up, and before the screen lights up, quickly hold down the Command, Option, P, and R keys, until the Mac has chimed twice more after the powerup chime. Then, before the screen lights up, hold down Command-Option-O-F until the Open Firmware screen appears. Then enter these lines, pressing Return after each one:






    "The reset-all command should restart your Mac. If so, you have successfully reset the Open Firmware settings."



    How to eject a cd from the internal cd drive:

    eject cd


    List of devices:



    List of variables:




    ( nvram is the equivalent Mac OS X terminal command. )


    Resetting your Mac's PRAM and NVRAM




    reset the PMU


  • -Rotten Apple- Level 1 (45 points)

    If you have integrated camera, then you have iSight model. I think that model does not have SMU reset button. There was also "ALS" which had similar case and build like the original model had but I don't know wheter or not it has that button, but the original model has.


    It might be power supply or capacitor problem. ALS and iSight models also suffered graphics card solder faults which can also cause reboots without any error messages. It is of course best to rule software and hard disk problems out first, but if hard disk is fine and you are sure you haven't messed OS I think it is caps, power supply or solder fault. All of those can be fixed, capacitors can be replaced as well as power supply, solder fault is trickier but the the solder can be relflown or the chip can be reballed or the eniter chip can be replaced if it is bad, but of course these solder repairs usually require a professional to do it.

  • DebWillard Level 1 (0 points)

    Hi again,

    Thanks for the detailed instructions last month. I may need them at some point. But I think my main problem is

    that the hard drive is nearly full.   I have a Western Digital EIDE Caviar SE 320GB Hard Drive, never used. According to documentation, I need a Mac-compatible ATA/100 controller card. 

    I have heard that there is some kind of USB adapter that I may be able to get in order to use this as an external drive.  Is there such a device? 

    thanks, Deb.

  • rccharles Level 5 (7,574 points)

    that the hard drive is nearly full.   I have a Western Digital EIDE Caviar SE 320GB Hard Drive, never used. According to documentation, I need a Mac-compatible ATA/100 controller card. 

    I have heard that there is some kind of USB adapter that I may be able to get in order to use this as an external drive.  Is there such a device? 

    thanks, Deb.

    If you want to boot off of external drive, you need a firewire drive.


    I recommend you do a google search on any external harddrive you are looking at.


    I bought a low cost external drive enclosure. When I started having trouble with it, I did a google search and found a lot of complaints about the drive enclosure. I ended up buying a new drive enclosure. On my second go around, I decided to buy a drive enclosure with a good history of working with Macs. The chip set seems to be the key ingredient. The Oxford line of chips seems to be good. I got the Oxford 911.


    Here is an external hd enclosure.



    Folks in these Mac forums recommend LaCie, OWC or G-Tech.

    Here is a list of recommended drives:



    FireWire compared to USB.  You will find that FireWire 400 is faster than USB 2.0 when used for a external harddrive connection.




    How to format an external harddrive:




    You need to delete some files ASAP.  Best to have greater than 2gig of free space.  Many posters to these forums state that you need much more free space: 5 gig to 10 gig or 10 percent of you hd size.



    Be careful when deleting files. A lot of people have trashed their system when deleting things. Place things in trash. Reboot & run your applications. Empty trash.


    Go after large files that you have created & know what they are.  Do not delete small files that are in a folder you do not know what the folder is for. Anything that is less than a megabyte is a small file these days.






    "The simple, fast way to save disk space"

    OmniDiskSweeper is now free!



    This will give you a list of files and folders sorted by size. Go after things you know that are big.