Power Macintosh G3 (Yosemite)
I still have a series of older Blue and White (Yosemite) G3 computers that we use to provide data recovery service for files that were left on either 3.5 inch disks, Zip Disks or even Syquest. One of my most reliable G3’s died on me late last week and I was concerned that it might be a power supply related issue. I remembered that the in most cases resetting the PRAM ( Parameter RAM ), PMU ( Power Management Unit ) and the NVRAM ( Non-volatile RAM ) usually resolves most issues when you think it might be something larger. In case you’re a long time users and forgot what the PRAM controls here’s a run down on how to reset Mac PMU:

  • Video and display setting associated with the screen resolution, number of colors and refresh rate.
  • Volume choice at Startup.
  • Default speaker volume.
  • Current or recent Kernel Panic information.
  • Default DVD region settings.

To reset the PRAM you simply:

  1. Shut down your Mac.
  2. Locate and hold simultaneously the Command (Apple), Option, P, and R keys.
  3. Turn on your Mac and hold the key combination before the gray screen appears.
  4. Only release the keys after the computer restarts and you hear the startup sound for a second time.

If this doesn’t fix your issue you can take the next step which resets the NVRAM.

  1. Restart your Mac.
  2. Locate and hold down the Command (Apple), Option, O, and F keys.
  3. Your Mac will boot into command line prompt.
  4. Type the following, hitting return after each line:


If all else fails (and in my case it did) I went for the PMU, also known as the “Cuda button” in these older models. The PMU is located inside the computer and has its own memory, firmware, software, CPU and in/outputs. Its functions include:

  • Telling the computer to turn off, on, wake, sleep, idle.
  • Managing system command line resets.
  • Maintaining the PRAM.
  • Managing the real time clock.
  • Mac not turning on.
  • Not waking from Sleep.
  • Devices plugged into the USB and FireWire ports not connecting.

Over time, the settings in the Power Manager may become unusable, which can result in operational anomalies with the computer. Examples include:

  • Not powering on
  • Not waking from sleep
  • Not charging the battery

My issue was related to it not turning on again and resetting the PRAM and the NVRAM didn’t cut it so resetting the PMU and replacing the battery was my best bet. To reset the PMU:

  1. Turn your Mac off if its on.
  2. DISCONNECT ALL CABLES from your computer to eliminate the possibility of getting shocked. This means all power and peripherals.
  3. If possible Apple recommends that you move it where it is not standing on carpet.
  4. Open the side door on your Mac and ground yourself by touching metal. (You can use the inside frame of the computer.)
  5. Find the Cuda Switch / PMU button. (Please see images below for PowerMac G3, G4 and G5)
  6. Press the button ONCE only. Do not press it more than that before starting the computer up again.
  7. Wait 10 seconds, then reconnect the primary connections first, leaving peripherals for a second startup.
  8. The computer should start up again, if it doesn’t then you might need a new system battery – or even worse a new power supply.


Power Macintosh G3 PMU Button

Power Macintosh G3 PMU

iMac G3 PMU Button

Resetting the PMU: iMac G3 500Mhz (2001) “Indigo”

Power Macintosh G4 PMU Button (Graphite)

Power Macintosh G4 PMU

Power Macintosh G4 PMU Button (Mirror Door)

Resetting the PMU: Power Macintosh G4 1.0Ghz Dual Processor (Mirrored Drive Doors)

Power Macintosh G4 PMU Button (Cube)

PowerMac G4 Cube PMU

Power Macintosh G5 PMU/SMU Button

Power Macintosh G5 PMU

iMac G5 PMU/SMU Button

iMac G5 Reset SMU: No longer termed iMac PMU reset button

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