Locating the iMac G5 logic board SMU
Recently a series of 20-inch iMac G5s I manage had either their power supply or graphics card fail due to a bad iMac G5 logic board controller. In one case I was able to reset the System Management Unit (SMU) and to correct the power supply issue I was having. The iMac G5’s advanced System Management Unit (SMU) manages the thermal and wattage conditions, while at the same time keeping the iMac’s noise to a minimum by regulating the speed of each fan. The SMU reset button (shown below) is similar to the PMU button on previous iMac, G4 and G3 computers.
The SMU supports the following functions:
- Initial configuration of system clocks.
- All power button and environmental user events.
- Controls fan speed (RPM).
- Provides processor Mode Ring and thermal data store.
- Monitors processor temperature and power consumption.
- Monitors VRD10-compliant processor power supply digital interface.
Resetting the SMU
Resetting the SMU will not resolve issues when your Macintosh is unresponsive – in these situations, restarting your Mac will generally resolve the issue. If your Mac isn’t responding, perform these steps one at a time, in this order, until the issue has been resolved:
- 1. Force Quit (Option-Command-Escape If Applications are unresponsive If Applications are unresponsive).
- 2. Restart (Control-Command-Power).
- 3. Force Shut Down (press the power button for 10 seconds).
[Note: Resetting the SMU does not reset the PRAM. ]
Resetting the SMU can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so on. If your Macintosh still exhibits these types of issues even after you’ve restarted the computer, try resetting the SMU. To reset the SMU on an iMac G5:
- 1. Turn off the computer by choosing Shut Down from the Apple menu, or by holding the power button until the computer turns off.
- 2. Unplug all cables from the computer, including the power cord.
- 3. Wait 10 seconds.
- 4. Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
- 5. Let go of the power button.
- 6. Press the power button once more to start up your iMac.
The original iMac G5 (2005) model has a small button on the main logic board next to the SMU, allowing you to reset both the SMU with the back cover removed and start up when pressing this button.