Please note that resetting the Power Manager also resets the date and time settings. After resetting, some iMacs may set the system clock to 00:00 (GMT), 01 Jan 1970 if it has Mac OS X, or 00:00, 01 Jan 1904 if it has Mac OS 9.
The PMU is a microcontroller chip that is present on the logic board in an iMac (Flat Panel). As the name implies, it's responsible for controlling all power functions for the computer such as power on/power off and some aspects of sleep/wake behaviors.
In rare occasions the settings in the Power Manager may become unusable, which can result in operational anomalies with the computer. For example, your computer might not turn on, not wake from sleep, or do (or not do) other things.
Resetting the PMU is not intended to resolve issues in which your computer stops responding or applications unexpectedly quit. In general, you shouldn't have to reset the PMU except as a last resort in cases where a power management system hardware error is suspected. Performing a PMU reset returns the iMac hardware, including NVRAM, to default settings and forces the computer to shut down.
Before resetting, try this first...
If you're experiencing issues with your iMac, for most situations, a restart is sufficient. If your computer has stopped responding, try these steps—in order—until the computer responds:
- Force Quit (Option-Command-Escape)
- Restart (Control-Command-Power)
- Force Shut Down (Press the power button for 10 seconds)
- Reset PRAM
You should only reset your computer's PMU if none of the steps above resolved your issue. If this is the case, see the instructions below to reset the PMU in your iMac (Flat Panel).
Important: Resetting the Power Manager on any iMac (Flat Panel) will permanently remove a RAM disk (if you are using Mac OS 9), if present, and all of its contents.
Resetting an iMac (Flat Panel) PMU
To perform a reset you will need a Phillips "0" screwdriver. The instructions in this article cover these iMac models:
- iMac (Flat Panel)
- iMac (17-inch Flat Panel)
- iMac (17-inch 1GHz)
- iMac (USB 2.0)
Which model do I have? Click here to find out.
- Unplug all cables from the computer except the power cord.
- Lay the computer face down on a soft cloth (see below).
- Remove the four screws from the computer base and gently remove the access panel.
- To avoid electrostatic discharge, touch a metal surface to "ground" yourself (see below).
Important: Make absolutely sure that you ground yourself by touching something metal before you touch any internal parts or install any components inside the computer. To avoid static electricity from building up in your body, do not walk around the room until you have completed this procedure and closed the computer.
- Unplug the power cord.
- Press the PMU reset button on the logic board once (in the pictures below, please note that the reset button is actually under the protective plastic - you can press the button through the plastic).
Important: Do NOT press the PMU reset button more than once.
The photo above shows where the PMU reset switch is for iMac (Flat Panel) and iMac (17-inch Flat Panel).
This picture (above) shows where the PMU reset switch is for iMac (17-inch 1GHz) and iMac (USB 2.0).
- Replace the access panel and the four screws, reconnect the power cord, and turn on the computer.
Important: Never turn on a computer unless all of its internal and external parts are in place and the computer's casing is closed. Operating the computer when it's open or missing parts can cause damage or injury.
Once your computer turns on, reconnect your cables. Be aware that resetting the PMU also resets your computer's PRAM, so you may need to reset your date and time settings.
If your computer doesn't turn on, you should take it to an Apple Authorized Service Provider for further help.