Specifications

iMac (21.5-inch, Late 2012) iMac (27-inch, Late 2012)
not connected to an AC power source. The RTC maintains the system date and time, while the PRAM stores information such as speaker volume,
screen resolution, startup disk selection, and recent kernel panics.
The coin battery is designed to last several years and does not normally require replacement. However, if issues with the functions listed above are
experienced, the RTC and PRAM may need to be reset, or the Coin Battery may need to be replaced. Test pads have been included on the iMac (21-
inch and 27-inch, Late 2012) logic boards to facilitate troubleshooting of the Coin Battery, RTC, and PRAM without removal of the logic board from
the system.
Coin Battery Voltage Test Instructions
1. Shut down and unplug the iMac. Allow approximately 2 minutes for the power supply to discharge.
2. Measure the coin battery voltage by using a voltmeter set for DC. Place probes on pads (negative probe - left pad, positive probe - right pad)
found in location B shown below. If the voltage is 2.7 volts DC or less, the coin battery should be replaced.
Be careful not to short the voltmeter test probes to other components, or damage delicate components on the logic board when
positioning the test probes.
Real Time Clock (RTC) Reset Instructions
1. Shut down and unplug the iMac. Allow approximately 2 minutes for the power supply to discharge.
2. Reset the RTC by shorting the pads found in location C shown above.
Parameter RAM (PRAM) Reset Instructions
1. Shut down the iMac.
2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.
Coin Battery Replacement Instructions
If the Coin Battery voltage tests below 2.7 volts DC, it will need to be replaced.
1. Shut down and unplug the iMac. Allow approximately 2 minutes for the power supply to discharge.
2. To replace the coin battery, use a black stick to push the battery further into the battery holder. The coin battery will lift up slightly and then
spring back out of the battery holder.
Starting Up in Safe Mode
A Safe Boot is a special way to start Mac OS X when troubleshooting. To start up into Safe Mode (Safe Boot):
1. Make sure the computer is shut down.
2. Press the power button.
3. Immediately after you hear the startup tone, press and hold the Shift key. Note: The Shift key should be held as soon as possible after the startup
tone but not before.
4. Release the Shift key when you see the screen with the gray Apple and progress indicator (looks like a spinning gear). During startup, ”Safe Boot”
appears on the Mac OS X startup screen.
To leave Safe Mode, restart the computer normally, without holding down any keys during startup.