Specifications

Logic Board Battery
Important: Apple highly recommends removing the battery when handling the logic board. Make
sure to use proper ESD protection when handling modules.
The battery on the logic board controls the stored system settings, such as date and time. It is
only necessary to test the battery when you can’t power on the computer, or the date and time are
reset every time the AC power is removed.
The battery is also used to power the PMU chip (because the PMU chip keeps time and must
always be running) when the computer is unplugged from the wall (AC power). The PMU is
very
sensitive and touching any circuitry that is connected to the PMU can cause it to crash. If the
PMU crashes, the battery life goes from about five years to about two days if the PMU is not
reset. Once the battery goes dead, the PMU will reset the time and date to 12:00 AM 1/1/04
every time the AC power is removed. To fix this situation, replace the battery and reset the PMU
(refer to “Resetting the PMU on the Logic Board” mentioned earlier in this chapter).
If the computer has a “No Power” situation, check the battery before replacing modules. A
drained battery may be indicative of a crashed Power Management Unit.
Does the battery measure at least +3.5v? If not, replace the battery and reset the PMU. If the
battery does measure over +3.5v, reinstall the battery and reset the PMU as above.
Connect the power cord and power up the system again.
Warning: Whenever the bottom housing is opened for service, you must do two things:
1. You must clean and reapply thermal paste to the thermal pipe surface.
2. You must tighten the four torx bolts on the base unit to a minimum of 17 in.– lbs. Failure to follow
either of these steps could cause the computer to overheat nd damage internal components.
Refer to the next topic, “Thermal Paste Application” for detailed information.
10 - iMac (Flat Panel) Troubleshooting
General Information