User Guide

Power Mac G4 Cube Troubleshooting -

General Information

Overview

This chapter includes symptom charts and other troubleshooting information for this

computer. Troubleshooting information can also be found online in the Knowledge Base

link at the Service Source site.

Resetting the PMU on the Logic Board

The PMU (Power Management Unit) is a microcontroller chip that controls all power

functions for this computer. The PMU is a computer within a computer. Its function is to:

• tell the computer to turn on, turn off, sleep, wake, idle, etc.

• manage system resets from various commands.

• maintain parameter RAM (PRAM).

• manage the real-time clock.

Important:

Be very careful when handling the logic board. The PMU is very sensitive and

touching the circuitry on the logic board can cause the PMU to crash. If the PMU crashes

and is not reset, the battery life goes from about ﬁve years to about two days.

Many system problems can be resolved by resetting the PMU chip. When you have a

computer that fails to power up, follow this procedure before replacing any modules:

1. Disconnect the power adapter from the computer and check the battery in the battery

holder. The battery should read 3.3 to 3.7 volts. If the battery does not have the

minimum voltage, replace it, wait ten seconds, and then reset the PMU (refer to the

next step). If the battery is good, go to the next step.