Testing the power supply on a Power Mac G4 is necessary if the computer does not turn on and there is no movement from the power supply fan. The Power Mac G4 AGP and PCI graphics models are impacted by power supply problems and, in fact, Apple instituted a power supply replacement program when the computers were still under warranty. Apple recommends that the Power Mac G4 power supply be tested only by Apple-authorised technicians, but electricians and others with knowledge of volt meter usage can complete the diagnostic tests.
Connect a working power cord to the back of the Power Mac G4, but do not power on the computer.
Open the case by lifting the access latch on the right side of the computer. Lift the side access door and lower it so that it is perpendicular to the computer tower.
Locate the power supply connector where it connects to the motherboard, and unplug the connector by pulling upward on it.
Connect the black volt meter lead to pin 16 of the power supply connector. Pin 16 is the fifth from the top when the connector is held vertically with the connector clip on the right side. Pin 16 is a ground and, as such, it has a black cable.
Measure the voltage. It should read +5V; if not, the power supply needs replacement.
Reconnect the power supply to connector J30 on the motherboard if your reading in Step 5 was +5V.
Press the power button to turn on the Power Mac G4. Regardless of the voltage reading, if the computer starts normally, the power supply is not in need of replacement. Check the function of the fan on the power supply. If it is not spinning, advance to Step 8.
Disconnect the power supply cable from the motherboard and connect the black volt meter lead to pin 16, fifth from the top on the right side when viewed with the connector clip on the right. Connect the red lead to pin 1, first from the bottom on the left side. Read the meter, which should have an approximate reading of +3.3V. If the reading is not near +3.3V, replace the power supply unit. If it is, continue to Step 9.
Connect the black lead to pin 16 and the red to pin 4, fourth from the bottom on the left side of the power connector. The reading should be near +5V. If not, replace the power supply. If the reading is approximately +5V, continue to Step 10.
Measure the voltage on pin 10 of the connector, the top pin on the left side, using the black lead. Voltage at pin 10 should be around +12V. If the reading is not near +12V, replace the power supply.
Reconnect the power supply connector and close the access panel if all measurements are normal. If all voltage readings are near the factory specs, the power supply is not the cause of the Power Mac G4 power problems.