About 2 years ago, I found a way to test a G4 MDD's power supply, what you need to do, is make sure it is plugged in, and connected to the motherboard. Using a volt meter and a paperclip, check the connection between pin 14, and any ground (Pin 13 for example) by probing into the top of the connector. If it works, you should get +5 volts. you can use this photo for reference of the PMG5's pinout.