In order to manually start up any PSU, you just ground the green wire. Here's the pinout for the G5 PSU-
Power Supply P1 Connector
Pin# Signal Color -- Pin# Signal Color
1 +5Vstb Purple -- 13 GND Black
2 GND Black -- 14 Power ON Green
3 FANtach White/Yellow -- 15 GND Black
4 GND Black --16 Reserved
5 Reserved --17 GND Black
6 GND Black - 18 RTNaud(GND) Black
7 +12Vaud (12V2) Yellow - 19 GND Black
8 GND Black -- 20 +5V sense Red
9 +3.3V sense Orange -- 21 GND Black
10 GND Black -- 22 -12V Blue
11 GND Black -- 23 GND Black
12 Reserved -- 24 Reserved
Power Supply P2 Connector
Pin# Signal Color -- Pin# Signal Color
1 +3.3V Orange -- 9 +5V Red
2 +3.3V Orange -- 10 +5V Red
3 +3.3V Orange -- 11 +5V Red
4 +3.3V Orange -- 12 +12V3 Yellow
5 Reserve -- 13 +12V3 Yellow
6 +12Vfan Yellow -- 14 +12V1 Yellow
7 +12Vfan(12V2) Yellow -- 15 +12V1 Yellow
8 +25V White -- 16 RTNfan(GND) Black
Power Supply P3 Connector
Pin# Signal Color -- Pin# Signal Color
1 +5V Red -- 5 +5V Red
2 GND Black -- 6 GND Black
3 GND Black -- 7 GND Black
4 12V2 Yellow -- 8 +12V2 Yellow