the fan/power cord are on a different set of cords not connected to the motherboard. They should connect and power up without the 20/24 pin connection to the motherboard i think.
buying the wrong size fans...i bought 80mm when i needed 120mm
If you are using an ATX (or any of the variants) then there is no way to turn on the power supply without having it connected to a motherboard (or tester). Unlike the old AT style power supplies which had two wires (carrying 120 volts!) running to a switch mounted on the computer tower, the new power supplies don't have any wires that carry more than 12 volts DC. In the 20/24 pin harness is a pin (pin #14, usually green) that carries a small voltage/current. When that pin is shorted to ground, the power supply "turns on" and provides it's usually assortment of 3.3/5/12 voltages on the appropriate wires. You will find some ATX power supplies with a rocker which built into them. This usually breaks the 120v AC coming into the power supply to completely kill it, as without that switch, the power supply is always "on" to some degree to be waiting for the motherboard to tell it to turn on.PS, Dell and Sony have had periods where their OEM power supplies/motherboards used retarded, non-standard pinouts. So, if you are ever working on an older Dell or Sony, make sure you verify the pin outs are correct before swapping power supplies or motherboards with non-OEM direct replacements.