|If you mean the old PS has two wires that are quite small in comparison to other wires coming from the power supply with a 3 position female connector that fits on the 3 pin header for the PS fan on the mboard, some better power supplies have that, so that the speed of one of the fans in the PS can be monitored by the mboard.|
You do not need to connect it, and you don't need such wires coming from the new PS.
The fan it monitors the rpm of will work in any case - it doesn't get it's power from the mboard.
Obtaining a cable with the right female connector on the end is of no use - the new PS does not have anywhere the wires can be connected to - the PS would have the wiring if it were capble of outputting the rpm of a fan.
However, in some bioses you can toggle on/off whether the speed of the PS fan is monitored by the hardware monitoring chipset, and the bios can be set to notify you some way if no rpm is detected from the PS fan - if your bios can be set like that, turn off the monitoring of the PS fan.
The important thing is that the new PS has all the same colors of wires in it's main connector from the PS in the same positions as the old PS, and that there are the same number of wires at each position (some have 2 wires), and that the new PS has the required extra power connectors that were used from the old PS to connect to the mboard, if that applies (e.g. 6 pin auxilary, 8 pin processor.
On some mboards, you MUST use a 24 position main connector from the PS into a 24 position main socket on the mboard - on other mboards they will work fine with a 20 position connector in the 24 position mboard socket - see the mboard or system manual.
Many recent PSs have a 20 position main connector and in the same wiring bundle a 4 position connector that can be clipped onto (or unclipped from) the 20 position one if you need to connect to all positions in a 24 position socket.