Can someone offer me advice? I have built a few machines but never had this problem before.

I have a new ECS K8 HT1600 nforce3 250 motherboard, with a retail boxed AMD Sempron 3000+ Processor, which came with heat sink and fan.

Both are approved AMD components for Sempron, but the supplied processor fan has a 4 pin power connector, and the board has 3.

The 4 pin connector has a dividing bar moulded on between pins 3 and 4. There are 4 wires to the connector. Would it work if i put it on the 3 pin connector on the board?

i think the 4 pin bit is for intel boards and the 3 pin bit with divider is amd.
so yes it should work (i have a couple of case fans in amd machine just like what u discribe)

.aaron

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Experience is a wonderful thing, it enables you recognise a mistake when you make it again