PSU works on old rig but not new!?!?

I currently have a gateway machine model GM5626
intel pentium e2180
gateway mini atx mobo
3gb ram
geforce 8500gt

the psu went dead on that machine so i opted to buy a new XFX 750w Black edition psu and it worked great...shortly after my friend decided to sell me his old rig for dirt cheap specs are :

intel q9550
abit ip35 pro
4gb 2x2gb OCZ sli-ready pc6400
visiontek 4870

so i opted to use the new xfx 750w psu on the new machine but after plugging in everything the 24pin and the 8pin it did not power up. Not even the power light or the fan spooled...NOTHING... so i thought i burnt up this psu during the switch or the mobo was bad...so i went out and gotten a new motherboard a gigabyte p43-ud3L assembled that and fired it up...still nothing...so i grabbed my friends old 20pin 450w enermax and plugged it in the new setup and it worked like a charm...so i returned the gigabyte mobo and reassembled the abit ip35 pro and tried again with the enermax 450w psu and it worked like a charm...so i ruled out the motherboard being a problem...i plugged in the xfx psu back into the gateway machine and it worked...it just wont work in my new rig...could somebody help? do i have a faulty psu or is there any incompatibilities withen my new system?

it could be the pins are differnt on that power supply but that is really weird

Wait, so the Gateway is out of the picture?

Just the XFX 750w

and this stuff?

intel q9550
abit ip35 pro
4gb 2x2gb OCZ sli-ready pc6400
visiontek 4870

pins are different? then wouldnt it not work with either system...this is so confusing!

well the xfx 750w works on the gateway but not on the new system...well once my new comp runs the gateway will go in the trash if thats what your asking?

yeah im trying to think why that would happen to you the pins are the only thing i can think of that makes sense to me

hmmmz what do you mean by pins? could you explain further on what your mean?

maby the pins are too short for that board idk take a mulitmeter and test it

seems to be the same length when i have it side by side...and i know the xfx is working but just on the gateway...and only the gateway machine...damn gotta check find my multimeter

Does your Gateway have a 4 pin or 8 pin 12v connector? After reading about the XFX PSU, It seems it has a 4+4 pin 12v connector and an 8 pin, I know you said you plugged in the 8 pin, Try first just the 4 pin, Then perhaps the 4+4. It doesn't make sense and about all I can come up with is if your Gateway only used a 4 pin, Perhaps your 8 pin is bad.

gateway is a 4pin. and on the new rig the abit ip35 pro has a 8pin but ive also tried just putting in the 4pin...and the other mobo i purchased yesterday to test out had a 4pin...i guess i have to rma this and hope for the best that it was this units fault...or if they find out this is a working unit ill be charged for 2 psu that dont work on my comp...ive never ran into a problem such as this and its just mind boggling

So the XFX PSU works fine with two other motherboards.....
.....and the Enermax PSU worked fine for the Abit motherboard

This is wierd? But I think you're on the right track. Keep us posted