Mac Pro 2008 with External PSU Problem?

[Deke29]

I use an external PSU to power a GTX 7780 ti card in my 2008 Mac Pro (Dual 3.2). For some reason the power supply will not turn on when it's plugged into the GPU. Unplugged the PSU fan will spin. Plugged in it doesn't. And I think with this PSU the fan always comes on. I've tried using both 8-pin cables from the CPU and just one from the CPU and one using the power supply from the board (two six-pins to one 8-pin adapter). The GPU worked in a another machine. Other GPU's work in that slot.

Why would plugging this card in make the PSU shut down?

 

[harryb2448]

G'day and welcome to the forums.

Incompatible card I guess. Old Mac Pros are pretty fussy about gfx cards. Have you tried plugging the cables into the power socket to see if the Mac Pro can boot it? This ain't windoze you know.

 

[Deke29]

I had been running two 680 cards in the same mac, with updated drivers, and extra juice from the same external PSU. I thought a 780 would take their place fine. I just don't know why a PSU would shut down when connected to the card. Is the 500w PSU not enough for that card? Not sure what you mean by plugging in the cables. I did try a different 750w PSU and that seemed to help...the mac started up without the GPU's fan going berserk, but it never got past the gray screen to actually boot up.