MP 1,1-5,1 MP 3,1 has stopped booting

[Phloem]

Sorry, another Mac Pro that doesn't boot with any beeps or blinking LED indication on the front, and the monitor display stays black.

I installed in an ATI Radeon HD 2600 XT (primary) and ATI Radeon X1900XT (with an aux 6-pin power lead plugged in) at the same time. It booted, and as I went to see if the ATI Radeon X1900XT had been recognised and then the Mac completely froze. I switched it off and left it a few days. I installed some extra ram (maybe incorrectly positioned), powered it on, and it wouldn't boot.

I took the ram risers out and graphics cards to eliminate them, and had a look at the diagnostic LEDs. All look fine except the "LED 7 [GPU Present]" which flickers rapidly (I imagine that's not normal!?) and "LED 8 [Power Good]" is off.

I have tried the SMC reset and changing the motherboard cell battery.

Anyone have any ideas? I imagine the PSU or logic board is fried, but I don't want to have to buy one to find it is the other as they currently cost the same.

 

[Phloem]

Is it an issue if you plug in the graphics card booster cable into the wrong relative slot?

EDIT: It apparently doesn't matter...

If the cable is connected properly, try connecting it to the other,
empty logic board booster power connector (if available) to see if the cause is related to
this connector.

 

[Ludacrisvp]

Check into pages 154+

http://tim.id.au/laptops/apple/macpro/macpro_early2008.pdf

 

[Phloem]

Thank you.

It would be nice if the guide had a Minimum Configuration Testing procedure like the mid 2010 guide has.