Mac Pro 2008 Won't turn on

So I have a Mac Pro 3,1 it has been off for several weeks since I upgraded to a newer system, and I was keeping around as a backup, also it has several HDDs in it. I went to turn it on today and to my dismay, when I hit the power button, I hear a click come from the power supply, the usual noise, and then a few seconds later it clicks off. I cannot find anything online about this,

the only thing I could fine was here

https://support.apple.com/kb/TS1367

which let me to try to reset the SMC here

http://support.apple.com/kb/HT3964

which did nothing, I had already done those steps and tried different cables. I am totally at a loss here, and Apple Car expired on this machine over a year ago, does anyone know whats going on?

Mactrunk said:

Hard to diagnose from afar.
I had a similar problem, but in my case it seemed to be temperature related.
I replaced the power supply and all is well.

Click to expand...

I was thinking it was the power supply, but how do you replace the powersupply on a Mac Pro?, it seems built into the chassis?

And I am looking online, $300 for a powersupply? that seems a bit pricey!
Is it possible it could be the battery in the motherboard? I really can't think of anything else to do to troubleshoot this, like what other diagnosis steps could I try? It just flat out is not turning on?