mac pro 2008 - Red LED on motherboard = ?? (not diagnostic LEDs)

Hi guys

I have probably the same problem.

I received from a friend a old mac pro 2008 3.1 aparently broke. He bought a newer iMac and he gave up to that mac pro. 

Machine was without graphic card, without hard drive and ram. Processors is dual quad core x5462 2.8ghz. Then i install original apple graphic card nvidia gt 120 in good contdition, 4 gb memory ram

and i try to run El Capitan from 120gb ssd already installed from other apple machine.

Computer start normal, boot with chime sound, i have boot screen with apple logo as well and when the line from logo start to go and grow to boot computer restart suddenly and fans start running very loud.

I force shutdown from start button and try again and againn but with the same issue. In logo bootscreen machine crash and restart and fans go run very fast and loud.

I try to boot from usb installer to see if the issue is from Osx, start boot good, installation start to go for 2,3 min and after computer again restart suddenly and fans go run.

I try everything other memory ram but anyway no issue with ram because when i start pc all red leds from memory bay stop after boot, mean memory ram is ok. I remove bluethoth and wifi, i check all cables and conections 

and everything was ok, no issue. 

But i notice one problem from begining is a red led open on the motherboard and i know from other mac pro machines 1.1 or 2.1 or even 5.1 when machine run well all leds must be off. But in this machine when i plug power 

cable that red led start to be on and never go off. Position for thet red led is between wifi and bluetooth ( see pictures i uploud with red led position). I am confused why that led start to be on when power go in motherboard 

I never seen any red led on in other mac pro machines, when i plug power cord, even if computer is off, before i press start button that red run when i plug power cord in machine. From my experience that led can be a issue from 

two reasons. First can be a issue from one or both processors or second can be from power supply issue because look to be in connection with power if start to be on when is power in motherboard.

So i remove processors i check in what conditions they are, i check sockets, i clean i reapply thermo paste and i put back. But same issue, nothing chance, same simptoms, same red led on.

Then i decide to replace processors with other two from a old mac pro 1.1. I am not sure exactly number but is 2x 3.0 ghz dual core but same socket 771. Machine start, boot and go finnaly in os very well, all run well i check all specs 

i run few applications and after 3,4 min computer stop, restart and fans go again with same issue. In all this time when computer run well that red led was on as weell but flashing exactly like a hard drive led when hard work.

Is clear now the issue is not processors but it is a difference betwen original processors ( x5462)) because with other processors ( 5160 i think) computer go in os run well few minutes and after crash again.

I run apple hardware test AHT, and in first short scan test he didn't find any issue with computer because scan was only around 1,2 min. When i run deep scan after 3,4 min scaning pc crash again.

Now in this moment when i write this, i have to check only power supply but unfortunately i don't have a spare one to do that. If the issue persist after i replace psu, probably i have to replace entire motherboard.

My curuiosity is to ask here if anybody have this issue with mac pro 3.1 and question for you guys is if that red led is normal to be on when power cord is pluged in computer even when is off.

Personal i dont't think so but i need confirmations from somebody else.

Thank you and any answer and advice are very welcome!