I have the above mentioned power supply and have found (http://pinoutsguide.com/Power/apple_mac_pro_psuj3_pinout.shtml) to be about my only reference after searching.
I would be very appreciative if anyone can join a few dots for me regarding what the "PSU ON" pin needs and and if they also could explain the function of the switch to the terminals it connects to i would see that as taking the inch, and the mile. I Know the bones of DC power supplies and basics in SMPS Power supplies, I've made a few before into suppliers of 12 for non PC applications, but not with the all black wiring plus i know what mac want to do to me overtime i bend over having owned many. (the balance comes from siri giving me the answer to 8x8 for the verification and keeping me dumb)
The problem of where the + and - wires sit can be solved with diagrams, and has been but for the potential possibility of me breaking something trying to get it to switch it on has been put off.
Thanks, any information would help
ILL TRANSFER 10 DOLLARS AUSTRALIAN TO THE FIRST PERSON TO REPLY WITH INFORMATION (SO ALSO SEND YOUR PAYPAL) TO SOLVE MY PROBLEM (not to best) CMON HAVE A STAB
this is the link if it didn't work to the diagram I'm referring to http://pinoutsguide.com/Power/apple_mac_pro_psuj3_pinout.shtml
I know this is old, but thought I'd post an answer just in case.
I think I understand that you want to turn on the PSU to check the voltages, but are afraid of cooking it?
The PSU ON pin needs to be grounded to turn on the PSU. This means that you should take a paper clip or other wire and connect the PSU ON pin to ground (plug an end into each of the PSU ON and GND pins) and then plug in the PSU to the mains power.
I'm sure you know that you can test all of the voltages by putting one multimeter probe on one pin and the other on a nearby ground pin (with the PSU on of course).
Hope that helps.