My friend has an old HP desktop with an i7 3770 and a GT 630(or something like that, it's a single slot turd of a GPU). We changed out his old power supply for a 600W silverstone and we gave him a GTX 555 to replace the GT 630, but when we installed everything and powered it on the monitor wasn't detecting a video source and it would display "no signal". The system was on because fans were spinning the HDD was spinning.
I then unplugged the DVI cable from the GTX 555 and plugged it into the motherboard to use the onboard graphics. I got the same result, "no signal." I put the GT 630 back in the system and plugged the DVI into that and "no signal." Plugged the DVI back into the MoBo while still having the GT 630 in the system and still no signal.
Now the weird part, I removed the graphics card so there was nothing in the PCI-E slots. I plugged the DVI cable into the MoBo and then it worked fine. We got a signal off the onboard graphics with no GPU installed. After that I put the GTX 555 in the PCI-E slot but I didn't connect the power connectors. The system was working fine.
I do believe this to be a driver issue but could it be something in the bios?
it's most likely a bios issue but it could also have todo with your PSU.
We have seen several power supplies that use one 12V rail for the 6-pin PEG connector and then a second 12V rail for the CPU and 24-pin ATX connector. That means if you have a graphics card that doesn't include a 6-pin jack, both the CPU and GPU will use the same 12V rail for power. In this case, the second 12V rail goes completely unused, and users risk drawing too much current on the remaining 12V rail. In addition, how much power a GPU draws from the 6-pin connector and how much it takes from the PEG slot varies. - .Anandtech.
this seems to explain why your gpu is working better without power connectors. but I could be wrong.
I just checked the bios and the only thing i could find was PCI/VGA palette snooping, it was originally disabled and i enabled it. Could this be the problem?
Plugged the old power supply into the CPU power and the new PSU powered the graphics card and everything else. Still the same result so I don't think its the new PSU