2011 iMac CPU Upgrade 4 LEDs, Chime, No display

Hello,

Hopefully I'm not over looking something, I've done iMac upgrades in the past and been very successful, but this one has stumped me.

I have an 2011 iMac that does work with the original i5 CPU, I can get into OS X Lion, look at the screen etc so everything is working there, but when I upgrade the CPU to a i7-2600s, I get no display, the 4 LEDs are on so I know that not an issue and connecting external display shows me Lion but the main display stays off.

Going back to the i5 gives me the main display again, but going to the i7 doesn't, I've rebuilt several times now, I know there's issues with the non S 2600 but that's not what I bought and I double check the CPU code and in Lion,

When I get a chance I'll replace the CMOS battery but what could be causing the issue? Would buying the non-S 2600 solve my problem?

Thank you