imac 27" a1312 black screen fans blasting

Ok I made a big boo boo while working on my iMac. I have had it open several times in the past with no problems but this time I pulled the larger ribbon cable in the middle, right above the video card, and the bracket broke off and pulled away from the pins. I Think I managed to push the bracket back onto the pins, it looks good and all pins are aligned into their respective holes.

I put it all back together and when I power up I get the gong right away but no video whatsoever and the fans are running full speed. My sync cable is also kind of worn but I don't think that is the problem, I should still have some video if the sync cable is bad or not hooked up.

My original thought was that the LCD temp sensor is bad, but of course that thing I did with the pin bracket on the logic board has me scared as well. Since I get a boot gong I assume the computer is working but I do not have cables to hook up an external monitor, nor do I have any sharing set up to be able to see it on my network so I don't know for a fact that it has booted and is fully working.

Does anyone have any idea on how to bypass the lcd temp sensor so I can check that component first? If it IS bad I will know to replace just it, and my sync cable while I am in it again.

I just cannot believe it would be the lcd cable, the fans should not run full speed if that cable was plugged in wrong, or not plugged in at all.

Thanks,
George

forgot to say this

I was replacing the superdrive that my 2 year old stuffed 5 disks into. It did not have any video problems before this.

an update on the situation. I examined everything inside again and found the LCD temp plug on the logic board had bent pins. I straightened them out and now the fan problem is solved but still no video at all. I did manage to finally get it to boot by holding down the option key and guessing that it was in the boot selection screen. I apparently was correct, it took off and started booting and now I am backing up everything via screen sharing. I held a flashlight up to the screen and I cannot see anything at all, not even a faint image. The screen is completely black, the LED's are not on, and of course no video.

How do I determine what component is bad now? I have had damaged LCD's in the past and they just don't go completely black. That usually indicates a power supply or video input problem.

Are you sure the LCD cable is connected properly? If the Backlight cables were not connected, you would see a faint image with your flashlight on the screen. That plug looks easy to break going to the Logicboard from the LCD.

I'm with Dennis on this one. You stated in your first post that the bracket holding the pins had come loose and you pushed it back on. From past experience with that kind of problem, it usually does not work even if the pins appear to line up. I'm thinking you're going to have to replace the entire cable.

Video Connector Problem

I've had to deal with a number of iMacs with the same problem.

Unfortunately the LVDS cable video connector on the motherboard can easily be pulled off. It's a horrible design and has since been changed.

Even though you put it back on, all 30 pins have to EXACTLY line up into the holes on the connector. If any of them are bent or touching or have any problem whatsoever you won't get any video when you connect the LVDS cable to the LCD screen.

So go back and try again to put the connector on but be VERY careful, if you damage any of the 30 pins beyond repair, you will have to send your motherboard in for a $200+ repair or possibly even replace the motherboard.

Once the connector is on perfect, I like to use a hot glue gun on the edges of the connector to make sure it doesn't easily pull off again.

The only other alternative, and not a very good one, is to use an external monitor.