Boots to bright white screen

I purchased a A1286 macbook pro 15" from a fellow for $200.00
The problem he described with it, and I can concur, is that it boots to a bright white screen. If I attach an external to it, the external will not work unless I disconnect the ribbon cable off the internal lcd screen first.
What could it be. I tried the internal lcd on another macbook and it worked fine.

Update

I have managed to boot up the mac with the ribbon cable still attached and using an external monitor.
I tried a screen from another macbook pro and it was white too.

I tried a known working lcd with cable and the result was the same.

If the gpu was bad then why does the external monitor work

I checked out the connector and it is tight to the motherboard.
How is it possible that the output is fine on a external and not on the macbook.
Is there anybody out there that had the same problem and how to solve it?