Why are fans speeding when screen disconnected?

I cracked the screen trying to remove it. It seemed to still work OK, so to prevent the crack spreading I used araldite to fix an aluminium strip right across the top of the screen and it ended up looking like an older, removable-glass iMac. Then the screen developed a broad grey vertical band top to bottom 4" wide when turned on. The warranty is void and a new screen is $1,000 Australian plus servicing costs.

So I disconnected the power and data connection strips to the screen and taped the loose connectors up with insulation tape. This was hopefully to prevent any more damage as the computer still worked OK using an external monitor.

However the fans are now going full speed.

Only the iMac screen is disconnected, so why is this?

Does the screen have a temperature sensor, same as the hard-drive? Does anyone know a fix?