Heat gauge question

[bigdon-70blazer]

My heat gauge on the instrument cluster does not seem to work now, did before the rebuild. The needle is pegged over past the hot area when I turn the key on but stays over there all the time unless I unhook it under the hood. Any simple fixes here? I did try to take some wire and ground it separately but no help. Any gurus know about this? I changed the original cluster lens and bezel and cleaned the rest very carefully. Help ! but almost done!