stock temperature gauge stuck on hot

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72 GMC 2500 402/700r4

my stock temperature gauge has been stuck on hot since i have had the truck. unhooking it from the sender it still stay on hot, checked for grounded wire with it unhooked from sender and its not grounded. removed gauge from cluster and manually pushed need back to cold, turned ignition on with sender wire disconnected and it stayed on cold, soon as i hook sender wire back up, it pegs to hot and stays there. I know it sounds like the sender is back, but why wont it drop back to cold when i unhook the wire. any ideas? thanks.