temp gauge pegged hot

[blue72k10]

I have a 72 K10 that I'm redoing. Finally got around to putting in a tach cluster I've had for 20+ yrs in the truck. The orig one the temp gauge never did work as I thought it should so I replaced the sender with one from LMC before I installed the tach cluster. Did an op check on new cluster before install to make sure it was good. Tach worked as it should and temp gauge pegged hot when wire was grounded and fuel gauge worked. Now when key is turned on the temp gauge pegs hot and stays there. Unplug sender and gauge goes cold. Tried the ground wire trick someone suggested that didn't help. Any thoughts or suggestions appreciated.