55.2-59 Temp Gauge on a 59

[dsraven]

the test lead attached to a good ground like directly to the battery neg, or the booster cable trick, attached directly to the sending unit outer housing, would be a good indicator if that changed the guage reading.
another thing that it could be is something is possibly stealing power from the bat pos side of the guage making the permanent magnet less strong. a quick check with a multimeter on the gauage might be in order. disconnect the sender wire so the circuit is open on the other side of the guage
if you have lots of wire laying around you could wire the guage independently of the truck. so a wire from the battery pos to the guage pos side. then another wire from the sending unit directly to the other side of the guage where the sending unit wire used to be connected. then start the truck and see what the guage does. this would require ensuring the guage is fully disconnected from the cluster power supply and original sending unit wire

doesthe rest of the cluster, including lighting, work properly? nothing different about the cluster, like it is a totally stock cluster with no add on stuff or LED bulbs etc?