I have a John Deere 5105. I have replaced the jig sensor and cleaned the battery ground.When I first start the tractor, the temperature gauge jumps to a little over 1/2.
After a couple of minutes of operation, the meter sets to the maximum or fluctuates between 3/4 and the maximum.I open the radiator cap and the tractor is not hot.
Make sure to clean the ground connection behind the battery.The chassis harness is grounded at that connection and is known to cause problems with the gauge.Remove the screw and clean the ends of the cable and sand the chassis until shiny.
Screw back in with a new screw and see if the problem goes away.