Most temp guages if you ground the single wire going to the sender unit the guage will swing to full hot, which indicates the guage and wiring are working.
If that worked then either the sender is bad or its doing its job if the engine temp is cold.
Worked on a 98 Explorer today whose guage was stuck on dead cold.
The system was indeed warm at ~138 degrees on an infrared thermometer.
I first thought the sender or guage must be bad since there is "some" heat and no reading whatsoever. I took out the thermostat to find it was broken apart. Replacement of thermostat and rechecking showed temp up to normal. Not a tractor, but same principle.
Fuel guages normally work the same where you'd remove sender signal wire, ground it and guage would swing to full. If it does I'd say you have a bad/ miscalibrated sender unit.