Well...I hope after 7 years you've been able to get it repaired...but just in case you're still asking.... Most electronic gauges receive 12 volts from the ign switch and then look for a ground thru the temp sending unit screwed into the water jacket somewhere. Turn the ign ON but don't start the tractor.... and test for 12 volts at the disconnected terminal at the sender. If you have batt voltage at the sender end of the wire... it's a bad sensor/sender.
If you don't have voltage there.... it's a bad gauge or wiring harness.
If it's a mechanical gauge... the sending bulb is filled with alcohol and when it heats up pressure builds and travels up the capillary-tube to move the needle by filling up and pressurizing an expanding diaphragm. Those types can be damaged by excessive movement of the capillary that might cause a small leak...and the alcohol evaporates and the gauge is inoperable. This can be caused by a worker who is too rough with the capillary-tube when working on a transmission clutch replacement.
The only fix for the latter situation is to replace the entire assembly. Don't overtighten the bulb when you screw it back into the water jacket or you can cut the "skirt" inside the fitting and ruin the new gauge as well as cause a leak.
