The temp lite is triggered by a sender in the water jacket, just behind were the injector lines cross the front of the engine from the injector pump to the injectors.
Check:
Radiator core cleanliness. Make sure prescreen and core are not plugged with residue or dust.
Fanbelt tension and general condition of fan.
Coolant level (with engine running and up to temp, radiator cap off)
Coolant circulation - look for coolant movement in the top tank of the radiator.
Waterpump - listen for noises indicating dry/loose bearings, look for weeping around fan shaft.
Wiring connection to temperature sensor - clean, dry and snug. Look at wiring between sensor and light, the tractor is old enough it could be chafed somewhere. IIRC, the sender turns the light on by providing a ground, so if you have a bare spot in the insulation, it could be vibrating against metal somewhere, especially at high RPM.
Verify coolant temp when light is flashing - an IR "temperature gun" is ideal for this.
If it actually IS running hot, it could be the thermostat.
If temp range is normal but the light is still coming on, it could be the sensor.
Let us know what you find.
