[shorod]

The heater would bring the temperature down because you were using the heater core and blower motor as an auxilliary radiator, radiating heat from the cooling system via the heater core.

When the A/C is turned on, the hydraulic cooling fan pump should enable the cooling fan to run.

Have the fins of the radiator and A/C condensor been inspected for debris and bent fins? If either are obstructed, there will not be sufficient airflow to properly cool the engine.

Also, consider having the system and cap pressure tested to make sure the cooling system is being properly pressurized. The system needs the proper amount of pressure to keep the boilover (and freezing) protection in spec.

Yes, if the thermostat is stuck closed, it could cause the engine to overheat as well.

The Cylinder Head Temperature (CHT) sensor is what triggers the engine to run in reduced power mode. When the CHT registers a head temperture above 250 degrees F it turns off the fuel injectors for half of the cylinders and uses those cylinders as heat pumps to help cool the engine.

-Rod