The primary objective for the thermostat heater control is for improvement in fuel economy and thermal efficiency. The system consists of a high temperature (98°C/208°F in lieu of a 90°C/194°F ) thermostat (Figure 51)that has a resistive heater within the wax element. The heater is controlled by the PCM dependent on engine speed, throttle position, engine load, vehicle speed, air charge temperature, transmission oil temperature and engine coolant temperature.
During low speed, low load and low air charge temperature conditions, the thermostat heater is OFF and the engine is allowed to operate at an elevated coolant temperature. This should result in lower internal friction and higher thermal efficiency, both leading to improved fuel economy.
During high speed, high load, high temperature conditions (air charge, transmission oil or engine coolant), the PCM output is energized with a duty cycle to the thermostat heater. This heats the wax and forces the thermostat to rapidly open wider allowing extra coolant to flow from the radiator. This will reduce the coolant temperature and improve with performance demand.
It should be noted that the heater is only capable of supplying a SMALL amount of additional heat to the wax element; it is NOT capable of opening the thermostat alone. The thermostat is 100% duty cycle for short calibrated time and than the duty cycle is reduce to a maximum of 70% on and 30% off.
Approximately, unheated, the thermostat will begin to open at a coolant temperature of 98°C (208°F) and will be fully open at 115°C (239°F) . Energizing the heater will reduce the opening temperature to about 80°C (176°F) and the fully open temperature to 110°C (230°F) .
