The evaporator temperature sensor is located in the air stream where the air leaves the evaporator and signals to the control unit the temperature of the air.
The coolant temperature sensor for the engine management system signals to the climate control ECU the temperature of the coolant flowing in the heater core. Some climate control systems have a separate coolant temperature sensor usually located near the engine management coolant sensor.
The signal from the coolant temperature sensor is used in conjunction with the signals from the ambient air sensor, the sun load sensor and the cabin air temperature sensor to control heater and fan operation.
For example if the electronic control unit registers the following readings: Coolant temperature - 16°C [or 60.8°F] Ambient Air temperature - 0°C [or 32°F] Sun Load - Zero Cabin air temperature - 0°C [or 32°F], the heater tap will open fully, the blend door will move to maximum air flow across the heater core and the fan speed will be slow and steady.
The sun load or solar sensor is located on the dash panel where it is exposed to light entering through the windscreen of the vehicle. This sensor is a photo diode. It becomes more conductive as the light intensity increases. When connected to the electronic control unit, a change in light intensity is registered as a change in voltage.
Other inputs to the electronic control unit come from the operator’s control panel. The operator can tell the electronic control unit where to deliver air using switches which produce an on-off type signal. The unit will also receive instructions from the operator regarding the desired cabin temperature required.
The desired temperature selection is used to tell the electronic control unit what temperature the operator wishes the inside of the vehicle to be. The electronic control unit then adjusts the position of valves and flaps to achieve the desired temperature.