Ready for Review ► Climate control systems can be manual, semiautomatic, or automatic.
► Manual climate control systems give the driver full control over blower motor speed, temperature of outlets, and direction of airflow.
► Semiautomatic control systems use an A/C ECU to turn on or off the A/C compressor, while the driver controls temperature request, blower motor speed, and airflow position.
► In automatic control systems the driver selects a temperature and the A/C ECU manages the subsystems necessary to achieve that temperature (heat, A/C, and air distribution).
► The A/C ECU requires input from electronic sensors to determine: vehicle heat load; desired temperature setting; and the temperature of ambient air, cabin air, evaporator, and engine coolant.
► An electric actuator or a vacuum servo may be used to move air box doors.
► Components of a climate control system include: blower motor, air box, heater core, evaporator, actuator, and control head.
► The blower motor is attached to a circular fan and provides the desired amount of airflow to the vehicle cabin.
► The doors that control airflow include: blend door, mode door, and recirculation door.
► A potentiometer is a three-wire sensor that signals the position of the blend door to the A/C ECU.
► A negative temperature coefficient thermistor (temperature sensor) gains resistance as the temperature drops, while a positive temperature coefficient thermistor loses resistance with lowered temperatures.
► Actuator feedback signals may be in the form of voltage signals.
► A pulse width modulation refers to a pulsing current flow that controls a blower motor’s speed and provides an infinite number of speeds.
► The sun load (or solar) sensor is a photo diode that reads light entering the windshield of the vehicle.
► The air conditioning pressure sensor monitors refrigerant pressure.
► The evaporator temperatures sensor ensures that the evaporator maintains the correct temperature by controlling compressor cycling.
► Control panels vary according to car type and A/C ECU system. Panels may have rotary switches, recirculation buttons, auto mode, or digital temperature display, and systems may offer dual temperature control or rear-climate control.
► Control panel assemblies may be mechanical or automatic. Mechanical systems may operate cables, vacuum valves, or electrical switches.
► The blower motor speed is determined by resistors connected to the fan control switch that feed voltage to the motor.
► In some vehicles the highest blower motor speed receives a full 12 volts from a dedicated fan relay.
► The A/C compressor is turned on/off by the A/C compressor cultch.