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► 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.

► Components of a compressor clutch include the pulley, clutch coil, compressor shaft, and a shim.

► Hybrid cars may use a standard A/C compressor clutch arrangement or may use a dual-drive A/C compressor.

► Common HVAC system issues are: electric actuator failure, vacuum hose failure, and sensor failure.

► Vehicles with HVAC systems equipped with self-diagnostics will provide a diagnostic trouble code that can help determine the cause of system failure.

► Use manufacturers’ troubleshooting charts when diagnosing HVAC controls malfunctions.

► Electrical malfunctions in HVAC systems may be due to failure of actuators, switches, connectors, wiring, grounds, or may be due to pin fit issues.

► Consider using an infrared thermometer to check coolant temperature without removing the radiator cap.

► Inspect and test blower motors for foreign objects, shorted motor, or binding of the fan motor shaft.

► Possible causes of the compressor clutch control system not working are: bad clutch coil, electrical fault in control circuits, large clutch air gap, or low refrigerant level.

► Inspect and test the heater control panel assembly and control cables if the system is not delivering proper air temperature.

► Air conditioning systems can grow mold and bacteria from trapped condensed moisture, which can then give off an unpleasant odor, necessitating use of an anti-odor kit.