Climate control systems of car

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Climate control systems of car

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►   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.
►   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.
►   Tools needed for inspection, maintenance, and repair of an HVAC system include: infrared thermometer, digital volt ohm meter (DVOM), back probes, fused jumper wire, and a scan tool.

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►   The heating, ventilation, and air conditioning (HVAC) system maintains a comfortable temperature inside a vehicle and uses an air filter to provide fresh, filtered air.
►   The vehicle heating system and automotive air conditioning system were developed in the 1930s.
►   The Environmental Protection Agency regulates automotive air conditioning via Section 609 of the Clean Air Act.
►   Automotive HVAC technicians must obtain a license to verify that they are familiar with federal rules and regulations.
►   HVAC systems use heat transfer to ensure temperature comfort inside a vehicle.
►   Heat transfer can take place via conduction, convection, or radiation.
►   A vehicle’s air conditioner transforms the refrigerant from a liquid to a gas and back to liquid.
►   Heat energy (the amount of heat needed to move a temperature up or down by degrees) is measured in British thermal units (BTUs).
►   The temperature at which a liquid changes to gas is called latent heat of evaporation; likewise, the temperature at which a liquid forms a solid (ice) is called latent heat of freezing.
►   Vehicle air conditioners use a refrigerant for the vaporization and condensation process.
►   HVAC systems have four major components: evaporator, compressor, condenser, and the restriction.
►   The compressor is designed to increase the pressure of the refrigerant (in gas form) and push it into the condenser.
►   A vehicle’s air conditioner is a closed loop system.
►   The condenser is designed to move gas through a series of coils and cool it into a liquid.
►   The restriction works to transform high pressure liquids into low pressure liquids.
►   The evaporator vaporizes liquid into gas as it removes heat energy from the passenger compartment.
►   Each HVAC system component requires the refrigerant to be delivered in the proper state and at the correct level.
►   Components of a compressor include: pulley, clutch, axial plate, piston, reed valves, and body.
►   One of the most common types of compressor is an axial piston compressor.
►   Refrigerant moves through the air conditioning system because of the high pressure and volume of gas leaving the compressor.
►   The condenser uses ram air flow and the engine cooling fan to cool the refrigerant.
►   A restriction can be a fixed orifice tube or a thermostatic expansion valve.
►   The velocity and pressure of the refrigerant changes when it moves through the restriction.
►   Bernoulli’s principle states that the speed/flow of a liquid changes in opposition to the liquid’s pressure (when one increases, the other decreases).
►   Pascal’s law states that liquid cannot be compressed.
►   The evaporator is a one-piece unit of copper or aluminum tubes, with no serviceable parts.
►   Automotive air conditioners can be fixed orifice tube systems or thermal expansion valve (TXV) systems.
►   Fixed orifice tube systems use an accumulator to prevent liquid from entering the compressor.
►   TXV systems use a receiver filter dryer to draw in and store liquid refrigerant.
►   Air doors are controlled by cables, vacuum servos, or electric servos.
►   Vehicle heating systems rely on a hot engine coolant flowing through a heater core—a small radiator comprising tubes, fins, and tanks.

Source : http://www.cdxauto.com/cw/fundamentals/docs/24118_CH43_Ready%20for%20Review.doc

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