afterblow • ambient air temperature sensor • aspirator tube • automatic temperature control (ATC) system cabin filter discharge air temperature (DAT) sensor • dual-position actuator hybrid electric vehicle (HEV)
The outside air temperature is 70°F (21°C) or higher.
The battery voltage is 12 volts or higher.
Afterblow is a term used to describe the operation of the blower motor after the ignition has been turned off. The purpose of afterblow is to dry the evaporator to help prevent the formation of mold and mildew in the evaporator case. The operation of the blower motor after the ignition is turned off has created some customer complaints. What is Afterblow? If the above conditions exist, the afterblow is commanded to be on for 20 seconds, off for 10 seconds, and back on for another 20 seconds. Check service information to be sure that the condition is normal or not on the vehicle being investigated. For example, in a typical General Motors system, the following conditions must be met for afterblow to occur:
Figure 49–2 The ambient temperature sensor in this system is located in the fresh air intake duct for the HVAC system.
Automatic temperature control ( ATC ) systems are similar to a normally adjusted system but with additional sensors.
Continued AUTOMATIC AIR-CONDITIONING Outside Air Temperature ( OAT ) Sensor Usually located at the front of the vehicle behind the grille but in front of the radiator. This sensor, commonly called the ambient air temperature sensor and is also supply temperature information for the driver on a display.
Inside Vehicle Temperature Sensor Older ATC systems used a sensor located behind the instrument panel. Air to the sensor was forced to flow past the sensor by using an aspirator tube , which was connected to the blower motor case. Discharge Air Temperature Sensor ( DAT ) Located at the outlet of the vents. The purpose of this sensor is to inform the controller of the actual temperature at the discharge ducts. Evaporator Outlet Temperature Sensor Used to control the AC compressor to keep the evaporative temperature within the specified temperature range for most efficient operation.
Sunload Sensor Mounted on the dash, they adjust temperature and fan speed to match increased heating through the windows from the sun. A common type of sunload sensor is a photo diode .
NOTE: Some vehicles are equipped with a dual-zone sunload sensor that has two sensors included. This sensor allows the system to automatically adjust the airflow and air temperature based on the actual sun intensity experienced by both the driver and the passenger.
Actuators move vanes or valves. Actuators in air-conditioning systems are electric or vacuum operated. Here are three types:
Dual-Position Actuator Able to move either open or closed. An example is the recirculation door, which can be open or closed. Three-Position Actuator Provides three air door positions, such as the bi-level door, to allow defrost only, floor only, or a mix. Variable-Position Actuator Capable of positioning a valve in any position. All variable-position actuators use a feedback potentiometer, which is used by the controller to detect the actual position of the door or valve. See Figure 49–3.
Figure 49–3 A block diagram showing the inputs to the electronic control assembly and the outputs; note that some of the outputs have feedback to the ECM. See the chart on Page 567 of your textbook.
Most late-model air-conditioning systems include a cabin filter , which is an air filter in the outside air inlet. The purpose of the cabin filter is to filter dirt and dust from the air before it enters the interior of the vehicle. Cabin air filters can be accessed either in the dash, usually behind the glove box, or from under the hood. Cabin air filters should be replaced regularly, usually every two years during normal service and more often if the vehicle is driven in dusty areas. See Figure 49–4.
Figure 49–4 A typical cabin filter being removed from behind the glove compartment. NOTE: Some cabin filters contain activated charcoal which absorbs hydrocarbons and helps to deodorize the air as it enters the interior. For best results, use the designated replacement filter.
Vacuum control circuits use vacuum created in the intake manifold of the engine.
Figure 49–5 With no vacuum signal, the spring extends the actuator shaft to place the door in a certain position (top). A vacuum signal pulls the shaft inward and moves the door to the other position (bottom). Because vacuum decreases close to zero during heavy acceleration, a vacuum accumulator is used to store vacuum during short periods of acceleration.
Most HVAC systems use electric motors to move valves and doors. Each servomotor contains a feedback potentiometer, which is used by the air conditioning control unit to indicate the actual position of the valve or door. If the commanded position and the actual position are not the same, then most systems are designed to store a diagnostic trouble code indicating which door is out of calibration. See Figures 49–6 and 49-7.
Figure 49–6 Three electric actuators can be easily seen on this demonstration unit. However, accessing these actuators in a vehicle can be difficult. Figure 49–7 The feedback circuit signals the AC control unit with the blend door position.
Blower motors are used to move air. The air is directed by the doors of the HVAC system. Most blower motors use resistors to control speed of the motors by dropping the amount of current flow through the motor at the lower speed. The resistor lowers voltage and current to the motor. The control allows full system voltage to be applied to the motor during high-speed operation. The blower motor resistor is always located in the plenum near the blower motor so that airflow past the resistor can help keep it cool. See Figures 49–8 and 49–9.
Figure 49–8 A typical blower motor assembly with attached squirrel cage blower. A replacement motor does not include the squirrel cage blower so it needs to be switched to the replacement. Figure 49–9 A “credit card” resistor used in the rear blower assembly in a Dodge minivan.
Dual-zone climate controls allow the driver and the passenger to select different temperatures, as much as a 30°F (17°C) difference. In a dual-zone climate control system, the ducts and airflow are split and two air mix doors are used, with each door being controlled by its own actuator. See Figure 49–10.
Figure 49–10 A dual climate control system showing the airflow and how it splits.
Many larger trucks, vans, and sport utility vehicles (SUVs) are equipped with rear heat and air-conditioning units. Many vehicles are equipped with ducts that route heated or cooled air to rear-seat passengers.
Continued Figure 49–11 A typical dual-zone climate control panel showing left and right side temperature control levers.
Lines and fittings connecting front heater and air-conditioning components to the rear system.
Rear controls for speed & temperature
Figure 49–12 Heated or cooled air is supplied to the rear seat passengers of most vehicles through ducts that run under the front seats. However, many larger vehicles require a separate heater core and air-conditioner evaporator in the rear to provide adequate heating and cooling. Most rear HVAC systems include the following components:
Figure 49–13 A sticker on a vehicle equipped with rear heat and air-conditioning warning that a special service procedure is needed when replacing engine coolant.
When recirculation is selected, about 90% of the air is drawn from the passenger compartment and the other 10% is drawn from outside air. The purpose is to speed up the cooling of the inside of the vehicle. However, the body control module may also select recirculation operation if the high-side air-conditioning system pressures exceed 320 psi (2,200 kPa) to help lower the high-side pressure using cooler inside air through the evaporator. This condition should not normally occur, but if it does, this could cause a customer concern because the blower noise is greatly increased in the recirculation position.
Hybrid electric vehicle ( HEV ) systems need to be different than conventional systems because the engine stops when at idle if the engine is warm. As a result, the engine-driven air-conditioning compressor will also stop. To allow idle-stop mode and still provide air conditioning, several methods are used:
HYBRID ELECTRIC VEHICLE HEATING AND COOLING SYSTEMS
Idle-stop mode is disabled if max cooling is selected.
Honda uses a hybrid compressor which has a smaller capacity, operated by an electric motor, powered by the high-voltage (HV) batteries.
Toyota uses an air-conditioning compressor entirely driven by the HV batteries, capable of providing cooling under all conditions, including when the engine is not operating.
HVAC systems are designed to be able to deliver airflow to the windshield for defogging or defrosting as well as to the floor or dash vents.
HVAC systems are designed to use outside air, recirculated air, or a combination of the two.
Sensors used in automotive air-conditioning systems include outside air temperature (OAT), inside vehicle temperature, discharge air temperature (DAT), evaporator outlet temperature, and sunload sensors.