Chevrolet Sonic Repair Manual: Manual HVAC Description and Operation

The air temperature and the air delivery description and operation are divided into five areas:

• HVAC Control Components
• Air Speed
• Air Delivery
• Heating and A/C Operation
• Recirculation Operation

HVAC Control Components

HVAC Control Assembly

The HVAC control assembly interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The air temperature and mode doors are controlled by cables. The recirculation actuator is electrically controlled.

There are two different HVAC control assemblies. The first type uses serial data to communicate system messages and requests to other module. The other uses discrete hard–wired signals to communicate commands. The hard–wired HVAC control assembly cannot communicate via serial data.

HVAC control module programming can not be performed on the hard–wired HVAC control assembly.

The HVAC control module supports the following features:

Feature	Availability
Afterblow	NO
Purge	NO
Personalization	NO
Actuator Calibration	NO

Evaporator Temperature Sensor

The evaporator temperature sensor is a two-wire negative temperature co-efficient thermistor. The sensor operates within a temperature range of -40 to +85°C (-40 to +185°F). The sensor is installed at the evaporator and measures its temperature. If the temperature drops under 1°C (34°F), the compressor will be switched off in order to prevent a frozen evaporator.

A/C Refrigerant Pressure Sensor

The engine control module (ECM) monitors the high side refrigerant pressure through the A/C refrigerant pressure sensor. The ECM supplies a 5V reference and a low reference to the sensor. Changes in the A/C refrigerant pressure cause the sensor signal to the ECM to vary. When the pressure is high, the signal voltage is high. When the pressure is low, the signal voltage is low. When pressure is high, the ECM commands the cooling fans on. When pressure is too high or too low, the ECM will inhibit activation of the A/C compressor.

A/C Compressor

The A/C compressor is belt driven and operates when the magnetic clutch is engaged. When the A/C switch is pressed, an A/C request message to the ECM. If the ECM determines that conditions are acceptable for A/C operation, it will ground the A/C compressor clutch relay control circuit, which activate the A/C compressor clutch relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch. The A/C compressor clutch will engage and the compressor will operate.

Air Speed

The HVAC control assembly applies a ground to the blower motor control circuit that corresponds to the selected blower speed. The resistors and the blower motor are in a series circuit. The following list represents the number of resistors in series with the blower motor per particular speed request:

• Low speed, 3 resistors
• Medium 1 speed, 2 resistors
• Medium 2 speed, 1 resistors

When blower switch is in the Hi speed position, ground is applied directly to the blower motor through the blower switch.

Air Delivery

The HVAC control assembly controls the distribution of air by the use of recirculation actuator and the mechanically controlled mode door. The modes that may be selected are:

• Defrost – Air flow is directed at the windshield
• Defog – Air flow is split between the windshield and floor
• Vent – Air flow is directed to the vents located in the instrument panel
• Bi–Level – Air flow is split between the instrument panel vents and the floor vents
• Floor – Air flow is directed to the floor

The desired air distribution mode can be selected with the air distribution knob at the HVAC control assembly. Depending on the position of the flap, air is distributed through various ducts leading to the outlets in the instrument panel. When the defrost position is selected, the HVAC control assembly will command the recirculation actuator to outside air, reducing window fogging.

Heating and A/C Operation

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. Regardless of the temperature setting, the following can affect the rate that the HVAC system can achieve the desired temperature:

• Recirculation actuator setting
• Difference between inside and desired temperature
• Blower motor speed setting
• Mode setting

When the A/C switch is pressed, an A/C request message to the ECM. If the ECM determines that conditions are acceptable for A/C operation, it will ground the A/C compressor clutch relay control circuit, which activate the A/C compressor clutch relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch. The A/C compressor clutch will engage and the compressor will operate.

The following conditions must be met in order to activate the A/C compressor:

• Battery voltage is between 9–18 V
• Engine coolant temperature is less than 120°C (248 °F)
• Engine speed is greater than 600 RPM
• Engine speed is less than 6200 RPM
• A/C high side pressure is between 220-3100 kPa (32-450 PSI)
• Throttle position is less than 95%
• Evaporator temperature is greater than 3°C (38°F)
• ECM does not detect an excessive torque load
• ECM does not detect insufficient idle quality
• Outside air temperature is greater than 3°C (38°F)
• Blower motor ON

The temperature door controls the temperature of the air entering the passenger compartment. Air entering the passenger compartment passes through the heater core and/or the evaporator core. The position of the temperature door determines how much air passes through the heater or evaporator core. As the temperature control is moved to a hotter setting, the temperature door will direct more air through the heater core. As the temperature control is moved to a cooler setting, more air will be passed through the evaporator core.

Recirculation Operation

Recirculation Actuator

The recirculation actuator is a two-wire bi-directional electric motor. There are two control circuits that allow actuator operation. The control circuits use either a 0 or 12-volt value to coordinate the actuator movement. When the actuator is at rest, both control circuits have a value of 0 volts.To move the actuator, the HVAC control module grounds one of the control circuits while providing the other with 12 volts. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction.

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