Chevrolet Trax: Description and operation

AUTOMATIC 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 Controls

The HVAC controls 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.

The HVAC control module supports the following features:

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 +85ºC (-40 to +185ºF). The sensor is installed at the evaporator and measures its temperature. If the temperature drops under 1ºC (34ºF), the A/C compressor will be commanded off in order to prevent icing of the evaporator.

A/C Refrigerant Pressure Sensor

The engine control module (ECM) monitors the high side refrigerant pressure through the signal circuit of 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, a serial data message is sent from the HVAC control module to the ECM indicating the A/C request. If the ECM determines that conditions are acceptable for A/C operation, it will apply ground to the A/C compressor clutch relay control circuit, energizing the A/C compressor clutch relay. With the relay contacts closed, battery voltage is applied through the control circuit to the A/C compressor clutch. The A/C compressor clutch will engage and the compressor will operate.

Windshield Temperature and Inside Moisture Sensor

The windshield temperature and inside moisture sensor includes the relative humidity sensor, windshield temperature sensor and humidity sensing element temperature sensor.

This sensor assembly provides information about:

• Relative humidity level at windshield (compartment side)
• Temperature of the windshield inside (compartment side)
• Temperature of the humidity sensor element

The relative humidity sensor measures the relative humidity of the compartment side of the windshield. It also detects the temperature of the windshield surface on the passenger compartment side. Both values are used as control inputs for the HVAC control module application to calculate the fog risk on windshield compartment side. The sensor will also enable partial recirculation mode in order to improve heat-up performance of the passenger compartment under cold ambient temperature conditions without the risk of mist build-up on the windshield. The humidity sensor element temperature sensor supplies the temperature of the humidity sensor element.

Ambient Light/Sunload Sensor

The ambient light/sunload sensor includes the sunload sensor and passenger compartment temperature sensor.

This sensor assembly provides information about:

• Sun heat intensity
• Passenger compartment temperature

The passenger compartment temperature sensor is a negative temperature co-efficient thermistor. A signal and low reference circuit enables the sensor to operate. As the air temperature increases, the sensor resistance decreases. The sensor signal varies between 0-5 V.

The sunload sensor provides a measurement of sun intensity. The sensor operates using a low reference, pulsed supply voltage, and signal circuits. A bright or high intensity light causes the vehicles interior temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle.

Blower Motor Control

The blower motor control processor controls the speed of the blower motor by increasing or decreasing the voltage drop on the ground side of the blower motor. The HVAC control module provides a low side pulse width modulation (PWM) signal to the blower motor control processor via the blower motor speed control circuit. As the requested blower speed increases, the HVAC control module increases the amount of time that the speed signal is modulated to ground. As the requested blower speed decreases, the HVAC control module decreases the amount of time that the signal is modulated to ground.

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 or 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 serial data 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, energizing 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 120ºC (248 ºF)
• 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 3ºC (38ºF)
• ECM does not detect an unacceptable torque load
• ECM does not detect insufficient idle quality

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 V value to coordinate the actuator movement. When the actuator is at rest, both control circuits have a value of 0 V. To move the actuator, the HVAC control module grounds one of the control circuits while providing the other with 12 V. The HVAC control module reverses the polarity of the control circuits to move the actuator in the opposite direction.

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