HVAC Control Assembly

The HVAC control assembly is a non-class 2 device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. An ignition voltage circuit provides power to the control assembly. The air temperature, mode and recirculation actuator are electrically controlled. The control assembly does support the following features:

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:

When the operator requests High speed, the HVAC control assembly activates a relay which applies direct ground to the blower motor.

Air Temperature and Mode Actuators

The actuators are 5-wire bi-directional electric motors that incorporate a feedback potentiometer. Low reference, control A , control B, 5-volt reference and position signal circuits enable the actuator to operate. Two control circuits enable the actuator to operate. The control circuits use either ground or a 12-volt value to coordinate the actuator movement. The HVAC control assembly reverses the polarity of the control circuits to move the actuator in the opposite direction. When the actuator shaft rotates, the potentiometer's adjustable contact changes the door position signal between 0-5 volts.

The HVAC control module uses a range of 0-255 counts to index the actuator position. The door position signal voltage is converted to a 0-255 count range. When the module sets a commanded, or targeted value, the control circuits are energized to rotate to reach the commanded value. As the actuator shaft rotates the changing position signal is sent to the module. Once the position signal and the commanded value are the same, the module opens the control circuits. There are two temperature actuators that run in sync with each other.

Recirculation Actuator

The recirculation actuator is a 2-wire bi-directional electric motor. Two control circuits enable the actuator to operate. The control circuits use either ground or a 12-volt value to coordinate the actuator movement. The HVAC control assembly reverses the polarity of the control circuits to move the actuator in the opposite direction. The recirculation operation can function with blower motor in either the ON or OFF position. Recirculation is not available in Defrost and Mix-Blend mode. If an operator requests recirc in defrost or mix blend the recirc LED will flash 3 times and then turn OFF. The system will remain in fresh air mode.

A/C Refrigerant Pressure Sensor

The A/C refrigerant pressure sensor is a 3-wire piezoelectric pressure transducer. A 5-volt reference, low reference, and signal circuits enable the sensor to operate. The A/C pressure signal can be between 0-5 volts. When the A/C refrigerant pressure is low, the signal value is near 0 volts. When the A/C refrigerant pressure is high, the signal value is near 5 volts.

Evaporator Temperature Sensor

The evaporator temperature sensor will not allow an A/C request to the body control module (BCM) at temperatures less than 0°C (32°F). The temperature is used to cycle the A/C compressor ON and OFF to prevent the evaporator core from freezing. A thermistor inside the sensor varies its resistance to monitor the evaporator air temperature.

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. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

The vehicle operator can activate the A/C system by pressing the A/C switch. The A/C system can operate regardless of the air temperature or mode setting. When the A/C switch is pressed, the HVAC control assembly applies 10 volts to the A/C request signal circuit. The voltage is then sent through the evaporator temperature sensor, if the evaporator temperature is above 0°C (32°F), then the voltage is applied to the other A/C request signal circuit to the BCM. The BCM receives the voltage input and sends a class 2 message to the PCM for an A/C request.

The PCM will operate the A/C system automatically in FRONT DEFROST mode to help reduce moisture inside the vehicle. The A/C LED will not illuminate unless the driver presses the A/C request switch on the HVAC control assembly. The A/C system maybe running without the A/C LED indicator illuminated when in FRONT DEFROST mode or Recirc. The following conditions must be met in order for the PCM to turn on the compressor clutch:

Once engaged, the compressor clutch will be disengaged for the following conditions:

When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.