The air temperature controls are divided into 4 areas:
| • | HVAC control components |
| • | Heating and A/C operation |
| • | Engine coolant |
| • | A/C cycle |
HVAC Control Components
HVAC Control Module
The HVAC control module is a GM LAN device that interfaces between the operator and the HVAC system to maintain air temperature and distribution settings. The battery positive and ignition 1 voltage circuits provide power to the control module. The control module supports the following features:
Feature | Availability |
|---|---|
Afterblow | Yes |
Purge | No |
Personalization | No |
Actuator Calibration | Yes |
Air Temperature Actuator
The air temperature actuator are 5-wire bi-directional electric motors that incorporate a feedback potentiometer. Low reference, 5-volt reference, position signal, and 2 control circuits enable the actuator to operate. 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 volt. In order 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. 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, one of the control circuits is grounded. 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 removes power from both control circuits.
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 volt. When the A/C refrigerant pressure is high, the signal value is near 5 volts.
The A/C refrigerant pressure sensor protects the A/C system from operating when an excessively high or low pressure condition exists. The engine control module (ECM) disables the compressor clutch under the following conditions:
| • | The A/C high side pressure is more than 2,929 kPa (425 psi). |
| • | The clutch will be enabled after the high side pressure decreases to less than 1,376 kPa (200 psi). |
| • | A/C low side pressure is less than 269 kPa (39 psi). |
| • | The clutch will be enabled or will allow engagement again after the low side pressure increases to more than 296 kPa (43 psi). |
Evaporator Low Ambient Protection
The refrigerant temperature at the temperature sensor on the evaporator case controls cycling of the compressor clutch to prevent freezing of the evaporator core. The compressor is disabled when the temperature goes below 3°C (37°F) and vehicle speed is greater than 8 km/h (5 mph). The compressor is enabled when the temperature exceeds 4°C (40°F). The minimum cycling time off is 4 seconds.
Ambient Air Temperature Sensor
The ambient air temperature sensor mounts underhood and can be affected by city traffic, by idling, and by restarting a hot engine. Therefore, the HVAC control module filters the value of the ambient air temperature sensor for temperature display. The ambient air temperature value is updated under the following conditions:
Condition | Display |
|---|---|
When the HVAC module A/C and RECIRCULATION buttons are pressed simultaneously | Updates temperature display instantly |
At start up with the engine off less than 2 hours | Displays last stored temperature unless temperature has decreased. The outside air temperature reading is always instantly updated if ambient air temperature has decreased. |
At start up with the engine off more than 2 hours | Displays actual outside temperature |
Vehicle speed above 32 km/h (20 mph) for a minimum of 80 seconds | Updates temperature display at a slow filtered rate |
Sensor ambient temperature reading is less than the last displayed value | Displays actual outside temperature |
When the ambient air decreases | Updates temperature display rapidly |
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 control. Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:
| • | Recirculation |
| • | Difference between inside and desired temperature |
| • | Difference between ambient and desired temperature |
| • | Blower motor speed setting |
| • | Mode setting |
The vehicle operator can activate the A/C system by pressing the A/C switch. The A/C system can operate regardless of the temperature setting.
The engine control module (ECM) 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 module. The A/C system maybe running without the A/C LED indicator illuminated when in FRONT DEFROST mode. The HVAC system uses a compressor that incorporates a thermal switch that opens once the compressor temperature exceeds 211-217°C (380-454°F) creating an open circuit. The following conditions must be met in order for the ECM to turn on the compressor clutch:
| • | BCM |
| - | Battery voltage between 11-16 volts |
| - | A/C request from the HVAC control module |
| • | ECM |
| - | Engine coolant temperature (ECT) is greater than 117°C (243°F). |
| - | Engine speed is less than 4,760 RPM. |
| - | A/C pressure is between 2,929-2,706 kPa (425-39 psi). |
Once engaged, the compressor clutch will be disengaged for the following conditions:
| • | Throttle position is 100 percent. |
| • | A/C pressure is more than 2,929 kPa (425 psi). |
| • | A/C pressure is less than 269 kPa (39 psi). |
| • | ECT is more than 120°C (248°F). |
| • | Engine speed is more than 6240 RPM. |
| • | Transmission shift |
| • | ECM detects excessive torque load. |
| • | ECM detects insufficient idle quality. |
| • | ECM detects a hard launch condition. |
When the compressor clutch disengages, the compressor clutch diode protects the electrical system from a voltage spike.
