The air temperature controls are divided into three primary areas. The first, Heater Mode, is related to how the heater system responds when the heater mode is selected, and how the HVAC system provides the desired temperature for each setting. The second, A/C Mode, is related to how the A/C system responds when an A/C mode is selected by the vehicle operator, and how the HVAC system provides the desired temperature for each setting. The third, A/C Cycle, describes the complete A/C cycle.

The purpose of the heater is to supply heat to the interior of the vehicle. The vehicle operator can determine the level of heat by turning the left temperature switch, located on the HVAC control module, to any setting. The temperature control can change the vehicle's air temperature regardless of the HVAC mode setting, heater or A/C. The vehicle passenger can adjust their temperature by toggling the right air temperature switch. Passenger temperatures can be offset 2°C (4°F) cooler or warmer than the drivers setting.

The HVAC control processor provides power to the left air temperature actuator through the off/run voltage circuit. Ground is provided by the low reference circuit and the HVAC control processor.

Air Temperature Actuators

When the HVAC system is in the OFF mode, pressing any HVAC switch, except the passenger temperature switch, will enable the automatic HVAC system. If the HVAC control module displays any temperature between 19-29°C (66-84°F), turning the temperature switch will increase/decrease the set temperature by 1°. If the temperature setting is 30°C (85°F), turning the temperature switch up increases the set temperature to 32°C (90°F).

The left air temperature actuator is an electronic stepper motor with feedback potentiometers. 0 volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, actuator rotation stops. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the HVAC control processor, to the left air temperature actuator. When a desired temperature setting is selected, whether manual or automatic, the solid state circuit is used to determine the left air temperature door position sensor signals value. A separate 5 volt reference is sent from the HVAC control processor to the solid state circuit. The HVAC control processor software uses this reference voltage to determine the left air temperature actuator position through the left air temperature door position signal circuit. The motor opens the left air temperature actuator to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature.

The right air temperature actuator operates the same as the left side. 0 volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, actuator rotation stops. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the HVAC control processor, to the right air temperature actuator. When a desired temperature setting is selected, whether manual or automatic, the solid state circuit is used to determine the right air temperature door position sensor signals value. A separate 5 volt reference is sent from the HVAC control processor to the solid state circuit. The HVAC control processor software uses this reference voltage to determine the right air temperature actuator position through the right air temperature door position signal circuit. The motor opens the air mixture door to a position to divert sufficient air past the heater core or evaporator to achieve the desired vehicle temperature.

Temperature Sensors

The automatic system uses multiple sensors to achieve and maintain the desired temperature. The inside air temperature sensor provides the HVAC control processor software with the temperature of the air drawn through the aspirator. A 5 volt reference signal is sent from the HVAC control processor to the inside air temperature sensor over the inside air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. As temperature increases, resistance decreases. That varied voltage provides a signal to the software inside the HVAC control processor. Remaining voltage from the inside air temperature sensor is sent back to the HVAC control processor ground through the low reference circuit.

The ambient air temperature sensor provides the HVAC control processor software with the temperature of the air outside the vehicle and displays that temperature on the HVAC control module. As temperature increases, resistance decreases. A 5 volt reference signal is sent from the HVAC control processor to the ambient air temperature sensor over the ambient air temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the HVAC control processor. Remaining voltage from the ambient air temperature sensor is sent back to the HVAC control processor ground through the low reference circuit.

Since the sensor is mounted underhood, it can be affected by city traffic, idling, and hot engine restarts. A temperature memory feature is used by the HVAC control processor ambient air temperature programming to help provide greater accuracy under engine restart conditions. If the engine coolant temperature is less than 10°C (50°F) above the ambient air temperature sensor reading, or if the engine has not been started in two hours, then the actual ambient air temperature sensor reading is displayed. However, if the engine coolant is more than 10°C (50°F) above the sensor reading, the memorized ambient air temperature is displayed. This is the last displayed temperature sensed when the vehicle was operating. At vehicle speeds greater than 32 km/h (20 mph), the ambient air temperature displayed may be allowed to increase, but only after a built-in 80 second time delay which allows for ambient air to cool the sensor. The time delay starts when vehicle speed reaches or maintains at least 32 km/h (20 mph). If the sensor reading is ever less than the displayed value or if the vehicle speed is 72 km/h (45 mph) or greater, then the ambient air temperature changes are displayed as rapidly as possible. The scan tool is able to provide an instant temperature update to the HVAC control processor.

Sunload Sensor

The sunload sensor provides the HVAC control processor software with the amount of sun light entering the driver and passenger side through the windshield. A 5 volt reference signal is sent from the HVAC control processor to the sunload sensor over the driver solar sensor signal circuit. A variable photo-diode resistor inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the HVAC control processor. Remaining voltage from the sunload sensor is sent back to the HVAC control processor ground through the low reference circuit.

Dual Zone Control Switch

The right air temperature switch is provided to allow the passenger to offset air discharge temperatures on the right side of the vehicle. Passenger temperatures can be offset 2°C (4°F) cooler or warmer than the primary setting. The temperature switches operate independently from each other. To activate the dual zone, the passenger toggles the right air temperature switch to the desired offset. A signal is sent from the HVAC control processor to the right air temperature actuator on the right air temperature door control circuit. The passenger side temperature will appear on the VF display as a red and blue bar graph. Temperature offset will be allowed as long as the driver set temperature is not in maximum hot or cold. If the right air temperature switch has been turned on, it can be turned off by pressing the right air temperature switch. Greater sunload on one side of the vehicle may cause the discharge air temperatures to be different, even when the HVAC system is not operating in a dual zone mode.

The HVAC control processor provides power to the right air temperature actuator through the vacuum control assembly supply voltage circuit. Ground is provided by the low reference circuit.

The right air temperature actuator is an electronic stepper motor with feedback potentiometers. 0 volts drives the actuator in one direction while 5 volts moves the actuator in the opposite direction. When the actuator receives 2.5 volts, actuator rotation stops. A 5 volt reference signal is sent out over the 5 volt reference circuit, from the HVAC control processor, to the right air temperature actuator. When a desired temperature offset is selected, whether manual or automatic, the solid state circuit is used to determine the right air temperature door position signals value. A separate 5 volt reference is sent from the HVAC control processor to the solid state circuit. The HVAC control processor software uses this reference voltage to determine the right air temperature actuators position through the right air temperature door position signal circuit. The motor moves the right air temperature actuator door to the desired position.

Steering Wheel Temperature Control Switch

A separate temperature switch is mounted on the steering wheel to allow the driver to adjust the temperature setting. Power to the steering wheel control switch is delivered from the rear fuse block, through the steering column fuse holder, and inflatable restraint steering wheel module coil on the battery positive voltage circuit. When the driver toggles the temperature switch up or down, the voltage is sent through a series of resistors. That varied voltage is sent back through the inflatable restraint steering wheel module coil to the instrument cluster through the remote radio control circuit. Once the instrument cluster receives the varied voltage signal, the information is sent out over the class 2 serial data to the HVAC control processor where the temperature setting is adjusted.

Engine Coolant

Engine coolant is the key element of the heating system. The normal engine operating coolant temperature is controlled by the thermostat. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation.

Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC module. The heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC module. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC module air temperature door. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.

The purpose of the air conditioning (A/C) system is to provide cool air and remove humidity from the interior of the vehicle. The A/C system is engaged when the HVAC control module is in any mode. The A/C system can operate regardless of the temperature setting, as long as ambient temperature is above 3°C (38°F). The passenger and can adjust the temperature offset by adjusting the right air temperature switch. Passenger temperatures can be offset 2°C (4°F) cooler or warmer than the drivers setting.

The HVAC control module is the interface between the vehicle operator and the HVAC control processor. When an A/C request is selected, a signal is sent from the HVAC control processor to the powertrain control module (PCM). The PCM monitors A/C refrigerant line pressure, A/C refrigerant temperatures and engine coolant temperature. If all the PCM signals are within operating range, the A/C clutch relay control circuit will be grounded, allowing the A/C compressor to engage. If there is a malfunction in the A/C system, the driver information center will read either SERVICE A/C SYSTEM or LOW RERFIG A/C OFF to alert the vehicle operator that there is a malfunction with the A/C system.

The PCM turns on the A/C compressor by providing a path to ground through the A/C clutch relay control circuit for the A/C compressor clutch relay. Power is provided to the A/C compressor clutch relay from the underhood fuse block on the A/C low pressure switch signal circuit and both A/C pressure switches. Once the relay closes its internal switch, power from the battery is provided to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. The ground circuit provides a path to ground for the compressor. The A/C compressor clutch relay control circuit is grounded internally within the PCM.

The PCM will engage the A/C compressor clutch any time the engine speed is below 5000 RPM and the A/C is requested unless any of the following conditions exist:

A/C Pressure Switches

The A/C system is protected by two A/C pressure switches. The A/C high pressure switch interrupts the A/C request signal when the A/C line pressure exceeds 2965 kPa (430 psi). When high side pressures drop down to 1379 kPa (200 psi), the A/C high pressure switch will close and allow compressor operation. The A/C low pressure switch interrupts the A/C low pressure switch signal when the A/C line pressure drops below 69 kPa (10 psi). When the PCM stops receiving the required signals, the A/C compressor clutch relay control circuit is no longer grounded, thus shutting off the compressor.

A/C Temperature Sensors

The HVAC control processor monitors the refrigerant temperature and sends the data to the PCM on the class 2 serial data circuit. The PCM monitors the A/C high temperature sensor signal along with the coolant temperature sensor in order to determine the need for the cooling fans and prevents the A/C compressor from operating at high discharge pressures. A 5 volt reference signal is sent from the HVAC control processor to the A/C refrigerant high temperature sensor over the A/C refrigerant high temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the HVAC control processor. Remaining voltage from the A/C refrigerant high temperature sensor is sent back to the HVAC control processor ground through the low reference circuit.

The HVAC control processor monitors the A/C low temperature sensor to determine the low side pressure based on the pressure/temperature relationship of the R-134a. A 5 volt reference signal is sent from the HVAC control processor to the A/C refrigerant low temperature sensor over the A/C refrigerant low temperature sensor signal circuit. A thermister inside the sensor varies the voltage. That varied voltage provides a signal to the software inside the HVAC control processor. Remaining voltage from the A/C refrigerant low temperature sensor is sent back to the HVAC control processor ground through the low reference circuit.

Extended Compressor at Idle

This mode allows for smoother idle conditions by increasing the minimum clutch ON time to a maximum of 45 seconds whenever the vehicle speed drops below 24 km/h (15 mph).

Minimum Compressor On Time

Vehicles operating above 32 km/h (20 mph) will be controlled to a minimum ON time of 4 to 6 seconds before the HVAC control processor will allow the clutch to disengage. After a minimum ON time has expired, the HVAC control processor will request the PCM to disengage the A/C compressor clutch only if the low side temperature is below -2°C (28°F). After reaching this turn-off temperature, the HVAC control processor waits for the low side to reach a turn-on temperature of 10°C (50°F) and then repeats the cycle. During the compressor ON cycle, any of the following conditions will force the PCM to disengage the A/C compressor clutch:

Except for wide open throttle (WOT), all of these above conditions will keep the compressor OFF until the condition no longer exists. The WOT condition will keep the compressor OFF for a maximum of 20 seconds. On initial crank, the HVAC control processor will check for indications of liquid refrigerant which may have collected at the compressor. If found, the A/C compressor clutch will engage. After the engine has started, the clutch will disengage and waits for normal calibrations and parameters to activate the clutch.

Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automotive use. R-134a is an very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then through the balance of the A/C system. The A/C system is mechanically protected with the use of a high pressure relief valve. If the high pressure switch were to fail or if the refrigerant system becomes restricted and refrigerant pressure continued to rise, the high pressure relief will pop open and release refrigerant from the system.

Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.

The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exits the condenser and flows through the liquid line, to the orifice tube.

The orifice tube is located in the liquid line between the condenser and the evaporator. The orifice tube is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the orifice tube, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to vaporize at the orifice tube. The orifice tube also meters the amount of liquid refrigerant that can flow into the evaporator.

Refrigerant exiting the orifice tube flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the HVAC module and passes through the evaporator core. Warm and moist air will cause the liquid refrigerant boil inside of the evaporator core. The boiling refrigerant absorbs heat from the ambient air and draws moisture onto the evaporator. The refrigerant exits the evaporator through the suction line and back to the compressor, in a vapor state, and completing the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.

The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture removed from the passenger compartment will also change form, or condense, and is discharged from the HVAC module as water.