Air Temperature Description and Operation CJ3

The air temperature controls are divided into three primary areas. The first, Heater Mode, is related to how the heater system responds when a 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 sliding the temperature control, 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 fuse block provides power to the left air temperature actuator through the ignition-3 voltage circuit. Power and ground are provided to the HVAC control module by the ignition-3 voltage and ground circuits.

When a desired temperature setting is selected, a variable resistor is used to determine the air temperature door control signals value. A resistor inside the HVAC control module provides a varying ground. This changes the 12-volt signal coming into the actuator and varies the voltage so the actuator is moved into the proper position. 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. Ground is provided by the ground circuit and HVAC control module.

Dual Zone

The HVAC control module has dual sliding temperature switches. The fuse block provides power to the right air temperature actuator through the ignition-3 voltage circuit.

When a desired offset temperature setting is selected, a variable resistor is used to determine the right air temperature door control signals value. A resistor inside the HVAC control module provides a varying ground. This changes the 12-volt signal coming into the actuator and varies the voltage so the actuator is moved into the proper position. The motor opens the right air temperature actuator to a position to divert sufficient air past the heater core or evaporator to achieve the desired vehicle temperature. Ground is provided by the ground circuit and HVAC control module.

The dual zone controls allows for maximum temperature offset between the driver and passenger. It is possible to select maximum airflow over the evaporator core with one dual zone switch along with maximum airflow over the heater core with the other dual zone switch. Each air temperature actuator is independent from the other and the passenger side is not limited in the range of temperature offset.

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 assembly. The heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC assembly. Heated air is distributed to the passenger compartment, through the HVAC assembly, for passenger comfort. The amount of heat delivered to the passenger compartment is controlled by opening or closing the HVAC assembly 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 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. A/C is available as long as ambient air temperatures are above 4°C (40°F).

When the vehicle operator presses the A/C switch, voltage is sent to the PCM through the A/C request signal circuit. When the PCM reads voltage on this circuit, the module knows that an A/C request has been made. The PCM will command on the A/C compressor clutch to help reduce moisture inside the vehicle. The A/C compressor is on in selected modes, even in cool weather conditions, to help eliminate moisture from fogging the windshield. The A/C LED will not illuminate unless the driver presses the A/C request switch on the HVAC control module. Otherwise, the A/C system may be running without the A/C LED indicator illuminated.

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 junction block. Once the relay closes its internal switch, power is provided to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. Whenever the compressor is turned off, the A/C compressor clutch diode prevents a voltage spike from burning up the compressor clutch coil. The ground circuit provides a path to ground for the compressor and relay. The A/C clutch relay control circuit is grounded internally within the PCM.

A/C Refrigerant Pressure Sensor

The A/C system is protected by the A/C refrigerant pressure sensor. The sensors' output to the PCM is variable and is dependent upon pressure inside the line. A higher pressure results in a higher voltage output. The A/C pressure is constantly monitored in order to allow the A/C compressor clutch to disengage as needed. If line pressures climb above 2979 kPa (432 psi), the PCM will turn off the A/C compressor clutch until the pressure lowers to 1510 kPa (219 psi). If line pressures fall below 186 kPa (27 psi), the PCM will turn off the A/C compressor clutch until the pressure raises to 207 kPa (30 psi).

A 5-volt reference signal is sent out over the 5-volt reference circuit, from the PCM, to the A/C refrigerant pressure sensor. The PCM monitors the A/C pressure by sending out a separate 5-volt signal on the A/C refrigerant pressure sensor signal circuit. This circuit is how the PCM monitors HVAC pressures. Ground for the A/C refrigerant pressure sensor is provided by the low reference circuit.

Recirculation Mode

Recirculation is only available in MAX. When MAX is selected, vacuum is applied to the recirculation actuator. The force of the vacuum overcomes the spring pressure of the spring inside the actuator. This moves the actuator, which is linked to the recirculation door, into recirculation mode. This brings air from inside the vehicle instead of fresh air from the outside.

Air Temperature Actuator

The vehicle operator can determine the level of A/C by sliding the temperature control, 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 fuse block provides power to the left air temperature actuator through the ignition-3 voltage circuit. Power and ground are provided to the HVAC control module by the ignition-3 voltage and ground circuits.

When a desired temperature setting is selected, a variable resistor is used to determine the air temperature door control signals value. A resistor inside the HVAC control module provides a varying ground. This changes the 12-volt signal coming into the actuator and varies the voltage so the actuator is moved into the proper position. 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. Ground is provided by the ground circuit and HVAC control module.

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 a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

The A/C system used on this vehicle is a non-cycling system. Non-cycling A/C systems use a high pressure switch to protect the A/C system from excessive pressure. The high pressure switch will OPEN the electrical signal to the compressor clutch, if the refrigerant pressure becomes excessive. After the high and the low sides of the A/C system pressure equalize, the high pressure switch will CLOSE. This completes the electrical circuit to the compressor clutch. The A/C system is also 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 continues to rise, the high pressure relief will pop open and release refrigerant from the system.

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. The refrigerant is discharged from the compressor through the discharge hose, and forced through the condenser and then through the balance of the A/C system.

Compressed refrigerant enters the condenser at a high-temperature, high-pressure vapor state. As the refrigerant flows through the condenser, the heat is transferred to the ambient air passing through the condenser. Cooling 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, causing the refrigerant to vaporize at the orifice tube. The orifice tube also measures 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 to boil inside 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 flows back to the compressor in a vapor state, 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 condenses, and discharges from the HVAC module as water.

Air Temperature Description and Operation CJ4

The air temperature controls are divided into three primary areas. The first, Heater Mode, is related to how the heater system responds when a 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 sliding the air temperature switch, located on the HVAC control module, to any setting. The temperature switch can change the vehicle's air temperature regardless of the HVAC mode setting, heater or A/C.

The HVAC control module receives power from the fuse block on the ignition-3 voltage circuit along with the battery positive circuit. The module is grounded by the ground circuit. The HVAC control module communicates directly to the powertrain control module (PCM) on the HVAC class 2 serial data circuit, through a splice pack, and through the ECM/PCM/VCM class 2 serial data circuit.

Air Temperature Actuator

When the temperature switch is positioned to the desired temperature, an internal resistor within the air temperature actuator varies the voltage signal from the ignition-3 voltage circuit. That varied voltage is sent to the controls circuit inside the air temperature actuator. The controls circuit moves the temperature door to divert air past the heater core. The controls circuit is grounded by the HVAC control module on the front temperature control signal circuit. The air temperature actuator is grounded by the ground circuit.

Temperature Sensor

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

Since the sensor is mounted in the front grille area under the front bumper and can be affected by city traffic, idling, and hot engine restarts, the ambient air temperature sensor could give a false temperature reading. If the ambient air temperature increases, the VF display will not update until the vehicle is driven at speeds greater than 32 km/h (20 mph), for one and a half minutes. The digital display will update sooner if the vehicle is driven at speeds greater than 72 km/h (45 mph), for one minute. If the sensor reading is ever cooler than the displayed value, then the ambient air temperature changes are displayed as rapidly as possible.

If the vehicle has been turned off for more than 3 hours, the current ambient temperature will be shown when you start the vehicle. If it has been turned off for less than 3 hours, the temperature will be recalled from the previous ignition cycle.

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 auxiliary heater is to supply heat to the back seat area of the vehicle. The back seat passengers can determine the level of heat by sliding the auxiliary air temperature switch, located on the auxiliary HVAC control module, to any setting. The temperature switch can change the back seats air temperature regardless of the HVAC control module setting.

The auxiliary HVAC control module receives power from the fuse block on the ignition-3 voltage circuit. The assembly is grounded by the ground circuit.

When the auxiliary temperature switch is positioned to the desired temperature, an internal resistor within the auxiliary air temperature actuator varies the voltage signal from the ignition-3 voltage circuit. That varied voltage is sent to the controls circuit inside the auxiliary air temperature actuator. The controls circuit moves the temperature door to divert air past the heater core. The controls circuit is grounded by the auxiliary HVAC control module on the auxiliary air temperature door control circuit. The auxiliary air temperature actuator is grounded by the ground circuit.

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 A/C switch is pressed or the HVAC control module is in FRONT DEFROST or MIX-BLEND mode. The A/C system can operate regardless of the temperature setting, as long as ambient temperature is above 4°C (40°F).

The HVAC control module receives power from the fuse block on the ignition-3 voltage circuit along with the battery positive circuit. The module is grounded by the ground circuit. The HVAC control module communicates directly to the powertrain control module (PCM) on the HVAC class 2 serial data circuit, through a splice pack, and through the ECM/PCM/VCM class 2 serial data circuit.

Regardless of the selected A/C mode setting, a request is made to the PCM to turn on the A/C compressor. The request is made to the PCM from the HVAC control module through the HVAC class 2 serial data circuit, through a splice pack, and through the ECM/PCM/VCM class 2 serial data circuit.

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 internally of the underhood junction block. Once the A/C compressor clutch 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. Whenever the compressor is turned off, the A/C compressor clutch diode prevents a voltage spike from burning up the compressor clutch coil. The ground circuit provides a path to ground for the compressor. The A/C clutch relay control circuit is grounded internally within the PCM.

A/C Refrigerant Pressure Sensor

The A/C system is protected by the A/C refrigerant pressure sensor. The sensors output to the PCM is variable and is dependent upon pressure inside the line. A higher pressure results in a higher voltage output. The A/C pressure is constantly monitored in order to allow the A/C compressor clutch to disengage as needed. If line pressures climb above 2826 kPa (410 psi) or fall below 207 kPa (30 psi), the PCM will turn off the A/C compressor clutch.

A 5-volt reference signal is sent out over the 5-volt reference circuit, from the PCM, to the A/C refrigerant pressure sensor. Voltage is dropped across the sensor and the PCM monitors the remaining voltage on the A/C refrigerant pressure sensor signal circuit. Ground for the A/C refrigerant pressure sensor is provided by the low reference circuit.

Dual Stage Orifice Solenoid

The dual stage orifice is an electric solenoid, which has two sizes, 1.092 mm (0.043 in) and 1.6 mm (0.063 in). The orifice changes positions from the larger size to the smaller size depending on the following conditions:

Power is provided to both sides of the dual stage orifice relay by the ignition-3 voltage circuit. When the HVAC control module activates the dual stage orifice, the HVAC control module grounds the dual stage orifice relay control circuit. Once the circuit is grounded, power is supplied directly to the dual stage orifice solenoid on the dual stage orifice solenoid supply voltage circuit. Ground is supplied to the solenoid by the ground circuit.

The purpose of the auxiliary A/C is to supply cool air to the back seat area of the vehicle. The back seat passengers can determine the level of A/C by sliding the auxiliary air temperature switch, located on the auxiliary HVAC control module, to any setting and pressing the A/C switch. The temperature switch can change the back seats air temperature regardless of the HVAC control module setting.

The auxiliary HVAC control module receives power from the fuse block on the ignition-3 voltage circuit. The assembly is grounded by the ground circuit.

When the auxiliary temperature switch is positioned to the desired temperature, an internal resistor within the auxiliary air temperature actuator varies the voltage signal from the ignition-3 voltage circuit. That varied voltage is sent to the controls circuit inside the auxiliary air temperature actuator. The controls circuit moves the temperature door to divert air past the heater core or evaporator. The controls circuit is grounded by the auxiliary HVAC control module on the auxiliary air temperature door control circuit. The auxiliary air temperature actuator is grounded by the ground circuit.

A/C can be requested by the back seat passenger by pressing the A/C switch on the auxiliary HVAC control module. An internal resistor varies the internal ignition-3 voltage signal. That varied voltage signal is sent to the HVAC control module on the recirculation/A/C request signal circuit. Once the signal is received by the HVAC control module, the A/C request is made to the PCM. The A/C LED will light on the switch indicating that A/C has been requested. Voltage is sent from the HVAC control module to the auxiliary HVAC control module on the auxiliary A/C request indicator signal circuit.

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 a very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.

The A/C system used on this vehicle is a non-cycling system. Non-cycling A/C systems use a high pressure switch to protect the A/C system from excessive pressure. The high pressure switch will OPEN the electrical signal to the compressor clutch, if the refrigerant pressure becomes excessive. After the high and the low sides of the A/C system pressure equalize, the high pressure switch will CLOSE. This completes the electrical circuit to the compressor clutch. The A/C system is also 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 continues to rise, the high pressure relief will pop open and release refrigerant from the system.

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. The refrigerant is discharged from the compressor through the discharge hose, and forced through the condenser and then through the balance of the A/C system.

Compressed refrigerant enters the condenser at a high-temperature, high-pressure vapor state. As the refrigerant flows through the condenser, the heat is transferred to the ambient air passing through the condenser. Cooling 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, causing the refrigerant to vaporize at the orifice tube. The orifice tube also measures 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 to boil inside 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 flows back to the compressor in a vapor state, 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 condenses, and discharges from the HVAC module as water.