Heater and Air Conditioning

HEATER AND AIR CONDITIONER
A manual temperature control type heating-air conditioning system is standard factory-installed equipment on this model. An electronically controlled Automatic Zone Control (AZC) type heating-air conditioning system is an available factory-installed option.

Fig.1 Blend-Air HVAC System - (typical):

All vehicles are equipped with a common HVAC housing assembly. The system combines air conditioning, heating, and ventilating capabilities in a single unit housing mounted under the instrument panel.

Outside fresh air enters the vehicle through the cowl top opening at the base of the windshield, and passes through a plenum chamber to the HVAC system blower housing. Air flow velocity can then be adjusted with the blower motor speed selector switch on the a/c heater control panel. The air intake openings must be kept free of snow, ice, leaves, and other obstructions for the HVAC system to receive a sufficient volume of outside air.

It is also important to keep the air intake openings clear of debris because leaf particles and other debris that is small enough to pass through the cowl plenum screen can accumulate within the HVAC housing. The closed, warm, damp and dark environment created within the HVAC housing is ideal for the growth of certain molds, mildews and other fungi. Any accumulation of decaying plant matter provides an additional food source for fungal spores, which enter the housing with the fresh air. Excess debris, as well as objectionable odors created by decaying plant matter and growing fungi can be discharged into the passenger compartment during HVAC system operation.

Both the manual and AZC heater and air conditioner are blend-air type systems. In a blend-air system, a blend door controls the amount of unconditioned air (or cooled air from the evaporator) that is allowed to flow through, or around, the heater core. A temperature control knob on the a/c heater control panel determines the discharge air temperature by energizing the blend door actuator, which operates the blend door. This allows an almost immediate control of the output air temperature of the system. The AZC system will have separate blend doors and temperature controls for each front seat occupant.

The mode control knob on the a/c heater control panel is used to direct the conditioned air to the selected system outlets. On manual temperature control systems, the mode control knob switches engine vacuum to control the mode doors, which are operated by vacuum actuators. On AZC systems, the mode control knob switches electrical current to control the mode doors, which are operated by electronic actuators.

The outside air intake can be shut off on manual temperature control systems by selecting the Recirculation Mode with the mode control knob. The outside air intake can be shut off on Automatic Zone Control (AZC) type system by pushing the Recirculation Mode button. This will operate the recirculation door that closes off the outside fresh air intake and recirculates the air that is already inside the vehicle.

The air conditioner for all models is designed for the use of non-CFC, R-134a refrigerant. The air conditioning system has an evaporator to cool and dehumidify the incoming air prior to blending it with the heated air. This air conditioning system uses a thermal expansion valve to meter refrigerant flow to the evaporator coil. To maintain minimum evaporator temperature and prevent evaporator freezing, the system utilizes an evaporator thermistor probe with the appropriate operating logic located in the body control module (BCM).

SYSTEM AVAILABILITY
- Depending on the model, either a Manual Temperature Control (MTC) or Automatic Zone Control (AZC) HVAC system is available in these vehicles.

SYSTEM CONTROLS
Manual Temperature Control (MTC)
- Refer to the Service Manual for MTC system description and operation information.

The Automatic Zone Control (AZC) Module:
- is fully addressable with the DRB III.
- communicates over the Programmable Communication Interface Multiplex System (PCI) Bus.
- has dual infrared sensors, mounted in the AZC module face, which independently measure surface temperature to maintain occupant comfort levels.
- has dual-zone temperature-controls to provide a wide side-to-side variation in temperature to meet the needs of either front seat occupant.
- can be operated in a manual mode.
- uses engine temperature data, received over the PCI Bus, for cold engine blower lock-out.
- uses engine RPM data, received over the PCI Bus, for wide-open throttle A/C cut-out.
- uses vehicle speed data, received over the PCI Bus, to determine air-flow over the condenser for optimum cooling performance.
- uses Evaporator Temperature Sensor data, received over the PCI Bus, to prevent evaporator freeze up while maintaining optimum cooling performance.
- uses Ambient Temperature Sensor data, received over the PCI Bus, to optimize comfort control.
- provides an A/C request to the BCM when compressor operation is desired.
- controls rear defogger operation.
- controls fan speed, providing 10 blower speeds in manual mode and infinite speeds in automatic mode.
- The Blower Motor Controller provides a 5.0 volt signal to the AZC Module over the Blower Motor Control circuit. The AZC Module provides a variable duty cycle ground to the 5.0 volt signal based on input from the blower switch. When the blower switch is set to LO speed, the AZC Module provides a short duty cycle (less time grounding the signal voltage). As higher blower speeds are requested, the AZC Module increases the duty cycle (more time grounding the signal voltage). When the blower switch reaches HI speed, the duty cycle increases to where the signal pattern is almost a flat line (with brief voltage spikes).
- controls electronic door actuator operation.
- A simplified control system for operation of the mode, recirculation, and temperature control actuators provides positive positioning without the complexity of feedback from position sensors. The AZC Module knows the number of operating actuator revolutions required for full door travel as well as the number of actuator commutator pulses per revolution. Using these parameters, the AZC Module runs the actuator for the number of commutator pulses that correspond to the desired door position. To maintain accuracy, the system recalibrates itself periodically at known zero and full travel conditions.
- Refer to the Service Manual for additional AZC system description and operation information.

The Dual-Zone AZC HVAC system uses:
- two, two-wire electronic blend door actuators.
- one, two-wire electronic mode door actuator.
- one, two-wire electronic recirculation door actuator.

SYSTEM REVISIONS
The 2002 WJ/WG HVAC systems remain mostly carryover from 2001. Revisions to the 2002 MTC and AZC HVAC systems include:
- the addition of an Evaporator Temperature Sensor which is hard wired to the BCM. Data from the sensor is used to prevent evaporator freeze up while maintaining optimum cooling performance.
- new Diagnostic Trouble Codes (DTCs) for a short low and for an open on the Evaporator Temperature Sensor circuit.

SYSTEM DIAGNOSTICS (MTC)
Fault detection is through Diagnostic Trouble Codes (DTCs)
- DTCs are displayed by the DRB III.
- DTCs pertaining to the Evaporator Temperature Sensor are stored in the BCM. Diagnostics for these DTCs can be found in the Heating & A/C category.
- Diagnostics for symptoms pertaining to A/C compressor operation can be found in the Heating & A/C category of this manual and in the Powertrain Diagnostic Procedures manual.
- DTCs pertaining to A/C compressor control circuits are stored in the PCM/ECM. Diagnostics for these DTCs can be found in the Powertrain Diagnostic Procedures manual
- Refer to the Service Manual for additional MTC system diagnosis and testing information.

SYSTEM DIAGNOSTICS (AZC)
Fault detection is through active and stored Diagnostic Trouble Codes (DTCs)
- DTCs are displayed by the DRB III.

AZC Module DTCs
- Active DTCs are those which currently exist in the system. The condition causing the fault must be repaired in order to clear this type of DTC.
- Active DTCs become stored DTCs when the condition that caused the active DTC no longer exists.
- Stored DTCs will be erased after 72 key cycles or by clearing them with the DRB III.
- Diagnostics for symptoms pertaining to A/C compressor operation can be found in the Heating & A/C category and in the Powertrain Diagnostic Procedures manual.
- Refer to the Service Manual for additional AZC system diagnosis and testing information.

BCM DTCs
- DTCs pertaining to the Evaporator Temperature Sensor are stored in the BCM. Diagnostics for these DTCs can be found in the Heating & A/C category.

PCM/ECM DTCs
- DTCs pertaining to A/C compressor control circuits are stored in the PCM/ECM. Diagnostics for these DTCs can be found in the Powertrain Diagnostic Procedures manual.

FOLLOWING A REPAIR (AZC)
The AZC Reset Module function:
- is actuated with the DRB III.
- must be actuated if the AZC Module or any door actuator is replaced.
- homes and repositions door actuators.