System Description

HEATING & AIR CONDITIONING

SYSTEM AVAILABILITY
- Depending on the model, either a Dual-Zone or Three-Zone HVAC system is available in these vehicles.

SYSTEM CONTROLS
The A/C-Heater Control Module:
- is fully addressable with the DRB III.
- communicates over the Programmable Communication Interface Multiplex System (PCI) Bus.
- uses input from the evaporator temperature sensor to prevent evaporator freeze up while maintaining optimum cooling performance.
- provides an A/C request to the Powertrain Control Module (PCM) over the PCI Bus when compressor operation is desired.
- controls front blower motor operation, providing four blower speeds (Low, M1, M2, & High).
- controls heated mirror and EBL operation.
- controls rear washer pump operation.
- on three-zone systems, activates and deactivates the Rear A/C-Heater Control.
- on three-zone systems, controls rear blower motor operation, providing three blower speeds (Low, Med, & High).
- on three-zone systems, controls rear heater coolant pump operation.
- To assist the HVAC system in maintaining a consistent and optimum heat output at low vehicle speeds, an electric coolant pump operates under specific conditions to keep a steady flow of hot coolant circulating through the heater circuit. The pump runs when all of the following conditions are met: When the vehicle's speed is below 27 km/h (17 mph), the coolant temperature is between 65.5 °C and 110 °C (150 °F and 230 °F), the front blower is turned ON, and the blend control is set above 60% reheat. The A/C - Heater Control Module turns the pump off if any one of the following conditions occur: The vehicle speed rises above 48.3 Km/h (30 mph). The coolant temperature drops below 65.5°C (150 °F). The coolant temperature rises above 110 °C (230 °F). The front blower is switched OFF. The blend control is set below 60% reheat.
- controls the front and three-zone system rear electronic door actuators' 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 A/C-Heater Control 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 A/C-Heater Control 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.

On Three-Zone systems, the Rear A/C-Heater Control:
- controls rear blower motor operation, providing three blower speeds (Low, Med, & High).
- provides desired rear blend and mode door position input to the A/C-Heater Control Module.

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

The Three-Zone HVAC system uses:
- two front, two-wire electronic blend door actuators.
- one front, two-wire electronic mode door actuator.
- one, two-wire electronic recirculation door actuator.
- one rear, two-wire electronic blend door actuator.
- one rear, two-wire electronic mode door actuator.

SYSTEM REVISIONS
The 2002 DN HVAC system remains mostly carryover from 2001. Revisions to the 2002 HVAC system include:
- new Diagnostic Trouble Code for a short high on the rear window defogger relay control circuit.
- new Diagnostic Trouble Codes for shorts high and low on the rear heater coolant pump circuit.
- a new System Test called the Actuator Circuit Test.

SYSTEM DIAGNOSTICS
Fault detection is through active and stored Diagnostic Trouble Codes (DTCs).
- DTCs are displayed by the DRB III.
- 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.
- Stored DTCs are those which occurred in the system since the A/C-Heater Control Module received the last "clear diagnostic info" message.

The A/C Cooldown Test:
- is actuated with the DRB III.
- checks A/C system performance based on evaporator temperature sensor input.
- will not run if ambient temperature is below 12.7 °C (55 °F)
- will pass if the evaporator temperature drops 6.7 °C (20 °F) within two minutes of starting the test.
- faults display on the DRB III as test messages only after running the test.
- faults will not display on the DRB III as Diagnostic Trouble Codes.
- will cause the A/C mode switch status indicator to flash while the test is running.
- If the test fails, the status indicator will continue to flash until the test returns passed. It will also prevent the EBL mode switch status indicator from indicating EBL operating status. However, the EBL mode switch will continue to function in this state.

The HVAC Door Recalibration function:
- is actuated with the DRB III.
- homes and repositions door actuators.
- monitors for faults on the actuator circuits.
- faults display on the DRB III as test messages only after running the test.
- faults will not display on the DRB III as Diagnostic Trouble Codes.
- will cause the EBL mode switch status indicator to flash while the test is running.
- If the test fails, the status indicator will continue to flash until the test returns passed. It will also prevent the A/C mode switch status indicator from indicating A/C status. However, the A/C mode switch will continue to function in this state.

The Actuator Circuit Test:
- is actuated with the DRB III.
- monitors for faults on the actuator circuits.
- allows service to easily diagnose and troubleshoot up to three simultaneous shorts.
- supplements the continuous diagnostics on the actuator drive system.
- faults display on the DRB III as test messages only after running the test.
- faults will not display on the DRB III as Diagnostic Trouble Codes.

When Performing The Actuator Circuit Test

CAUTION:
- Shorted rear or common door driver circuits can cause additional Actuator Circuit Test messages to set for circuits where no condition exists to cause a fault.
- To ensure a proper diagnosis, repair all Short Too Complex messages first, all rear door driver circuit related messages second, all common door driver circuit related messages third, and all front door driver circuit related messages last.
- The DRB III can display up to three Actuator Circuit Test messages at a time. After repairing each Actuator Circuit Test message, cycle the ignition switch, then rerun the Actuator Circuit Test to ensure no new messages exist.

- The Short Too Complex message:
- indicates that a specific determination of which lines are shorted could not be made.
- is caused by more than three drivers being shorted in the same direction. For example, four drivers all shorted to ground, or two or more drivers shorted with at least one driver shorted to ignition/battery and one driver shorted to ground.

Messages displaying:
- XXX Driver/Circuit Shorted to Ignition Battery will set on a per-driver basis.
- XXX Driver/Circuit Shorted to Ground will set on a per-driver basis.
- the same two drivers/circuits shorted to ignition/battery as-well-as shorted to ground indicates that two actuator driver circuits are shorted together.

After the Actuator Circuit Test is run on systems with less than six actuators:
- the actuator drive system is left in a state where further actuator positioning requests are ignored from both the panel and the DRB III. This state is cleared by cycling the ignition switch (cycling power to the control).
- If the test returns passed, then troubleshooting should proceed to clearing faults and running the HVAC Door Recalibration system test as a final check of system health.

COOLING SYSTEM REQUIREMENTS
To maintain the performance level of the heating-air conditioning system, the engine cooling system must be properly maintained. The use of a bug screen is not recommended. Any obstructions in front of the radiator or condenser will reduce the performance of the air conditioning and engine cooling systems.

The engine cooling system includes the heater core and the heater hoses.

REFRIGERANT HOSES/LINES/TUBES PRECAUTIONS
Kinks or sharp bends in the refrigerant plumbing will reduce the capacity of the entire system. High pressures are produced in the system when it is operating. Extreme care must be exercised to make sure that all refrigerant system connections are pressure tight.

A good rule for the flexible hose refrigerant lines is to keep the radius of all bends at least ten times the diameter of the hose. Sharp bends will reduce the flow of refrigerant. The flexible hose lines should be routed so they are at least 80 millimeters (3 inches) from the exhaust manifold. It is a good practice to inspect all flexible refrigerant system hose lines at least once a year to make sure they are in good condition and properly routed.

There are two types of refrigerant fittings:
- All fittings with O-rings need to be coated with refrigerant oil before installation. Use only O-rings that are the correct size and approved for use with R-134a refrigerant. Failure to do so may result in a leak.
- Unified plumbing connections with gaskets cannot be serviced with O-rings. The gaskets are not reusable and new gaskets do not require lubrication before installing.

Using the proper tools when making a refrigerant plumbing connection is very important. Improper tools or improper use of the tools can damage the refrigerant fittings.

The refrigerant must be recovered completely from the system before opening any fitting or connection. Open the fittings with caution, even after the refrigerant has been recovered. If any pressure is noticed as a fitting is loosened, tighten the fitting and recover the refrigerant from the system again.

Do not discharge refrigerant into the atmosphere. Use an R-134a refrigerant recovery/recycling device that meets SAE Standard J2210.

The refrigerant system will remain chemically stable as long as pure, moisture-free R-134a refrigerant and refrigerant oil is used. Dirt, moisture, or air can upset this chemical stability. Operational troubles or serious damage can occur if foreign material is present in the refrigerant system.

When it is necessary to open the refrigerant system, have everything needed to service the system ready. The refrigerant system should not be left open to the atmosphere any longer than necessary Cap or plug all lines and fittings as soon as they are opened to prevent the entrance of dirt and moisture. All lines and components in parts stock should be capped or sealed until they are to be installed.

All tools, including the refrigerant recycling equipment, the manifold gauge set, and test hoses should be kept clean and dry All tools and equipment must be designed for R-134a refrigerant.