Heating & A/C System

HEATING & A/C SYSTEM

CAUTION: Do not remove the A/C-Heater Control Module from one vehicle and install it in another vehicle. The module's software configures differently for JTEC and NGC, and for vehicles with and without a CMTC. Failure to follow these instructions can either cause CMTC display problems or an inaccurate temperature to display. It can also cause an ambient temperature sensor open DTC to set when support should not be operative or it can completely prevent the A/C-Heater Control Module from reporting ambient temperature sensor circuit DTCs.

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 DRBIII.
- 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, Medium, & 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 and 110 °C (150 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 2003 DN HVAC system remains mostly carryover from 2002. Revisions to the 2003 HVAC system include:
- a change in A/C mode switch status indicator operation when performing the A/C Cooldown test. Refer to The A/C Cooldown Test under System Diagnostics for more information.
- a change in EBL mode switch status indicator operation when performing the HVAC Door Recalibration. Refer to The HVAC Door Recalibration Function under System Diagnostics for more information.
- updated diagnostic procedures using span data to diagnose HVAC Door Recalibration fault messages.

SYSTEM DIAGNOSTICS
Fault detection is through active and stored Diagnostic Trouble Codes (DTCs).
- DTCs are displayed by the DRBIII.
- 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 DRBIII.
- checks A/C system performance based on evaporator temperature sensor input.
- will fail if evaporator temperature is below 18.3 °C (65 °F) when initiating the test.
- will pass if the evaporator temperature drops 6.7 °C (20 °F).
- faults display on the DRBIII as test messages only after running the test.
- faults will not display on the DRBIII 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 either the test returns passed or the ignition key is cycled. 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 DRBIII,
- After completing HVAC Door Recalibration, the DRBIII will display the current total span of each door actuator. It is important to note this information before proceeding with diagnosing door calibration fault messages.

- homes and repositions door actuators.
- monitors for door span faults.
- faults display on the DRBIII as test messages only after running the test.
- faults will not display on the DRBIII 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 either the test returns passed or the ignition key is cycled. 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 DRBIII.
- monitors for shorted 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 DRBIII as test messages only after running the test.
- faults will not display on the DRBIII as Diagnostic Trouble Codes.

When Performing The Actuator Circuit Test:

CAUTION
- Shorted rear 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 DRBIII 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.

- When 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.