Heating and A/C System

AUTOMATIC TEMPERATURE CONTROL (ATC)

SYSTEM AVAILABILITY
- The Automatic Temperature Control (ATC) system is a Dual-Zone Air Conditioning System with a Rear Booster Blower.

SYSTEM CONTROLS
The ATC Module:
- Is fully addressable with the DRBIII(R).
- Communicates over the Programmable Communication Interface Multiplex System (PCI) Bus.
- Provides an A/C request over the PCI Bus to the Powertrain Control Module (PCM) when compressor operation is desired.
- Uses input from the evaporator temperature sensor to prevent evaporator freeze up while maintaining optimum cooling performance.
- Has dual-zone temperature-controls to provide a wide side-to-side variation in temperature to meet the needs of driver and the front seat passenger.
- Has dual infrared (I/R) sensors mounted in the ATC Module's face which independently measure surface temperature to maintain occupant comfort levels.
- Can be operated in a manual mode.
- Provides an EBL relay on request over the PCI Bus to the Integrated Power Module (IPM) when EBL operation is desired.
- Provides a Blower Relay on request over the PCI Bus to the Integrated Power Module (IPM).
- Controls front blower operating speed, providing 6 blower speeds in manual mode and infinite blower speeds in automatic mode.
- Provides a blower delay function for cold engine blower lock out.
- Controls rear booster fan operating speed in automatic mode, providing infinite fan speeds.
- Activates and deactivates the Rear Booster Fan Rear Control Switch.
- The separate Front Blower Power Module and Rear Booster Fan Power Module operate as follows: when blower operation is desired, the power module provides a 12.0 volt signal to the ATC Module over a control circuit. The ATC Module provides a variable duty cycle ground to the 12.0-volt signal based on input from the blower switch. When the blower speed is set to low, the ATC Module provides a short duty cycle (less time grounding the signal voltage). As higher blower speeds are requested, the ATC Module increases the duty cycle (more time grounding the signal voltage). When the highest blower speed is requested, the duty cycle increases to where the signal pattern is almost a flat line (with brief voltage spikes).
- Provides a power after run feature.
- The blower after run feature evacuates the HVAC evaporator of condensate to prevent odor. This feature operates the front blower motor at a minimal speed for no more than three minutes (luring a two-hour period after the ignition is turned off. The after run feature will only operate if the A/C system has been on for a minimal amount of time, and ambient air temperature and vehicle voltage are within a specific range.
- Controls the electric door actuators operation.
- A simplified control system for operation of the front mode, recirculation, and temperature control actuators provides positive positioning without the complexity of feedback from position sensors. The ATC 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 ATC Module runs the actuator for the nun)- her 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.
- When a mode change is requested through the MODE switch, the ATC drives the Rear Mode Door Actuator for 10 seconds. The Panel and the Floor/Mix/Defrost mode settings direct rear air flow to the rear panel or rear floor air outlets respectively. There 15 no Bi-Level position for the rear mode door. When the MODE switch setting is changed to Bi-Level the rear mode door remains in the last known position (panel or floor). Unlike the front door actuators, the Automatic Temperature Control will not set Diagnostic Trouble Codes for faults pertaining to the Rear Mode Door or its electrical circuits.
- Uses Ambient Temperature Sensor data received over the PCI Bus to display outside temperature. This signal is filtered to compensate for hot engine heating effects which can cause erroneous temperature display values.
The Rear Booster Fan Rear Control Switch:
- Provides desired rear booster fan speed input to the ATC Module. The Dual-Zone ATC system uses:
- Two two-wire electric blend door actuators.
- One, two-wire electric front mode door actuator.
- One, two-wire electric recirculation door actuator.
- One, two-wire electric rear mode door actuator.
System Relays
- The Integrated Power Mode (IPM) houses and provides power to the A/C Clutch Relay, EBL Relay, Front Blower Motor Relay, and Rear Booster Fan Relay.

Ambient Temperature Sensor
- The ambient air temperature is monitored by the NGC and displayed by the ATC. The NGC receives a hardwire input from the ambient temperature sensor (ATS). The ATS is a vanable resistor that operates on a 5-volt reference signal circuit hardwired from the NGC. The resistance in the ATS changes as the outside temperature rises or falls. The NGC senses the change in reference voltage through the ATS resistor. Based on the resistance of the ATS, the NGC is programmed to correspond to a specific temperature. The NGC stores and filters the ambient temperature data and transmits this data to the ATC via the PCI Bus. The ATS cannot he adjusted or repaired and, if fault or damaged, it must be replaced.

SYSTEM DIAGNOSTICS
Fault detection is through active and stored diagnostic Trouble Codes (DTCs)
- DTCs are displayed by the DRBIII(R)
- Active DTCs are those which currently exist ill the system. The condition causing the fault must he repaired in order to clear this type of DTC.
- Stored DTCs are those that occurred in the system since the ATC received the last "clear diagnostic info" message.

The AC Cooldown Test:
- Is actuated with the DRBIII(R)
- Checks A/C system performance based on evaporator temperature sensor input
- will not run:
- if ambient temperature is below 12° C (53 F)
- if A/C pressure is below 50 psi or above 300 psi
- If the evaporator temperature sensor/circuit is open.
- If the evaporator temperature sensor/circuit is shorted.
- Will pass if the evaporator temperature drops 6.7° C (20° F) within two minutes of starting this test.
- Will pass if the evaporator temperature drops either 6.7° C (20° F) within two minutes at ambient temperatures of 21° C (70° F) or above, or if the evaporator temperature drops 9.4° C (15° F) within two minutes at ambient temperatures below 21° C (70° F).
- Messages display on the DRBIII(R) after running this test.
- These messages will clear after paging back out of this test. Therefore, it is important to note all of the AC Cooldown test messages before doing so.
- Will cause the DELAY VF segment and Snow-flake LED on the ATC to flash. If the test fails the VF segment and LED will continue to flash until either the vehicle is driven more than three miles or a successful AC Cooldown Test is performed.

Actuator Calibration:
- Occurs after turning on the ignition after a battery reconnect.
- Occurs after ATC installation or replacement.
- Occurs after 20 ignition cycles.
- Can be initiated by the DRBIII(R) by selecting Body, Automatic Temperature Control, Miscellaneous, and Reset ATC Head.
- Can be initiated by pressing the PWR and RECIRC buttons simultaneously for five seconds.
- Manual motor calibration will cause the DELAY and RECIRC VF segments on the ATC to flash. If the test fails the VF segments will continue to flash until either the vehicle is driven more than three miles or a successful calibration occurs.
Ambient Temperature Sensor Faults:

NOTE:
- The Outside Air Temperature (OAT) display can be activated and de-activated by pressing the Outside Temp button on the ATC.
- Anytime the vehicle is turned oft for more than 2 hours, the OAT display will update instantly upon ignition on. However, if the vehicle is turned off for less than 2 hours and the outside air temperature increases, the OAT display may display a value that is colder than the actual outside air temperature. This is due to filtering which does not allow for instantaneous updates for warmer outside air temperatures. Also, if the vehicle is driven for an extended period of time below 20 mph the OAT display will not update for warmer outside air temperatures. If after the extended period of time driving below 20 mph the vehicle speed is increased to above 50 mph the OAT display may display an erroneous "too warm" value. This is due to extended idle speeds that create hot sensors due to engine heat.

The OAT function is supported by the Ambient Temperature Sensor (ATS), a signal circuit and a ground circuit which is hardwired to the NGC and OAT data that is bussed from the NGC to the ATC where the outside air temperature is displayed. If the ATS sense circuit is shorted to ground, the OAT display will display 130° F (54° C). If the ATS sense circuit is open, the OAT display will display -40° F (-40° C). When diagnosing OAT display related symptoms, always check the ATS and related circuits before testing the ATC. Ambient Temperature Sensor Diagnostic Trouble Codes (DTCs) are stored in the NGC. The ATS can be checked using the following Ambient Temperature Sensor Test.

Ambient Temperature Sensor Test
1. Turn the ignition OFF.
2. Disconnect the ATS harness connector.
3. Measure the resistance of the ATS using the following mm/max values:
- 32° F (0° C) Sensor Resistance 29.33-35.99 Kiloohms
- 50° F (10° C) Sensor Resistance 17.99 -21.81 Kilohms
- 68° F (20° C) Sensor Resistance = 11.37 -13.61 kiloohms
- 77° F (25° C) Sensor Resistance = 912 - 10.86 Kilohms
- 86° F (30° C) Sensor Resistance = 7.37 - 8.75 Kilohms
- 104° F (40° C) Sensor Resistance = 4.90 -5.75 Kiloohms
The sensor resistance should read between these min/max values. If the resistance values are not OK, replace the Sensor.