System Description

The all new heating and air conditioning system for AN and AN84 provides more heating and cooling capacity than in previous years.

A simplified control system for operation of the mode, recirculation, and temperature control actuators provides positive positioning without the complexity of feedback from the position sensors. Knowing the number of operating actuator revolutions required to provide full door travel, and the number of actuator commutator pulses per revolution, the control head 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.

The new system also employs a new resistor technology to control blower speed. Called the "credit card" resistor, the device looks much like its name in that it is thin and compact like a credit card. The credit card resistor, like the coil resistor used in previous years, mounts inside the HVAC housing assembly in the airflow path. However, unlike the coil resistor, the credit card resistor offers much less airflow restriction.

The HVAC control monitors the temperature of the evaporator to keep it just above freezing for maximum cooling and dehumidification. A thermistor (temperature variable resistor) placed between the evaporator fins provides the signal for the control. The powertrain control module cycles the compressor ON and OFF with a signal from the HVAC control.

The HVAC control module is fully addressable with the DRB III. Two of control module's diagnostic capabilities, which the DRB III will actuate, include the A/C Cooldown Test, to test A/C system performance, and the HVAC Door Recalibration Test, to determine actuator travel span. After performing either test, the control module will display one or more messages on the DRB III screen. The message may indicate that the HVAC system passed the test, or that there is a fault currently active in the HVAC system. The DRB III can also extract active and stored Diagnostic Trouble Codes from the control module. Active DTCs are faults that currently exist in the HVAC system. Stored DTCs are faults that occurred in the HVAC system since the control module received the last "clear diagnostic info" message.