General Description

The Heating, Ventilation and Air Conditioning (HVAC) system is designed to provide a comfortable environment inside the vehicle passenger compartment, regardless of weather conditions outside the vehicle. The HVAC system is most easily understood if you break it up into its major functional systems:

1. Air Distribution System.
2. Temperature Control system.
3. Refrigeration System.
4. Heater System.

NOTICE: The refrigeration system on this vehicle uses Refrigerant-134a (R-134a) which is not compatible with refrigerant R-12. Before servicing the system always make sure the proper servicing equipment is used or the system could become severely damaged. Always refer to the service text and manufacturers instructions included with service equipment before proceeding.

Air Control

There are four actuators that are used to control the air flow in and out of the HVAC module. The actuators are as follows:

^ Driver air mix door actuator.
^ Passenger mix door actuator.
^ Mode door actuator.
^ Air inlet actuator.

The actuators are specifically designed for the right and left side of the heater and A/C module. When servicing an actuator, be sure to replace it with the correct replacement actuator for the position being serviced.

A reversible DC motor in the actuator operates the door within the HVAC module. The actuators contain a feedback potentiometer that informs the HVAC programmer of door position.

Compressor Control

The compressor for the air conditioning system is belt-driven by the engine through the A/C compressor clutch. The clutch allows the compressor to be cycled on and off. During normal operation the compressor will be on the majority of the time, but will disengage when the air conditioning is not requested or disengaged when engine load must be reduced.

The HVAC programmer sends a message requesting compressor over class 2 serial data communications to the Powertrain Control Module (PCM) when the system FRONT DEFROST or if the A/C is requested through the automatic system. The A/C pressure switch also sends a signal to the PCM which provides A/C pressure information to the PCM.

Under normal operating conditions, the PCM then grounds the A/C compressor control relay coil and the relay energizes. Voltage is applied to the A/C compressor clutch through relay center fuse 5 and the A/C compressor control relay contacts. The A/C compressor clutch engages and the compressor runs.

If the PCM determines that engine load should be reduced, such as during full throttle conditions, it will de-energize the A/C compressor control relay to disengage the compressor, even though the voltage signal from the HVAC programmer is still present.

The A/C compressor clutch diode, connected across the A/C compressor clutch terminals, provides a path for the current resulting from voltage spikes. The voltage spikes are generated by the collapsing magnetic field of the A/C compressor clutch coil, and occurs every time the coil is de-energized.