The heating, ventilation and air conditioning (HVAC) system
provides a comfortable environment inside the vehicle passenger compartment,
regardless of the weather conditions outside of the vehicle. The HVAC system
consists of the following major functional systems:
| • | The air distribution system |
| • | The temperature control system |
| • | The refrigeration system |
Air Control
The following 4 actuators control the air flow into and out of
the HVAC module:
| • | The driver air mix actuator |
| • | The passenger air mix actuator |
The right and left sides of the heater and A/C module have specifically
designed actuators. When servicing an actuator, replace the actuator with
the correct actuator for the serviced position.
A reversible DC motor in the actuator operates the valve within the
HVAC module. The actuators contain a feedback potentiometer. The feedback
potentiometer informs the heater and A/C programmer of the door position.
Compressor Control
The engine belt-drives the compressor for the air conditioning system
through the A/C compressor clutch. The clutch allows the compressor to cycle
on and off. During most normal operation, the compressor will be in the
ON position. The compressor will disengage under the following conditions:
| • | When the air conditioning is OFF |
| • | When the engine load must be reduced |
When the automatic system requests the system FRONT DEFROST or A/C,
the heater and A/C programmer sends a request message for the compressor.
The request message travels over class 2 serial data communications
to the powertrain control module (PCM).
Under normal operating conditions, the PCM supplies a ground to the
A/C compressor control relay coil, and the relay energizes. The A/C CLU fuse
and the A/C compressor control relay contacts supply voltage to the A/C
compressor clutch. The A/C compressor clutch engages, and the compressor
runs.
If the PCM determines that the engine load needs reducing, such as during
full throttle conditions, the PCM will de-energize the A/C compressor control
relay. De-energizing the relay will disengage the compressor, even though
the voltage signal from the heater and A/C programmer is still present.
The A/C compressor clutch diode is connected across the A/C compressor
clutch terminals. The A/C compressor clutch diode provides a path for the
current that results from the voltage spikes. The collapsing magnetic field
of the A/C compressor clutch coil generates the voltage spikes. Voltage spikes
occur every time the coil energizes.
