AUTOMATIC A/C-HEATER SYSTEM -COMPONENT DISCRIPTION and OPERATION
 

A/C COMPRESSOR CLUTCH CONTROLS

A/C Compressor Control Relay
The A/C compressor Control relay controls power to the A/C compressor clutch.
The PCM controls the A/C compressor relay. A/C-Heater (HVAC) Programmer
receives signals from A/C-heater control, solar sensor, ambient temperature sensor,
in-car temperature sensor, blower motor control module, PCM, and air mix valve actuators.
Programmer contains a microprocessor that processes signals and then controls
A/C system components. System components control A/C compressor clutch
operation, intake air location, discharge air location and temperature, and blower speed.

There are 4 solenoid actuated vacuum valves, located inside the
A/C-heater programmer, that control intake air and discharge locations.
When different combinations of vacuum valves are actuated,
A/C-heater programmer can control whether air comes out the floor outlets,
panel outlets or defroster outlets, and whether outside air is drawn into
system or inside air is re-circulated.

Programmer also performs self-diagnosis functions.
Microprocessor identifies Diagnostic Trouble Codes (DTCs) when system
is operating and stores DTCs in memory.

A/C Pressure Transducer
The A/C pressure transducer is mounted on compressor discharge line
and is used to monitor A/C system pressure. Pressure transducer sends
a variable voltage signal to Powertrain Control Module (PCM).
Higher A/C system pressures result in higher voltage outputs and lower A/C system
pressures result in lower voltage output. PCM uses voltage inputs to control
compressor clutch operation and cooling fan low and high speed operations.
PCM will disengage compressor clutch when A/C system pressure is greater
than 430 psi (30.2 kg/cm_ ) or less than 35 psi (2.5 kg/cm)

Powertrain Control Module (PCM)
The PCM controls power to the A/C compressor clutch through A/C
compressor relay. When PCM receives an A/C request signal from the
A/C-heater programmer, the PCM grounds the coil circuit of the A/C
compressor control relay. This energizes the A/C compressor control
relay (the relay contacts close), allowing power to the A/C compressor
clutch. Under various engine operating conditions (such as wide open
throttle), the PCM prevents compressor clutch engagement.

A/C-HEATER CONTROL PANEL

English/Metric Display Button
Pressing ENGLISH/METRIC display switch will change preset or outside
temperature displayed value from English-to-Metric or Metricto-
English.

External Temperature Display Button
With A/C system operating and preset temperature displayed, pressing external
temperature display switch changes display to outside temperature.
Pressing switch again will change display back to preset temperature.
With A/C system off, pressing external temperature display switch will turn
external temperature display off. Pressing switch again will turn external temperature display on.

Fan Speed Increase/Decrease Rocker
Button Pressing either end of fan speed rocker button once, will
increase or decrease fan speed one level. Pressing and holding fan
speed rocker button will increase or decrease fan speed until maximum
or minimum speed is reached. If A/C-heater system is in AUTO mode,
pressing fan speed rocker button will stop automatic control and begin
manual fan speed control. Fan speed will be constant until fan speed
rocker button is pressed again. With system OFF, pressing either end
of fan speed rocker button will activate AUTO mode with automatic fan
speed control.

Information Display Panel
With A/C system off, outside temperature is displayed continuously.
When any button, except OFF, is selected, SET TEMP is
displayed for 5 seconds. Arrows appearing in center of display are
used to indicate air discharge location relative to seat. Word AUTO
will illuminate at right of display panel when system is operating in
AUTO mode.

With system off, word OFF and OUTSIDE will illuminate and outside temperature will display.
System will turn on when any button, except A/C, recirculation or rear defroster is pressed.
Fan speed is displayed while fan speed is in manual mode. Number of bars
illuminated indicates fan speed. When system is in AUTO mode, with AUTO fan speed control,
no bar segments are illuminated.

Mode Selector Buttons
Pressing AUTO button tells A/C-heater programmer to select appropriate
air intake and discharge points and activate or deactivate A/C compressor.
System temperature is determined by operator selected
preset temperature and system sensors.

Temperature Increase/Decrease Rocker Button
With system in any mode except OFF and preset temperature 66-
84 _ F (19-29 _ C), pressing increase/decrease end of rocker button once
will change preset temperature one degree. If decrease end of button
is pressed with preset temperature at 65 _ F (18 _ C), preset temperature
will change to 60 _ F (16 _ C). If increase end of button is pressed with
preset temperature at 85 _ F (29 _ C), preset temperature will change to
90 _ F (32 _ C).
An increase or decrease in preset temperature signals A/C heater
programmer to change blower speed, air outlet location, and
outlet temperature. Programmer will continue changes until sensors
indicate preset temperature has been reached. With system OFF,
pressing either end of temperature rocker button will activate AUTO
mode.

AIRFLOW CONTROL

Outside/Re-circulated Air Door
This door controls whether fresh (outside) or recirculated
(inside) air enters the system. A vacuum actuator controls the
position of the door. Vacuum actuator is located to the right of the
accelerator pedal, on the heater case. A solenoid in the A/C-heater
programmer controls vacuum supply to the actuator.

Air Mix (Temperature) Door
This door controls the volume of air flowing through the heater core.
An electric actuator (part of the A/C-heater programmer)
controls the position of the door. The electric actuator and the A/C
heater programmer can be serviced separately. The A/C-heater
programmer also monitors the position of the door by applying a
reference voltage to a position sensor (variable resistor) in the
actuator motor.

Mode Door
The position of this door determines how much air will be
diverted to the floor outlets. A vacuum actuator controls the position
of the door. The vacuum actuator is located to the right of the
accelerator pedal, on the heater case. A solenoid in the A/C-heater
programmer controls vacuum supply to the actuator.

Defrost-A/C Door
This door directs air to the defroster vents or the dash
panel vents, depending on its position. A vacuum actuator controls the
position of the door. The vacuum actuator is located to the right of
the accelerator pedal, on the heater case. A solenoid in the A/C heater
programmer controls vacuum supply to the actuator.

BLOWER CONTROL MODULE
Blower control module is a solid state device. It controls
blower motor by varying voltage to blower motor between 0 and 12 volts
(off-maximum speed). To determine appropriate output voltage, blower
control module monitors a varying voltage signal (between 2-8 volts)
from A/C-heater programmer. The blower control module then amplifies
A/C-heater programmer signal and apples voltage directly to blower
motor. A/C-heater programmer monitors signals from blower control
module.

BLOWER MOTOR
In manual blower mode, 6 blower motor speeds are available.
In automatic mode, blower speed is infinitely variable between minimum
and maximum speeds based on sensor signals to A/C-heater programmer.
Sensors that A/C-heater programmer uses for blower speed control are
in-car and ambient temperature sensors, solar sensor, coolant sensor
and vehicle speed sensor.

ORIFICE (EXPANSION) TUBE
The orifice tube controls liquid refrigerant flow into
evaporator. The orifice tube has filter screens on both inlet and
outlet sides. The tube has no moving parts and is not adjustable.
Replace orifice tube if plugged or defective.
When engine is turned off with A/C system on, refrigerant in
system will flow from high pressure side to low pressure side across
orifice tube to stabilize system. During this stabilization period, a
faint hissing sound will normally be heard for 30-60 seconds.

PRESSURE RELIEF VALVE
A pressure relief valve on the rear of the compressor
prevents the system from being damaged due to excessive pressure in
the high side of the system. If pressure increases to about 461-599
psi (32-42 kg/cm _ ), the valve opens, venting refrigerant to the
atmosphere.

SENSORS

Ambient Temperature Sensor
The A/C-heater programmer monitors the temperature of the air
outside of the vehicle by applying a reference voltage to the
thermistor ambient temperature sensor.

To help eliminate false readings of temperature variations,
during city traffic and idle conditions, programming techniques are
used. A temperature memory feature is used in ambient temperature
programming to help provide greater accuracy during engine restart. If
engine coolant temperature is 18 _ F (8 _ C) less than ambient temperature
sensor reading, actual ambient temperature is displayed. If engine
coolant is 18 _ F (8 _ C) greater than ambient sensor reading, memorized
outside temperature is displayed. This temperature was memorized
during vehicles last operation time. If vehicle speed is maintained at
more than 20 MPH for more that 80 seconds, ambient temperature display
may be allowed to increase. If ambient temperature sensor reading is
less than displayed value, or vehicle speed is more than 45 MPH,
real-time ambient temperature will be displayed.

Ambient temperature sensor display can be updated by pressing
AUTO, A/C and RECIRC buttons simultaneously. Temperature sensor is
located in front of radiator.

In-Car Temperature Sensor
The A/C-heater programmer monitors the temperature of the air
inside of the vehicle by applying a reference voltage to the in-car
thermistor temperature sensor. A slight vacuum in the A/C-heater
plenum draws passenger compartment air through an aspirator hose to
the in-car temperature sensor.

Solar Sensor
The A/C-heater programmer monitors the intensity of sunlight
by applying a reference voltage to the photo-diode solar sensor.
Programmer compares inside air temperature to solar sensor signal to
determine how much cooling is required to maintain desired preset
temperature. Solar sensor operation will be affected if it is covered,
keeping sunlight from reaching solar sensor. Solar sensor is located
on drivers side of vehicle, under defroster grill.

SYSTEM PURGING

When A/C is turned on, A/C-heater programmer will purge stale
air out of the system. When system is in AUTO mode, the A/C-heater
programmer will determine if any undesirable conditions exist, and
will chose one of 3 different purge modes to eliminate any undesirable
conditions.

Normal Purge
A/C-heater programmer delays blower operations for 5 seconds.
A/C-heater programmer energizes heater solenoid to direct air toward
floor and runs blower at low speed for fifteen seconds. Normal purge
can be bypassed if fan is manually turned on, or defrost button is
depressed.

Cold Purge
A/C-heater programmer energizes A/C solenoid to direct air
toward windshield outlets and runs blower 60 percent of maximum speed
for 20 seconds. Cold purge can be by-passed if fan is manually turned
on, or defrost button is depressed.

A/C Purge
A/C-heater programmer energizes A/C solenoid to direct air
toward floor and runs blower 80 percent of maximum speed for 4
seconds. Cold purge can be by-passed if fan is manually turned on, or
defrost button is depressed.