With Automatic A/C

Automatic Heater A/C Contol:

Heater A/C Control (Removal):

The automatic heater A/C control, uses short-travel, soft-touch buttons for selecting the mode temperature and blower speeds manually, if desired. In the automatic mode, the desired temperature is set by the driver and a microprocessor utilizes this information along with the two air temperature sensor inputs to provide the desired comfort level. The driver can override the auto blower speed and mode selection, if desired. For example, if a different mode is selected, the microprocessor will continue to moderate the blower speed and temperature.

The microprocessor also provides for system self-tests which result in diagnostic codes being displayed. Faults in the inside and outside temperature sensor circuits and temperature valve control circuit are identified when the control assembly is set to the diagnostic mode.

Basic control operation is shown in the first illustration.

BLOWER MOTOR POWER MODULE

The blower motor power module contains solid-state circuitry which utilizes a large heat sink positioned in the evaporator air flow. The module receives commands from the heater A/C control, amplifies them, and in turn varies the speed of the blower motor. Command signals are based on the blower and mode selections and the user-set temperature, and the difference between in-vehicle and outside air temperatures. If the in-vehicle temperature is much hotter or colder than the user-set temperature, and the "AUTO" mode has been selected, the control module enables the blower motor to run at high speed, and as the set temperature is approached, cuts the voltage to the motor such that the speed decreases.

TEMPERATURE SENSORS

Automatic control of in-vehicle temperatures depends on the functioning of an in-vehicle sensor and an outside air sensor. Both sensors are temperature sensitive thermistors whose resistance varies inversely to the temperature. The resistance value determines the signal level which is sent to the heater A/C control. In turn, the heater A/C control microprocessor utilizes this information to develop command signals for the vacuum solenoid module, temperature motor actuator and blower motor power module.

A hose duct connects the in-vehicle sensor housing to an aspirator, and air flow from the HVAC module creates a slight vacuum at the aspirator end of the hose. This vacuum draws in-vehicle air over the in vehicle sensor and improves the accuracy of the sensor's ability to predict cabin temperature.

A/C VACUUM SOLENOID MODULE

The vacuum solenoid module converts electrical signals from the heater A/C control microprocessor into vacuum commands which position the mode, A/C-defrost, and outside air valves in the air distributor. Vacuum is constantly applied to the module which distributes the vacuum to four solenoid-controlled valves in the module. When the heater A/C control signals the module to change an air distributor valve position, the associated solenoid energizes and vacuum is applied to the proper actuator.

TEMPERATURE VALVE ELECTRIC ACTUATORS

The system also utilizes two bi-directional DC mode valve actuators. The actuators operate on signals from the A/C control that are inputs from the in-vehicle, outside air temperature, solar sensor, and the set temperature. As the actuators turn they move the temperature valves in the HVAC module to obtain the correct blend of heated air from the heater and cooled air from the A/C evaporator.

COMPRESSOR CONTROLS

Compressor control relays and switches are similar to those in the manual HVAC system described in Heating and Air Conditioning (With A/C).