Automatic Climate Control, Acc: Overview

Automatic HVAC Description and Operation

The air temperature and the air delivery description and operation are divided into eight areas

HVAC Control Components
Air Speed (Front)
Air Speed (Rear)
Air Delivery (Front)
Air Delivery (Rear)
Heating and A/C Operation
Recirculation Operation
Automatic Operation
Engine Coolant
A/C Cycle

Heating and A/C Operation

The purpose of the heating and A/C system is to provide heated and cooled air to the interior of the vehicle. The A/C system will also remove humidity from the interior and reduce windshield fogging. Regardless of the temperature setting, the following can affect the rate that the HVAC system can achieve the desired temperature

Recirculation actuator setting
Difference between inside and desired temperature
Blower motor speed setting
Mode setting

When the A/C switch or the AUTO switch is pressed, the Radio/HVAC control sends a signal to the HVAC control module via LIN-Bus. The HVAC control module evaluates this signal and sends an A/C request signal to the ECM via CAN-Bus. The ECM checks all preconditions before approving. If all conditions are met sends a release signal back to the HVAC control module. The A/C compressor is activated by the HVAC control module. The HVAC control module supplies battery voltage to the A/C compressor. When the A/C switch is pressed, the HVAC control module provides a pulse width modulation (PWM) signal to the A/C compressor in order to command the performance of the A/C compressor. The performance of the A/C compressor is regulated according to adjusted interior temperature on the basis of characteristic performance data. Therefore the HVAC control module grounds the A/C compressor with the PWM signal.

The following conditions must be met in order to activate the A/C compressor

Battery voltage is between 9-18 V
Engine coolant temperature is less than 124°C (255°F)
Engine speed is greater than 600 RPM
Engine speed is less than 5 500 RPM
A/C high side pressure is between 269-2 929 kPa (39-425 PSI)
Throttle position is less than 100%
Evaporator temperature is greater than 3°C (38°F)
ECM does not detect immoderate torque load
ECM does not detect insufficient idle quality
The ambient temperature is above 1°C (34°F)

The sensor information is used by the ECM to determine the following

The A/C high side pressure
An A/C system load on the engine
An immoderate A/C high side pressure
The heat load at the A/C condenser

The air streams into the passenger compartment through the heater core and the evaporator core. The air temperature actuator drives the mixed air flap to induce the airflow. If the interior temperature should be increased, the mixed air flap is put into the position in which more air streams through the heater core. If the interior temperature should be decreased, the mixed air flap is put into the position in which more air streams through the evaporator core.

Recirculation Operation

The recirculation switch is integrated into the Radio/HVAC control. The selected recirculation switch position is sent to the HVAC control module via LIN-Bus. The HVAC control module controls the air intake through the air inlet actuator and recirculation actuator. In the recirculation mode the air inlet flap closes and the recirculation flap opens in order to circulate the air within the vehicle. In fresh air mode the air inlet flap opens and the recirculation flap is closed again in order to route outside air into the vehicle. Recirculation is only available if the defrost mode is not active. When the defrost mode is active, the recirculation actuator opens the recirculation flap and the air inlet actuator opens the air inlet flap and outside air is circulated to the windshield to reduce fogging.

In automatic mode the values of the windshield temperature and inside moisture sensor are used as control inputs for the HVAC control module to calculate the fog risk on the inside of the windshield. The A/C compressor and the defrost mode are activated to prevent or remove fog on the inside of the windshield.

Automatic Operation

In automatic operation, the HVAC control module maintains the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation actuator.

To put the HVAC system in automatic mode, the following is required

The auto switch must be activated.
The air temperature switches must be in any other position than full hot or full cold position.

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators automatically adjust to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature

Monitors the following sensors: Ambient air temperature sensor Lower left air temperature sensor Lower right air temperature sensor Lower rear air temperature sensor (only CJ4) Upper left air temperature sensor Upper right air temperature sensor Upper rear air temperature sensor (only CJ4) Windshield temperature and inside moisture sensor Ambient light/sunload sensor
Regulate the blower motor speed
Regulate the rear blower motor speed (only CJ4)
Position the air temperature actuators
Position the rear air temperature actuators (only CJ4)
Position the mode door actuator
Position the rear mode door actuator (only CJ4)
Position the recirculation actuator
Position the air inlet actuator
Request A/C operation
Control of the A/C compressor

When the warmest position is selected in automatic operation the blower speed will increase gradually until the vehicle reaches normal operating temperature. When normal operating temperature is reached the blower stays on high speed and the air temperature actuators stays in the full heat position.

When the coldest position is selected in automatic operation the blower stays on high and the air temperature actuators stay in full cold position. The mode actuator remains in the panel position and the recirculation actuator will remain in the recirculation position.

Under cold ambient temperatures, the automatic HVAC system provides heat in the most efficient manner. The operator can select an extreme temperature setting but the system will not warm the vehicle any faster. Under warm ambient temperatures, the automatic HVAC system also provides air conditioning in the most efficient manner. Selecting an extreme cool temperature will not cool the vehicle any faster.

In automatic mode the values of the windshield temperature and inside moisture sensor are used as control inputs for the HVAC control module application to calculate the fog risk on the inside of the windshield and ability to reduce fuel consumption by decreasing A/C compressor power to a minimum without causing any fog. The A/C compressor and the defrost mode are activated to prevent or remove fog on the inside of the windshield. The sensor will also enable partial recirculation mode in order to improve heat-up performance of the passenger compartment under cold ambient temperature conditions without the risk of mist build-up on the windshield.

Circuit/System Description

The ambient light/sunload sensor includes the sunload sensor and passenger compartment temperature sensor.

This sensor assembly provides information about

Sun heat intensity
Elevation
Azimuth
Passenger compartment temperature

The sunload sensor is connected to ground and to a 12 V clocked power supply through the HVAC control module. This clocked power supply is to power the sensor electronics and to work as a clock generator to the sunload sensor micro controller. The sensor uses a pulse signal for data identification and transferring the sun intensity measurement. At each positive transition from the clocked supply input, the sunload sensor micro controller will shift channels enabling new intensity measurement on the signal output to the HVAC control module. The signal voltage varies between 0-4 V.

The passenger compartment temperature sensor is a negative temperature co-efficient thermistor. A signal and low reference circuit enables the sensor to operate. As the air temperature increases, the sensor resistance decreases. The sensor signal varies between 0-5 V.

Bright or high intensity light causes the vehicles interior temperature to increase. The HVAC system compensates for the increased temperature by diverting additional cool air into the vehicle.