Part 3

Auxiliary Heating and A/C Operation

There are 2 separate controls for the auxiliary HVAC system. There is the front auxiliary blower motor switch and the auxiliary HVAC control module. If the front auxiliary blower motor switch is in any other position than OFF or REAR, then the auxiliary air temperature actuator mimics the set passenger temperature. The auxiliary mode will mimic the primary mode. If the front auxiliary blower motor switch is in the REAR position, then the system will only function with inputs to the auxiliary HVAC control module. If the front auxiliary blower motor switch is in the OFF position, then the auxiliary HVAC control module does not respond to input. The auxiliary HVAC control module can not request A/C operation from the powertrain control module (PCM).

Pressing the UP air temperature switch to the warmest position diverts most of the airflow through the heater core, which increases the outlet air temperature. Pressing the DOWN air temperature switch to the coolest position diverts most of the airflow around the heater core, which decreases the outlet air temperature.

The auxiliary air temperature actuator shares a control circuit with the auxiliary mode actuator. If change of position is required for both actuators, then the module positions the auxiliary air temperature actuator first. All control circuits for the auxiliary actuators are at a low voltage potential until a change of position is required. The module then applies a high voltage potential to the appropriate control circuit, which will rotate the actuator.

The coolant bypass valve controls coolant flow to the auxiliary heater core. If a cool air temperature is selected, the auxiliary HVAC control module sends a class 2 message to the HVAC control module to close the valve. When the HVAC control module applies 12 volts to the coolant bypass solenoid control circuit, the solenoid opens. When the solenoid is open, a diaphragm closes the water valve when vacuum is applied. This action restricts coolant flow to the auxiliary heater core. The coolant bypass valve is a normally open valve.

Automatic Operation

In automatic operation, the HVAC control module will maintain 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.

To place the HVAC system in Automatic mode, the following is required:

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

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

* Monitor the following sensors:

- Inside air temperature sensor
- Ambient air temperature sensor
- Lower left air temperature sensor
- Lower right air temperature sensor
- Upper left air temperature sensor
- Upper right air temperature sensor

* Regulate blower motor speed
* Position the air temperature actuator
* Position the mode actuator
* Position the recirculation actuator
* Request A/C operation

Steering Wheel Controls

The steering wheel controls for the HVAC system include air temperature and blower motor speed adjustments. Pressing the up arrow on the air temperature switch increases the outlet air temperature. Pressing the down arrow on the air temperature switch decreases the outlet air temperature. The body control module (BCM) receives the input from the steering wheel controls. Pressing one of the steering wheel switches enables an in line resistor to drop voltage on the remote radio control signal circuit. The BCM then interprets this voltage signal and sends a class 2 message to the HVAC control module for the desired change.

Engine Coolant

Engine coolant is the essential element of the heating system. The thermostat controls the normal engine operating coolant temperature. The thermostat also creates a restriction for the cooling system that promotes a positive coolant flow and helps prevent cavitation.

Coolant enters the heater core through the inlet heater hose, in a pressurized state. The heater core is located inside the HVAC module. The ambient air drawn through the HVAC module absorbs the heat of the coolant flowing through the heater core. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. Opening or closing the air temperature door controls the amount of heat delivered to the passenger compartment. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.