Part 2

Air Temperature Description and Operation

A/C Low Pressure Switch

The A/C low pressure switch protects the A/C system from a low pressure condition that could damage the A/C compressor or cause evaporator icing. The HVAC control module applies 5 volts to the A/C low pressure switch signal circuit. The switch will open when the A/C low side pressure reaches 137-165 kPa (20-24 psi). This prevents the A/C compressor from operating. The switch will then close when A/C low pressure side reaches 275-310 kPa (40-45 psi). This enables the A/C compressor to turn back ON.

The low pressure switch uses refrigerant pressure to open and close a set of electrical contacts. When A/C request is authorized, the switch is closed and shows normal status. During this state, the switch will show 0 volts on the A/C low pressure sensor signal circuit. When A/C request is denied due to a low pressure condition, the switch will be open. During this state, the switch will show 5 volts on the A/C low pressure sensor signal circuit.

Coolant Bypass Valve

The coolant bypass valve controls coolant flow to the auxiliary heater core. Integral to the coolant bypass valve is an electric solenoid that controls vacuum flow to open and close the valve. When the HVAC control module applies 12 volts to the integral solenoid, the solenoid applies vacuum to a diaphragm that closes the water valve. This action restricts coolant flow to the auxiliary heater core. The coolant bypass valve is a normally open valve. If there is a concern with control of the valve or with its vacuum source the valve will still be able to supply heated coolant to the auxiliary heater core.

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. The vehicle operator can determine the passenger compartment temperature by adjusting the air temperature switch. The vehicle passenger can offset the passenger temperature as much as 16.7°C (30°F). Regardless of the temperature setting, the following can effect the rate that the HVAC system can achieve the desired temperature:

* Recirculation actuator setting
* Difference between inside and desired temperature
* Difference between ambient and desired temperature
* Blower motor speed setting
* Mode setting
* Auxiliary HVAC settings

The control module makes the following actions when automatic operation is not selected, and an air temperature setting is selected:

* When the air temperature switch is placed in the warmest position, the control module commands the air temperature door to divert maximum air past the heater core.
* When the air temperature switch is placed in the coldest position, the control module commands the air temperature door to direct air to bypass the heater core.
* When the air temperature switch is placed between the warmest and coldest positions, the control module monitors the following sensor inputs to determine the air temperature door position that diverts the appropriate amount of air past the heater core in order to achieve the desired temperature:

- Sunload
- Duct temperatures
- Ambient temperature
- Inside temperature

The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a class 2 message to the powertrain control module (PCM) for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.

The following conditions must be met in order for the A/C compressor clutch to turn ON:

* The ambient air temperature is above 1°C (35°F).
* The A/C low pressure switch signal circuit is grounded.
* The A/C refrigerant pressure sensor parameter is less than 2 413 kPa (350 psi).
* The A/C compressor temperature switch contacts are closed.
* The PCM receives an A/C request from the HVAC control module.
* The engine coolant temperature (ECT) is less than 123°C (253°F).
* The engine RPM is more than 550 RPM.
* The throttle position is less than 100 percent.

The HVAC control module monitors the A/C low pressure switch signal circuit. If the voltage signal on this circuit has no voltage drop, the module will interpret this condition as a low pressure, disabling the A/C request. The A/C low pressure switch will open its internal contacts at 151 kPa (22 psi). Then close the contacts at 275 kPa (40 psi) to resume A/C operation. This switch assists in cycling the A/C compressor and prevents A/C compressor operation if system has a low refrigerant level.

The PCM monitors the A/C refrigerant pressure sensor signal circuit. The voltage signal on this circuit is proportional to the refrigerant pressure inside the A/C high side pressure line. As the pressure inside the line increases, so does the voltage signal. If the pressure is above 2 413 kPa (350 psi), the A/C compressor output is disabled. When the pressure lowers to 1 578 kPa (229 psi), the PCM enables the compressor to operate.

The sensor information is used by the PCM to determine the following:

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

The A/C compressor has an A/C compressor temperature switch. This switch protects the compressor from over heating. The switch interrupts power to the compressor clutch coil. When the compressor core temperature rises above 135°C (275°F), the switch opens, disabling the compressor clutch coil. When the temperature lowers to 120°C (248°F), the switch closes, enabling the compressor clutch coil. This switch is not a serviceable part, it is integral to the A/C compressor.

Once engaged, the compressor clutch will be disengaged for the following conditions:

* The ambient air temperature is less than 1°C (35°F).
* The A/C compressor temperature switch contacts are open.
* The throttle position is 100 percent.
* The A/C low pressure switch is open.
* The A/C high side pressure is more than 2 413 kPa (350 psi).
* The A/C low side pressure is less than 151 kPa (22 psi).
* The engine coolant temperature (ECT) is more than 123°C (253°F).
* The engine speed is more than 5,500 RPM.
* Transmission shift
* The PCM detects excessive torque load.
* The PCM detects insufficient idle quality.
* The PCM detects a hard launch condition.

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