Intermittent
Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to TESTING FOR INTERMITTENT & POOR CONNECTIONS .
Recalibrating Actuators
The HVAC control module will relearn the actuator limits whenever power is disconnected and re-connected to the module.
Use the following steps to perform the calibration update
- Turn OFF the ignition.
- Remove the battery positive voltage circuit fuse of the HVAC Control Module. Important: The scan tool must be disconnected from the vehicle to properly perform the calibration procedure. If a scan tool is connected before this procedure is completed, then new calibration values will not be stored. Important: Failure to wait 60 seconds will not allow the HVAC control module sufficient time to clear the old calibration values.
- Wait 60 seconds.
- Install the fuse.
- Turn ON the ignition.
- Wait 60 seconds before installing a scan tool.
Scheme 205
- Remove the instrument panel accessory trim plate.
- Remove the screws from the HVAC control module.
- Depress the control assembly retaining tabs and remove the HVAC control module from the instrument panel.
- Disconnect the electrical connectors from the HVAC control module.
Air Speed-Front Control
The blower motor forces air to circulate within the vehicle's interior. The vehicle operator determines the blower motor's speed by placing the blower motor switch in a desired speed position or by selecting automatic operation. The blower motor will only operate if the blower motor switch is in any position other than OFF, and the ignition switch is in the RUN position. The blower motor and mode switches are located on the front of the HVAC control module.
Power is provided to the blower motor from the underhood junction block through the battery positive voltage circuit. The HVAC control module receives power from the ignition 3 voltage and battery positive voltage circuits. Ground is provided by the right instrument panel junction block through the ground circuits.
When any blower speed is selected, the HVAC control module sends a pulse width modulated (PWM) signal to the blower motor on the blower speed control circuit. In manual operation, once a blower speed is selected, the blower speed remains constant until a new speed is selected. In automatic operation, the HVAC control module will determine what blower speed is necessary in order to achieve or maintain a desired temperature.
When any blower speed is selected on either of the auxiliary control modules send a pulse width modulated (PWM) signal to the auxiliary blower motor speed control circuit. In manual operation, once a blower speed is selected, the auxiliary blower speed remains constant until a new speed is selected. In automatic operation, the auxiliary HVAC module will determine what auxiliary blower speed is necessary in order to maintain desired temperatures. At start-up in colder temperatures, the auxiliary blower motor will not begin operation at the same time as the primary blower motor. Warm coolant is circulated to the auxiliary heater core before the blower motor begins operating.
Air Distribution - Front Control
When a mode switch position is selected, a signal is sent from the HVAC control module to the mode actuator through the mode door control circuit. The mode actuator moves the mode door to the desired position.
When the mode door moves to a desired position, a variable resistor within the actuator is used to create the mode door position signal. The HVAC module uses the mode door position signal to determine the actual mode door position. The left instrument panel fuse block provides power to the mode actuator through the ignition 3 voltage circuit and the IP wiring harness junction block. Power and ground are provided to the HVAC control module by the fuse block through the ignition 3 voltage circuit and the ground circuits through the right instrument panel junction block. A 5 volt reference signal is sent to the actuator through the 5 volt reference circuit, through the right instrument panel junction block, to the mode actuator.
Recirculation Mode
When air recirculation is selected, a signal is sent from the HVAC control module to the recirculation actuator through the recirculation door control circuit. The recirculation actuator closes the recirculation door in order to circulate the air within the vehicle. The outside air switch opens the recirculation door in order to route outside air into the vehicle.
Regardless of the blower motor switch position, recirculation is available only in the vent and bi-level mode switch positions. Including the OFF position. The mode switch must be placed in either the vent or bi-level position before the blower motor switch is placed in the OFF position. In order to reduce windshield fogging, outside air is circulated when the mode switch is in the defrost, floor, and mix-blend positions. If the recirculation switch is pressed into the ON position when the mode switch is in an unavailable mode position, then the recirculation switch LED will flash 3 times.
When the recirculation door moves to a desired position, a variable resistor within the actuator is used to create the recirculation door position signal. The HVAC module uses the recirculation door position signal to determine the actual recirculation door position. The left instrument panel fuse block provides power to the recirculation actuator through the ignition 3 voltage circuit and the IP wiring harness junction block. Power and ground are provided to the HVAC control module by the fuse block through the ignition 3 voltage circuit and the ground circuits through the right instrument panel junction block. A 5 volt reference signal is sent to the actuator through the 5 volt reference circuit, through the right instrument panel junction block, to the recirculation actuator.
When the automatic setting has been selected, the HVAC control module will recirculate air whenever system temperature performance is insufficient to provide the desired temperature. The recirculation switch LED will not be illuminated. If either the outside air or recirculation switch is pressed during automatic operation, then automatic operation will be over ridden. The appropriate recirculation or outside air switch LED will be illuminated. The recirculation door will be opened or closed depending upon the selected override. When the ambient air temperature is below 4°C (40°F) the recirculation override of the automatic setting will only be available for 10 minutes.
When the automatic setting has not been selected, only the outside air or recirculation switch will be used to determine the recirculation door position. When the ambient air temperature is below 4°C (40°F) any recirculation selection will only be available for 10 minutes.
The A/C high pressure recirculation switch can cause the HVAC system to recirculate air. When the high side pressure reaches 2206-2620 kPa (320-380 psi), the PCM will place the HVAC system in recirculation mode. The high side pressure is lowered when the inside air cools the refrigerant within the A/C evaporator. When the high side pressure reaches 1447-1861 kPa (210-270 psi), the PCM will place the HVAC system out of recirculation mode.
Automatic Mode
The automatic HVAC system will maintain the interior temperature of the vehicle by controlling the blower motor, air temperature, mode and recirculation actuators to achieve the desired temperature. For full automatic operation, both the blower and mode switches must be in the AUTO position and the air temperature set between 19°C (66°F) and 28°C (82°F). Blower speeds will be gradually increased or decreased, and can be reduced based on vehicle speed in order to maintain airflow into the vehicle. At start up, the automatic HVAC system will use to the last settings used prior to vehicle shut down.
In cold temperatures, the automatic HVAC system will provide heat in the most efficient manner. At initial vehicle start up, the blower motor is not turned on until the coolant reaches a predetermined temperature. If the coolant does not warm up within a preset time, the blower will be activated. The HVAC system can quickly warm up the vehicle when the temperature and blower motor settings are set to maximum and outside air and floor modes are selected. Once the desired temperature is reached, the blower motor will slow down, discharge temperatures will be lowered, and the mode actuator will remain in the floor setting. The system can be placed in bi-level if the ambient temperature is near 7-10°C (45-50°F). Regardless of mode selection, some air will be diverted to the defrost setting to keep window fogging at a minimum.
If the ambient air temperature is less than 2°C (35°F) and the coolant temperature is less than 10°C (50°F), then a 75 second purge will occur. The purge is performed in order to reduce window fogging until the coolant warms up. The purge will begin as soon as the coolant temperature reaches 4°C (40°F). The mode door will move to the defrost position and the blower motor will be near half speed.
In warm temperatures, the automatic HVAC system will provide A/C in the most efficient manner. The HVAC system can quickly cool down the vehicle when the temperature is set to minimum, blower motor is set to maximum, and recirculation and vent modes are selected. Once the desired temperature is reached, the blower motor will slow down, discharge temperatures will be raised, and the mode actuator will be switched to bi-level for better passenger comfort. The recirculation actuator can be partially opened to assist in temperature control. Regardless of mode selection, some air will be diverted to the floor setting to keep occupant comfort at a maximum.
HVAC Control Module
The HVAC control module receives power from two separate sources. The underhood junction block provides keep alive memory (KAM) power through the battery positive circuit. The left instrument panel (I/P) fuse block provides a device on signal to the HVAC control module through the ignition 3 voltage circuit. The ignition 3 voltage circuit also powers the 5 volt regulator. The Class 2 serial data circuit provides a data circuit for scan tool communication and transmit and receive Class 2 messages. Personalization of HVAC operation is not available with this vehicle.
The HVAC control module processes information provided from various air temperature sensors, actuators and driver inputs to ensure that accurate HVAC operation is provided. The HVAC system can work in manual mode and automatic mode. The vehicle operator selects which mode is selected through the HVAC control module switch inputs.
When in automatic mode, blower speed, mode position and air temperature settings are calculated based on the air temperature setting. When in manual mode the blower speed and air delivery mode can be changed. The HVAC control module still tries to maintain the air temperature unless it is set to full cold or full hot temperatures. When at these full extremes the HVAC control module will position the air temperature actuator to its full cold or hot position depending on the selected air temperature.
Air Temperature Actuator
The air temperature actuator opens the air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. The air temperature actuator is a 5 wire actuator that incorporates a electric motor with feed back capability. Power is provided by ignition 3 voltage circuit. Ground is provided by the ground circuit through the HVAC control module. The air temperature actuator has a potentiometer integral to it. A 5-volt reference signal is sent out over the 5-volt reference circuit to the air temperature actuator. A feed back signal is provided by the air temperature door position signal circuit. As the actuator moves the voltage on the door position signal circuit changes. The HVAC control module monitors this signal to calculate the actual door position.
The control of the air temperature actuator is provided by the air temperature door control circuit. when a request of actuator position change from the HVAC control module, the air temperature door control circuit voltage is varied. A 2.5 volt signal from the HVAC control module keeps the actuator stationary. A 0 volt or 5 volt signal from the HVAC control module allows the actuator to rotate to a position determined by the HVAC control module.
The HVAC control module will check the range of the actuator when a calibration of the actuators is performed or it loses its keep alive memory (KAM). The HVAC control module, when checking the range, will rotate the actuator to one extreme and then to the other to ensure the actuator is working within its full range.
Duct Air Temperature Sensors
The HVAC control module receives inputs for the air duct outlet temperature from the upper and lower air temperature sensors. The HVAC control module uses these inputs to position the air temperature actuator to achieve and maintain the set temperature on the HVAC control module.
The upper and lower air temperature sensors are Negative Temperature Coefficient (NTC) thermistors, when the temperature of the sensor changes so does the resistance across the thermistor. When the air temperature is warm, the sensor resistance and signal voltage is low. When the air temperature is cool, the sensor resistance and signal voltage is high. Inside the HVAC control module 5 volts is supplied to the air temperature signal circuit through a fixed resistance. The fixed resistance inside the HVAC control module makes the signal circuit a series circuit. As the resistance of the sensor changes, the amount of voltage it drops also changes since it is in series with the fixed resistance inside the HVAC control module. The HVAC control module monitors the voltage drop of the circuit which is needed to calculate the air temperature. The ground for the upper and lower air temperature sensor is provided by the low reference circuit.
The HVAC control module will use a default value for the upper and lower air temperature signal if there is a fault with the input. The HVAC control module will use this default to ensure HVAC operation is still performed. The scan tool value will be the actual reading of the signal circuit. This means if signal circuit is shorted to a ground then the scan tool will read 0 Counts. If the signal circuit is more than 5 volts than the scan tool will read 255 Counts.
Ambient Air Temperature Sensor
The HVAC control module receives an input of the ambient air temperature from the ambient air temperature sensor. The HVAC control module uses this input for determining heating and cooling requirements. The ambient air temperature sensor is mounted in the grill area of the vehicle. In this position, it is exposed to the airflow through the grill before is reaches the radiator.
The ambient air temperature sensor is a Negative Temperature Coefficient (NTC) thermistor, when the temperature of the sensor changes so does the resistance across the thermistor. When the air temperature is warm, the sensor resistance and signal voltage is low. When the air temperature is cool, the sensor resistance and signal voltage is high. Inside the HVAC control module 5 volts is supplied to the ambient air temperature signal circuit through a fixed resistance. The fixed resistance inside the HVAC control module makes the signal circuit a series circuit. As the resistance of the sensor changes, the amount of voltage it drops also changes since it is in series with the fixed resistance inside the HVAC control module. The HVAC control module monitors the voltage drop of the circuit which is needed to calculate the air temperature. The ground for the ambient air temperature sensor is provided by the low reference circuit.
The signal provided by this sensor is filtered. There are conditions which cause the sensor to produce a signal that is not proportional to the actual ambient air temperature. The HVAC control module will use a default value for the ambient air temperature signal if there is a fault with the input. This value will be displayed on the scan tool. The HVAC control module will use this default to ensure HVAC operation is still performed.
Sunload Sensor
The sunload sensor provides the HVAC module software with the amount of sun light entering the passenger compartment through the windshield. With this input the HVAC control module can adjust cooling requirements based on the amount of heat load that the sun is placing on the vehicle.
The sunload sensor is a photoconductive diode, meaning that it is sensitive to light. When the sensor is in direct sunlight the signal voltage is low. When the sensor is in dark conditions the signal voltage is high. Inside the HVAC control module 5 volts is supplied to the sunload sensor signal circuit through a fixed resistance. The fixed resistance inside the HVAC control module makes the signal circuit a series circuit. As the resistance of the sensor changes, the amount of voltage it drops also changes since it is in series with the fixed resistance inside the HVAC control module. The HVAC control module monitors the voltage drop of the circuit which is needed to calculate the sunload. The ground for the sunload sensor is provided by the low reference circuit.
The HVAC control module will use a default value for the sunload sensor signal if there is a fault with the input. This value will be displayed on the scan tool. The HVAC control module will use this default to ensure HVAC operation is still performed. A resistance check of the sunload sensor should not be performed as it will damage the sensor.
A/C Pressure Sensors
The A/C system is protected by two pressure switches. The A/C high pressure switch interrupts the A/C request signal when the A/C line pressure exceeds 2896 kPa (420 psi). The A/C low pressure switch interrupts the A/C low pressure switch signal when the A/C line pressure falls below 145-172 kPa (21-25 psi). When the PCM sees an open in either signal, the A/C clutch relay control circuit is no longer grounded, thus shutting off the compressor. The low pressure switch will close when pressure reaches 262-290 kPa (38-42 psi).
The recirculation door will move automatically with an input from the A/C high pressure recirculation switch. The PCM will place the A/C system in recirculation mode when a signal is sent over the A/C refrigerant high pressure cut-out switch signal circuit. This allows for the cooler inside air to flow over the A/C evaporator and cool the refrigerant temperature, until the high side pressure returns to normal. The PCM sends a Class 2 message to command full recirculation at 2896 kPa (420 psi).
Engine Coolant
Engine coolant is the key element of the heating system. The normal engine operating coolant temperature is controlled by the thermostat. 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 heat of the coolant flowing through the heater core is absorbed by the ambient air drawn through the HVAC module. Heated air is distributed to the passenger compartment, through the HVAC module, for passenger comfort. The amount of heat delivered to the passenger compartment is controlled by opening or closing the air temperature door. The coolant exits the heater core through the return heater hose and recirculated back through the engine cooling system.
A/C Cycle
Refrigerant is the key element in an air conditioning system. R-134a is presently the only EPA approved refrigerant for automotive use. R-134a is an very low temperature gas that can transfer the undesirable heat and moisture from the passenger compartment to the outside air.
The A/C compressor is belt driven and operates when the magnetic clutch is engaged. The compressor builds pressure on the vapor refrigerant. Compressing the refrigerant also adds heat to the refrigerant. The refrigerant is discharged from the compressor, through the discharge hose, and forced to flow to the condenser and then through the balance of the A/C system. The A/C system is mechanically protected with the use of a high pressure relief valve. If the high pressure switch were to fail or if the refrigerant system becomes restricted and refrigerant pressure continued to rise, the high pressure relief will pop open and release refrigerant from the system.
Compressed refrigerant enters the condenser in a high temperature, high pressure vapor state. As the refrigerant flows through the condenser, the heat of the refrigerant is transferred to the ambient air passing through the condenser. Cooling the refrigerant causes the refrigerant to condense and change from a vapor to a liquid state.
The condenser is located in front of the radiator for maximum heat transfer. The condenser is made of aluminum tubing and aluminum cooling fins, which allows rapid heat transfer for the refrigerant. The semi-cooled liquid refrigerant exists the condenser and flows through the liquid line, to the orifice tube.
The orifice tube is located in the liquid line between the condenser and the evaporator. The orifice tube is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the orifice tube, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to vaporize at the orifice tube. The orifice tube also meters the amount of liquid refrigerant that can flow into the evaporator.
Refrigerant exiting the orifice tube flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the HVAC module and passes through the evaporator core. Warm and moist air will cause the liquid refrigerant boil inside of the evaporator core. The boiling refrigerant absorbs the moisture and heat from the ambient air. The refrigerant exits the evaporator through the suction line and back to the compressor, in a vapor state, and completing the A/C cycle of heat removal. At the compressor, the refrigerant is compressed again and the cycle of heat removal is repeated.
The conditioned air is distributed through the HVAC module for passenger comfort. The heat and moisture removed from the passenger compartment will also change form, or condense, and is discharged from the HVAC module as water.
