HVAC Systems - Automatic: Other Chevrolet Tahoe II

Sensor Resistance Table (Ambient and Inside Air Temp - ohms)

Sensor Resistance Table (Ambient and Inside Air Temp - ohms)

Sensor Resistance Table (Upper, Lower and Aux Air Temp - ohms)

Sensor Resistance Table (Upper, Lower and Aux Air Temp - ohms)

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Intermittent

Faulty electrical connections or wiring may be the cause of intermittent conditions. Refer to TESTING FOR INTERMITTENT AND POOR CONNECTIONS .

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.

Rear Auxiliary HVAC Control Module

The rear auxiliary HVAC control module processes and controls all aspects of the automatic auxiliary HVAC system. The module has communication with the HVAC control module via 2 keyboard data display (KDD) communication circuits. The system receives inputs from the auxiliary upper air temperature sensor, auxiliary lower air temperature sensor, infrared temperature sensor, and feed back signals from auxiliary mode actuator and the auxiliary air temperature actuator. Along with inputs from the front auxiliary HVAC control assembly. The outputs are the auxiliary air temperature actuator, auxiliary mode actuator, auxiliary blower motor control processor and data communication with the HVAC control module. The module is turned on by ignition voltage from the ignition 3 voltage circuit.

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.

Air Speed-Manual Auxiliary HVAC Control

The auxiliary blower motor circulates the air at the rear of the vehicle. The vehicle operator determines the auxiliary blower motor's speed by placing the blower motor switch in a desired speed position. The auxiliary blower motor switch controls the auxiliary blower motor speed. Three relay blower motor speed control circuits enable one of three relays to provide power to the blower motor. The blower motor switch grounds the selected relay. Each relay's control and load power is provided by the ignition and battery positive voltage circuits. Both the low and medium speed relays connect to the auxiliary blower motor through a resistor assembly. The resistor assembly creates a voltage divider circuit with the auxiliary blower motor to select the auxiliary blower motor speed. The high speed relay is connected directly to the auxiliary blower motor. The primary HVAC control module located in the dash does not control the auxiliary HVAC system.

Air Speed-Automatic Auxiliary HVAC Control

The auxiliary blower motor circulates the air at the rear of the vehicle. The auxiliary blower motor can be controlled by either of the auxiliary HVAC controls. The primary HVAC control module located in the dash does not control the auxiliary HVAC system. If the vehicle does not have a sunroof (CF5), then the front auxiliary controls must be set to the rear position for the rear auxiliary controls to operate. The vehicle operator determines the blower motor's speed by placing a blower motor switch in a desired speed position or by selecting automatic operation. The auxiliary blower motor will only operate when the ignition is in the RUN position and an auxiliary HVAC control is set in any position other than OFF.

Power is provided to the auxiliary blower motor from the underhood junction block through the battery positive voltage circuit and IP wiring harness junction block. The auxiliary HVAC control module receives power by the ignition 3 voltage circuit through the right instrument panel junction block through the ground circuits.

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.

Air Distribution-Manual Auxiliary HVAC Control

If the vehicle has a sunroof (CF5), then the auxiliary mode actuator can only be controlled by the front auxiliary HVAC control assembly. The primary HVAC control module located in the dash cannot control the auxiliary HVAC system. The vehicle operator determines the mode setting by placing a mode switch in a desired mode position. Only upper and lower vent positions are available at the rear of the vehicle. When a mode setting is selected, a signal is sent from the front auxiliary control assembly to the mode actuator through the mode door control circuit. The auxiliary mode actuator moves the auxiliary mode door to the desired position. Power and ground is supplied to the mode actuator by the ignition and ground circuits.

Air Distribution-Automatic Auxiliary HVAC Control

Depending upon the auxiliary HVAC system content, the auxiliary mode actuator can be controlled by either of the auxiliary HVAC controls. The primary HVAC control module located in the dash does not control the auxiliary HVAC system. If the vehicle does not have a sunroof (CF5), then the front auxiliary controls must be set to the rear position for the rear auxiliary controls to operate. The vehicle operator determines the mode setting by placing a mode switch in a desired speed position. Only upper and lower vent positions are available in the rear of the vehicle.

When a mode position is selected on either of the auxiliary HVAC controls, a signal is sent from the auxiliary HVAC controls to the auxiliary mode actuator through the mode door control circuit. The auxiliary mode actuator moves the auxiliary mode door to the desired position. When the auxiliary 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 auxiliary mode door position. 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 auxiliary mode actuator.

Auxiliary HVAC Configurations

The Automatic HVAC system has the option of being configured with either a manual or automatic auxiliary system. The number of auxiliary HVAC controls is dependent upon whether or not the vehicle is equipped with a sunroof (CF5). An automatic auxiliary HVAC system is only available with an automatic primary HVAC system without a sunroof. A manual auxiliary HVAC system is only available with an automatic auxiliary primary HVAC system with a sunroof.

The automatic primary and auxiliary HVAC systems communicate using keyboard data display (KDD) protocol. The automatic primary system communicates with the rear auxiliary HVAC control module. The manual auxiliary system is independent of the automatic primary HVAC system.

Air Delivery Description and Operation

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 it 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).

Recirculation Mode

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 exits 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 heat from the ambient air and draws moisture onto the evaporator. 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.

A/C Cycle with Auxiliary

The auxiliary A/C system operates from the vehicles primary A/C system. The front or primary A/C system must be ON to allow the rear A/C system to function.

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 system used on this vehicle is a non cycling system. Non cycling A/C systems use a high pressure switch to protect the A/C system from excessive pressure. The high pressure switch will OPEN the electrical signal, to the compressor clutch, in the event that the refrigerant pressure becomes excessive. After the high and low side of the A/C system pressure equalize, the high pressure switch will CLOSE. Closing the high pressure switch will complete the electrical circuit to the compressor clutch. The A/C system is also 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.

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.

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 exits the condenser and flows through the liquid line. The liquid line flow is split and the liquid refrigerant flows to both the front or primary A/C system, and to the liquid line for the rear A/C system.

The liquid refrigerant, flowing to the rear A/C system, flows into the rear TXV. The rear TXV is located at the rear evaporator inlet. The TXV is the dividing point for the high and the low pressure sides of the rear A/C system. As the refrigerant passes through the TXV, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to boil at the expansion device. The TXV also meters the amount of liquid refrigerant that can flow into the evaporator.

Refrigerant exiting the TXV flows into the evaporator core in a low pressure, liquid state. Ambient air is drawn through the rear A/C 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 heat from the ambient air and draws moisture onto the evaporator. The refrigerant exits the evaporator through the suction line and back to the primary A/C systems suction line. Refrigerant in the primary A/C system suction line flows back to the compressor, in a vapor state, and completes 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 rear A/C module for passenger comfort. The heat and moisture removed from the rear passenger compartment will also change form, or condense, and is discharged from the rear A/C module as water.

Auxiliary HVAC Combinations

The table below represents the different auxiliary HVAC combinations. The table will help to identify the auxiliary HVAC control devices used for the different RPO configurations.

Air Temperature Description and Operation

Automatic Auxiliary HVAC System

This HVAC system can be identified by the wording Computer Climate Control on the front face plate of the auxiliary control modules.

The rear auxiliary HVAC control module processes and controls all aspects of the automatic auxiliary HVAC system. The module has communication with the HVAC control module via 2 keyboard data display (KDD) communication circuits. The system receives inputs from the auxiliary upper air temperature sensor, auxiliary lower air temperature sensor, infrared temperature sensor, and feed back signals from auxiliary mode actuator and the auxiliary air temperature actuator. Along with inputs from the front auxiliary HVAC control assembly. The outputs are the auxiliary air temperature actuator, auxiliary mode actuator, auxiliary blower motor control processor and data communication with the HVAC control module. The module is turned on by ignition voltage from the ignition 3 voltage circuit.

Auxiliary Air Temperature Actuator

The auxiliary air temperature actuator opens or closes the auxiliary air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. The auxiliary air temperature actuator is a 5 wire actuator that incorporates a electric motor with feed back capability. The auxiliary air temperature actuator has a potentiometer integral to it. A 5-volt reference signal is sent out over the 5-volt reference circuit, through the instrument panel junction block, to the air temperature actuator. A feed back signal is provided by the air temperature door position signal circuit. The control of the auxiliary air temperature actuator is provided by the air temperature door control circuit. As the actuator moves, the voltage on the door position signal circuit changes. The rear auxiliary HVAC control module monitors this signal to calculate the actual door position.

When a request of actuator position change from the rear auxiliary HVAC control module, the auxiliary air temperature door control circuit voltage is varied. A 2.5 volt signal from the rear auxiliary HVAC control module keeps the actuator stationary. A 0 volt or 5 volt signal from the rear auxiliary HVAC control module allows the actuator to rotate to a position determined by the HVAC control module. The feed back potentiometer provides the position of the actuator through a varied 5 volt signal for reference of the rear auxiliary HVAC control module.

Auxiliary Duct Air Temperature Sensors

The rear auxiliary HVAC control module receives inputs for the auxiliary air duct outlet temperature from the upper and lower auxiliary air temperature sensors. The rear auxiliary HVAC control module uses these inputs to position the auxiliary air temperature actuator to achieve and maintain the set temperature.

The upper and lower auxiliary 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 rear auxiliary HVAC control module 5 volts is supplied to the auxiliary air temperature signal circuit through a fixed resistance. The fixed resistance inside the rear auxiliary 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 rear auxiliary HVAC control module. The rear auxiliary 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 auxiliary air temperature sensors is provided by the low reference circuit.

The rear auxiliary HVAC control module will use a default value for the upper and lower auxiliary air temperature signal if there is a fault with the input. The rear auxiliary HVAC control module will use this default to ensure auxiliary 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 it will read 255 Counts.

Auxiliary Inside Air Temperature Sensor

The auxiliary inside air temperature sensor is an infrared sensor. This component is integral to the rear auxiliary HVAC control module. There is a lens on the front face plate of the rear auxiliary HVAC control module to cover the sensor. If the sensor lens is covered, the sensor can not make a proper indication of heat. The sensor does not set a DTC. The sensor helps in making the proper automatic calculations to position the auxiliary mode and temperature doors.

Manual Auxiliary HVAC w/o CF5

This system incorporates a front and rear auxiliary HVAC control assemblies that provide inputs to the auxiliary HVAC control processor.

Auxiliary HVAC Control Processor

The auxiliary HVAC control processor controls all outputs for the auxiliary HVAC system. It receives inputs from the front and rear auxiliary HVAC control assemblies. The processor positions the auxiliary air temperature actuator and auxiliary mode actuator based on these inputs. This system does not have Class 2 communication available.

The auxiliary HVAC control processor receives power from the ignition 3 voltage circuit. Ground is provided by the ground circuit through rear auxiliary HVAC control assembly and a splice pack. The system receives 12 volt varied voltage input for auxiliary air temperature change request. Then the processor creates a 12 volt varied output for control of the auxiliary air temperature actuator. When the voltage signal is low a cool air request is made and when the voltage signal is high a warm air request is made.

The auxiliary air temperature actuator opens or closes the auxiliary air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. The auxiliary air temperature actuator is a 3 wire actuator that incorporates a bi-directional permanent magnet electric motor.

The auxiliary air temperature actuator receives power from the ignition 3 voltage circuit. Ground is provided by the ground circuit through a splice pack. The control of the air temperature actuator is provided by the auxiliary air temperature door control circuit. The auxiliary HVAC control processor provides a varied 12 volt signal to the actuator. This signal is monitored by the logic incorporated in the actuator and it will move the actuator in the desired direction when a position change is requested. When the voltage signal is low a cool air request is made and when the voltage signal is high a warm air request is made.

The auxiliary air temperature actuator opens or closes the auxiliary air mixture door to a position to divert sufficient air past the heater core to achieve the desired vehicle temperature. The auxiliary air temperature actuator is a 3 wire actuator that incorporates a bi-directional permanent magnet electric motor.

The auxiliary air temperature actuator receives power from the ignition 3 voltage circuit. Ground is provided by the ground circuit through a splice pack. The control of the air temperature actuator is provided by the auxiliary air temperature door control circuit. The auxiliary HVAC control processor provides a varied 12 volt signal to the actuator. This signal is monitored by the logic incorporated in the actuator and it will move the actuator in the desired direction when a position change is requested. When the voltage signal is low a cool air request is made and when the voltage signal is high a warm air request is made.