HVAC System - Automatic: Diagnosis GMC Yukon Denali

Diagnostic Starting Point

Begin the system diagnosis with the Diagnostic System Check . The Diagnostic System Check will provide the following information

1. The identification of the control module(s) which command the system.
2. The ability of the control module(s) to communicate through the serial data circuit.
3. The identification of any stored diagnostic trouble codes (DTCs) and their status.

The use of the Diagnostic System Check will identify the correct procedure for diagnosing the system and where the procedure is located.

Scan Tool Diagnostics

Use the Scan Tool Data Display Values and Definitions Information in order to assist in diagnosing the electronic climate control (ECC) module problems. Compare the vehicles actual scan tool data with the typical data display value table information. Use the data information in order to aid in understanding the nature of the problem when the vehicle does not match with the typical data display values.

The scan tool data values were taken from a known good vehicle under the following conditions

1. The ignition switch is in the ON position.
2. The engine is running at idle.
3. The vehicle is in PARK.
4. The doors are closed.
5. The windows are closed.
6. The ambient air temperatures are at 22-27°C (70-80°F).

1. AUTO - The electronic climate control (ECC) module control the discharge of air flow through the heater and A/C control system.
2. UPPER OUTLET - The air is flowing out the 4 upper outlets.
3. BI-LEVEL - The air is flowing out the 4 panel vents and the 2 floor vents.
4. FLOOR - The air is flowing out the 2 floor vents.
5. DEFOG - The air is flowing out the defrost vent at the top of the instrument panel.
6. DEFROST - The air is flowing out the defrost vent at the top of the instrument panel.

Scan Tool Data List

HVAC Scan Tool Data List

Circuit Description

The HVAC control module monitors the outside ambient air temperature via a temperature sensor. When the air is cold, the sensor resistance and signal voltage are high. When the air is warm, the sensor resistance and signal voltage are low.

Conditions for Running the DTC

The ignition is turned ON.

Conditions for Setting the DTC

The HVAC control module detects the signal circuit is less than 0.09 V (5 counts) or greater than 4.90 V (250 counts).

Action Taken When the DTC Sets

A default value of 2.50 V (127 counts) will be used for the sensor data by the HVAC control module. The actual sensor reading will be displayed on the scan tool in the HVAC data list.

Conditions for Clearing the DTC

1. The DTC will become history if the HVAC Control Module no longer detects a failure.
2. The history DTC will clear after 50 fault free ignition cycles.
3. The DTC can be cleared with a scan tool.

Diagnostic Aids

Check the following conditions

1. Visually inspect the sensor for contamination or damage.
2. Inspect for the following that may affect the sensors accuracy. Exposure to direct sunlight. Exposure to direct air flow. Exposure to heat created by the A/C system, cooling system or engine compartment.
3. Verify that the sensor is installed properly.

The HVAC Control Module monitors the passenger compartment inside air temperature via a temperature sensor. When the air is cold, the sensor resistance and signal voltage are high. When the air is warm, the sensor resistance and signal voltage are low.

The ignition is turned ON.

The HVAC Control Module detects the signal circuit is less than 0.09 V, (5 counts), or greater than 4.90 V, (250 counts).

A default value of 250 V (127 counts) will be used for the sensor data by the HVAC Control Module. The sensor reading displayed on the scan tool in the HVAC data list will be this default value.

1. The DTC will become history if the HVAC Control Module no longer detects a failure.
2. The history DTC will clear after 50 fault free ignition cycles.
3. The DTC can be cleared with a scan tool.

Check the following conditions

1. Visually inspect the sensor for contamination or damage.
2. Verify that air flow to sensor is not restricted.
3. Verify that the sensor is installed properly.

The HVAC Control Module monitors the temperature in the upper center A/C discharge duct via a temperature sensor. When the air is cold, the sensor resistance and signal voltage are high. When the air is warm, the sensor resistance and signal voltage are low.

The ignition is turned ON.

The HVAC Control Module detects the signal circuit is less than 0.09 V (5 counts) or greater than 4.90 V (250 counts).

A default value of 2.50 V (127 counts) will be used for the sensor data by the HVAC control module. The sensor reading displayed on the scan tool in the HVAC data list will be this default value.

1. The DTC will become history if the HVAC Control Module no longer detects a failure.
2. The history DTC will clear after 50 fault free ignition cycles.
3. The DTC can be cleared with a scan tool.

Check the following conditions

1. Visually inspect the sensor for contamination or damage.
2. Verify that air flow to sensor is not restricted.
3. Verify that the sensor is installed properly.

The HVAC control module monitors the air temperature in the floor discharge duct via a temperature sensor. When the air is cold, the sensor resistance and signal voltage are high. When the air is warm, the sensor resistance and signal voltage are low.

The ignition is turned ON.

The HVAC control module detects the signal circuit is less than 0.09 V (5 counts) or greater than 4.90 V (250 counts).

A default value of 2.50 V (127 counts). Will be used for the sensor data by the HVAC control module. The sensor reading displayed on the scan tool in the HVAC data list will be this default.

1. The DTC will become history if the HVAC control module no longer detects a failure.
2. The history DTC will clear after 50 fault free ignition cycles.
3. The DTC can be cleared with a scan tool.

Check the following conditions

1. Visually inspect the sensor for contamination or damage.
2. Verify that air flow to sensor is not restricted.
3. Verify that the sensor is installed properly.

The HVAC control module monitors the ambient light via a light sensitive photodiode. The HVAC control module uses this information to calculate a solar correction that compensates for the effect of the sun on the vehicle inside air temperature. When the sensor is in direct sunlight, the signal voltage is low. When the sensor is shaded, the signal voltage is high.

The ignition is turned ON.

The HVAC control module detects the signal circuit is less than 0.09 V (5 counts) or greater than 4.90 V (250 counts).

A default value of 0 counts will be used for the sensor data by the HVAC control module. The sensor reading displayed on the scan tool in the HVAC data list will be this default value.

1. The DTC will become history if the HVAC Control Module no longer detects a failure.
2. The history DTC will clear after 50 fault free ignition cycles.
3. The DTC can be cleared with a scan tool.

Check the following conditions

1. Visually inspect the sensor for contamination or damage.
2. Verify that exposure to sunlight is not obstructed.
3. Verify that the sensor is installed properly.

The recirculation actuator is an electronic device that incorporates a stepper motor and feedback potentiometer. The HVAC control module controls the recirculation door position by sending a control signal to the actuator. The signal's value is dependent upon the desired mode selected by the vehicle occupants. As the recirculation actuator moves, the potentiometer produces the actual door position signal that is sent to the HVAC control module. The HVAC control module continues to command the actuator to move until the desired and actual values are equal.

After a calibration or recalibration procedure is performed, the HVAC control module calculates the recirculation actuator travel range. The HVAC control module commands the actuator through it's full range. The HVAC control module stores the learned minimum and maximum position values. The travel range value is calculated by subtracting the minimum and maximum position values. The HVAC control module compares the travel range, minimum and maximum values to calibrated value ranges. If the travel range value is less than or equal to the maximum calibrated limit and greater than or equal to the minimum calibrated limit, then the calibration is considered successful. The HVAC control module continuously compares the actual actuator position to the calibrated minimum and maximum position values.

1. The ignition is ON.
2. Ignition voltage is between 9-16 volts.
3. The HVAC control module keep alive memory battery power must be interrupted to run the range check (recalibration procedure).

1. Recirculation actuator position signal is less than 5 counts.
2. Recirculation actuator position signal is more than 250 counts.
3. The recirculation actuators total travel range (min-max) is less than or greater than the calibrated limits.

1. The DTC is stored in DTC information.
2. The HVAC control module will continue to make use of whatever travel range is still available.

1. If the HVAC control no longer detects a failure, then the DTC will become history.
2. The history DTC will clear after 50 ignition cycles without a failure.
3. The DTC can be cleared with a scan tool.

1. A short to voltage on the 5 volt reference circuit of the recirculation actuator will cause DTC B0229 and DTC B0414 to set.
2. A damaged or broken recirculation door and or actuator can cause this DTC to set.
3. The range fault can only be detected following a check of the actuator travel range. The actuator travel range check (recalibration procedure) can only be initiated by disrupting power to the HVAC control module.

DTC B0229 Air Flow Control #7 (Recirc) Feedback Circuit Range/Performance

The mode actuator is an electronic device that incorporates a stepper motor and feedback potentiometer. The HVAC control module controls the mode door position by sending a control signal to the actuator. The signal's value is dependent upon the desired mode selected by the vehicle occupants. As the actuator moves, the potentiometer produces the actual door position signal that is sent to the HVAC control module. The HVAC control module continues to command the actuator to move until the desired and actual values are equal. The door directs airflow through the outlets as selected by the vehicles occupants.

1. The ignition is ON.
2. Ignition voltage is between 9-16 volts.
3. An mode actuator positional change is attempted.

The actual mode door position differs from the commanded mode door position by 4 counts or more.

The mode actuator is considered stalled and the HVAC control module will command the mode actuator to stop rotation (2.5 V).

1. If the HVAC control no longer detects a failure, then the DTC will become history.
2. The history DTC will clear after 50 ignition cycles without a failure.
3. The DTC can be cleared with a scan tool.

1. A dead zone exists in a range 10 counts above or below the mode door max or mode door min positions. When the mode door actual data parameter is within the dead zone, the DTC will not set. If the mode door commanded data parameter differs from the mode door actual data parameter by 4 counts or more outside the dead zone, the DTC will set.
2. A damaged or broken mode door and or actuator can cause this DTC to set.

DTC B0248 Mode Door Inoperative Error

The recirculation actuator is an electronic device that incorporates a stepper motor and feedback potentiometer. The HVAC control module controls the recirculation door position by sending a control signal to the actuator. The signal's value is dependent upon the desired mode selected by the vehicle occupants. As the actuator moves, the potentiometer produces the actual door position signal that is sent to the HVAC control module. The HVAC control module continues to command the actuator to move until the desired and actual values are equal. The door directs airflow through the outlets as selected by the vehicles occupants.

1. The ignition is ON.
2. Ignition voltage is between 9-16 volts.
3. An mode actuator positional change is attempted.

The actual mode door position differs from the commanded mode door position by 4 counts or more.

The mode actuator is considered stalled and the HVAC control module will command the mode actuator to stop rotation (2.5 V).

1. If the HVAC control module no longer detects a failure, then the DTC will become history.
2. The history DTC will clear after 50 ignition cycles without a failure.
3. The DTC can be cleared with a scan tool.

1. A dead zone exists in a range 10 counts above or below the mode door max or mode door min positions. When the mode door actual data parameter is within the dead zone, the DTC will not set. If the mode door commanded data parameter differs from the mode door actual data parameter by 4 counts or more outside the dead zone, the DTC will set.
2. A damaged or broken recirculation door and or actuator can cause this DTC to set.

DTC B0268 Air Flow Control #7 (Recirc) Circuit Malfunction

The air temperature actuator is an electronic device that incorporates a stepper motor and feedback potentiometer. The HVAC control module determines the position of the air temperature actuator based on the voltage from the potentiometer. When a change in air temperature is required (greater than 4 counts) the HVAC control module will drive the air temperature actuator in the appropriate direction to achieve the desired setting while comparing the actual position to the commanded position. If the actual value differs from the commanded value by 4 or more counts, the actuator is considered stalled and the DTC will set.

1. The ignition is ON.
2. Ignition voltage is between 9-16 volts.
3. An actuator positional change is attempted.

The actual air mix door position differs from the commanded air mix door position by 4 counts or more.

The actuator is considered stalled and the HVAC control module will command the actuator to stop rotation (2.5 V).

1. If the HVAC control no longer detects a failure, then the DTC will become history.
2. The history DTC will clear after 50 ignition cycles without a failure.
3. The DTC can be cleared with a scan tool.

1. A dead zone exists in a range 10 counts above or below the mix door max or mix door min positions. When the air mix door actual data parameter is within the dead zone, the DTC will not set. If the air mix door commanded data parameter differs from the air mix door actual data parameter by 4 counts or more outside the dead zone, the DTC will set.
2. A damaged or broken air temperature door and or actuator can cause this DTC to set.

DTC B0408 Temperature Control #1 (Main/Front) Circuit Malfunction

The air temperature actuator is an electronic device that incorporates a stepper motor and feedback potentiometer. The HVAC control module determines the position of the air temperature actuator based on the voltage from the potentiometer. Whenever the HVAC control modules keep alive memory battery power is interrupted, the HVAC control module will perform a recalibration of the actuators. The HVAC control module will perform a travel range comparison of the calculated minimum and maximum extremes of the actuator to the calibrated minimum and maximum range extremes within the HVAC control module. If the calculated range does not equal the calibrated expected range the actuator will be considered not calibrated and the DTC will set. The HVAC control module will also perform a continuous test on the current feedback value. If the feedback value does not equal or fall within the calibrated range the DTC will set.

1. The ignition is ON.
2. Ignition voltage is between 9-16 volts.
3. The HVAC control modules keep alive memory battery power must be interrupted to run the range check.

1. The air temperature door position signal is less than 5 counts or greater than 250 counts.
2. The air temperature actuators total travel range (max-min) is less than or greater than the calibrated limits.

1. The HVAC control module will continue to make use of whatever travel range is still available.
2. Each time the ignition switch is turned ON the HVAC control module will recheck the actuator travel range.

1. If the HVAC control no longer detects a failure, then the DTC will become history.
2. The history DTC will clear after 50 ignition cycles without a failure.
3. The DTC can be cleared with a scan tool.

1. The range fault can only be detected following a check of the actuator travel range. The actuator travel range check can only be initiated by disrupting power to the HVAC control module.
2. The calibration limit is 64 to 216 counts. An actual range below this limit suggests an obstruction (something stuck in the door limiting travel). An actual range above this limit suggests an over travel condition (damaged or missing foam seals).
3. A damaged or broken air temperature door and or actuator can cause this DTC to set.
4. If none of the above conditions exist check for Intermittent Conditions and Poor Connections.

DTC B0414 Temperature Control #1 (Main/Front) Feedback Circuit Range/Performance

The auxiliary air temperature actuator is an electronic device that incorporates a stepper motor and feedback potentiometer. The position of the auxiliary air temperature actuator is based on the auxiliary air temperature door position signal voltage from the potentiometer. The rear auxiliary HVAC module converts this voltage to a digital signal that can be transmitted over the keyboard display driver (KDD) data bus to the HVAC control module. The HVAC control module contains the logic that controls the operation of the auxiliary air temperature actuator. When a change in air temperature is required (greater than 4 counts) the HVAC control module will drive the air temperature actuator in the appropriate direction to achieve the desired setting while comparing the actual position to the commanded position. If the actual value differs from the commanded value by 4 or more counts, the actuator is considered stalled and the DTC will set.

1. The ignition is ON.
2. Ignition voltage is between 9-16 volts.
3. An actuator positional change is attempted.

The actual air mix door position differs from the commanded air mix door position by 4 counts or more.

The actuator is considered stalled and the HVAC control module will command the actuator to stop rotation (2.5 V).

1. If the HVAC control no longer detects a failure, then the DTC will become history.
2. The history DTC will clear after 50 ignition cycles without a failure.
3. The DTC can be cleared with a scan tool.

1. A dead zone exists in a range 10 counts above or below the mix door max or mix door min positions. When the air mix door actual data parameter is within the dead zone, the DTC will not set. If the air mix door commanded data parameter differs from the air mix door actual data parameter by 4 counts or more outside the dead zone, the DTC will set.
2. A damaged or broken auxiliary air temperature door and or actuator can cause this DTC to set.
3. If none of the above conditions exist check for Intermittent Conditions and Poor Connections.

DTC B0429 Air Mix Door #3 Range Error

The auxiliary air temperature actuator is an electronic device that incorporates a stepper motor and feedback potentiometer. The position of the auxiliary air temperature actuator is based on the auxiliary air temperature door position signal voltage from the potentiometer. The rear auxiliary HVAC module converts this voltage to a digital signal that can be transmitted over the keyboard display driver (KDD) data bus to the HVAC control module. The HVAC control module contains the logic that controls the operation of the auxiliary air temperature actuator. Whenever the HVAC control modules keep alive memory battery power is interrupted, the HVAC control module will perform a recalibration of the actuators. The HVAC control module will perform a travel range comparison of the calculated minimum and maximum extremes of the actuator to the calibrated minimum and maximum range extremes within the HVAC control module. If the calculated range does not equal the calibrated expected range the actuator will be considered not calibrated and the DTC will set. The HVAC control module will also perform a continuous test on the current feedback value. If the feedback value does not equal or fall within the calibrated range the DTC will set.

1. The ignition is ON.
2. Ignition voltage is between 9-16 volts.
3. The HVAC control modules keep alive memory battery power must be interrupted to run the range check.

1. The auxiliary air temperature door position signal is less than 5 counts or greater than 250 counts.
2. The auxiliary air temperature actuators total travel range (max-min) is less than or greater than the calibrated limits.

1. The HVAC control module will continue to make use of whatever travel range is still available.
2. Each time the ignition switch is turned ON the HVAC control module will recheck the actuator travel range.

1. If the HVAC control no longer detects a failure, then the DTC will become history.
2. The history DTC will clear after 50 ignition cycles without a failure.
3. The DTC can be cleared with a scan tool.

1. The range fault can only be detected following a check of the actuator travel range. The actuator travel range check can only be initiated by disrupting power to the HVAC control module.
2. The calibration limit is 64 to 216 counts. An actual range below this limit suggests an obstruction (something stuck in the door limiting travel). An actual range above this limit suggests an over travel condition (damaged or missing foam seals).
3. A damaged or broken auxiliary air temperature door and or actuator can cause this DTC to set.
4. If none of the above conditions exist check for intermittent conditions and poor connections.

DTC B0434 Temperature Control #3 Rear Feedback Circuit Range

Symptoms

1. Perform «Diagnostic System Check»(ref-235302-S27396764382006061900000) before using the Symptom Tables in order to verify that all of the following conditions are true: There are no DTCs set. The control module(s) can communicate via the serial data link.
2. Review the system operation in order to familiarize yourself with the system functions. Refer to the following descriptions: «HVAC Compressor Controls Circuit Description»(ref-235302-S08540770122006061900000) «HVAC Blower Controls Circuit Description»(ref-235302-S27959997722006061900000) «HVAC Air Delivery/Temperature Control Circuit Description»(ref-235302-S35280782272006061900000)

Visual/Physical Inspection

1. Inspect for aftermarket devices which could affect the operation of the HVAC Automatic System. Refer to Checking Aftermarket Accessories in Wiring Systems.
2. Inspect the easily accessible or visible system components for obvious damage or conditions which could cause the symptom.
3. Inspect for the proper refrigerant level.

Symptom List

Refer to a symptom diagnostic procedure from the following list in order to diagnose the symptom

1. «Blower Motor Inoperative in Auto Mode»(ref-235302-S26542198582006061900000)
2. «Defrosting Insufficient»(ref-235302-S05902953202006061900000)
3. «HVAC Compressor Clutch Does Not Engage»(ref-235302-S01943709572006061900000)
4. «HVAC Compressor Clutch Does Not Disengage»(ref-235302-S13379121852006061900000)
5. «System Performance Test»(ref-235302-S24013618682006061900000)
6. «Heating Insufficient»(ref-235302-S07083774882006061900000)

System Performance Test

Perform the following test if you suspect a malfunction in the refrigerant system due to abnormal system pressures.

1. Park the vehicle inside, or in a shaded area.
2. Open the doors or the windows of the vehicle.
3. Vent the engine exhaust if necessary.
4. Open the hood.
5. Install the high and low side pressure gauges. Route the lines over the rear hood seal. Tape the gauges to the windshield in order to view the gauges.
6. Close the hood to the safety latch position.
7. Record the ambient temperature at the vehicle.
8. Record the relative humidity. Use a psychrometer or consult the local weather bureau.
9. Close the doors or the windows of the vehicle.
10. Set the mode control to MAX A/C.
11. Adjust the blower speed to HI.
12. Adjust the temperature to full COLD.
13. Open the air conditioning outlets.
14. Install a temperature tester into the right center air conditioning outlet.
15. Place the transmission in PARK or NEUTRAL.
16. Start the engine.
17. Stabilize the engine speed at 1500 RPM. Start the timer.
18. Run the air conditioning system until the outlet air reaches the lowest temperature (about 3 minutes).
19. Record the following statistics: The outlet air temperature The high-side pressure The low side-pressure
20. Turn off the engine.
21. Compare the readings. Normal operating air conditioning systems should not exceed normal levels. Refer to the following diagnostic aids: Pressure Temperature Relationship of R-134a System Performance Test With 1,500 RPM Engine Speed
22. Inspect the outer surfaces of the radiator and the condenser cores. Inspect between the condenser and the radiator as well as the outer surfaces. Ensure that the airflow is not blocked by the following materials: Dirt Leaves Other foreign matter
23. Inspect the following components for restrictions or kinks: The evaporator core The condenser core The hoses The tubes
24. Inspect for refrigerant leaks.
25. Inspect all of the air ducts for leaks or restrictions. Low air flow may indicate a restricted evaporator core.
26. Inspect the compressor clutch for slippage.
27. Inspect the drive belt for improper tension.

HVAC Compressor Controls Circuit Description

The HVAC air delivery electrical system consists of a control module, blower switch, mode and temperature potentiometers & (motors), recirculating outside air control push button, temperature, mode and recirculating motors that operate their respective actuators and doors.

The mode, recirculating and temperature door motors are powered by ignition 3 voltage when the ignition switch is in the RUN position. The ground path is provided by the body wiring harness junction block to ground located in the left hand upper plenum area. The HVAC control module is powered by ignition 3 voltage from the HVAC fuse located in the fuse block when the ignition switch is in the RUN position. Grounding for the module is through the same circuit utilized for the door motors.

The recirculating door motor is controlled by a signal from the HVAC control module via recirculation actuator control when the push button is activated on the control head. Also, the recirculation door motor can be controlled by the PCM which monitors the automatic recirculation switch activity (located at the bottom right hand side of the air conditioning (A/C) condenser) and will signal the door motor to the recirculating mode when the switch closes due to high refrigerant pressure. This will allow cooler air from inside the vehicle to flow over the A/C evaporator thus cooling the refrigerant temperature.

The mode and temperature door motors are positioned in proportion to the variable voltage signal received from the HVAC control module. Adjusting the temperature dial (potentiometer) in the control head provided a variable (0-5 volts) voltage to the door motor via temp actuator control. There is a constant 5 volt reference sent to both motors via 5 volt reference from the control module. The module uses this reference voltage to accurately determine the door motor position through a feed back temp actuator signal which also varies from 0-5 volts.

The air delivery mode functions consist of vent, bi-level, heater, defog and defrost (non rear HVAC vehicles) and upper or lower (Rear HVAC vehicles). The mode door motor operates in the same manner as the temperature door except for the signal and feed back circuits which are mode actuator control and mode actuator signal respectively. The HVAC control module receives a mode signal from the control head and will adjust the voltage control signal to the motor in accordance with the mode selected.

HVAC Blower Controls Circuit Description

The electronic climate control (ECC) modules (front and rear) serves as an interface between the passenger and the blower motor. System voltage is supplied to the ECC modules by the following sources

1. Continuous battery voltage source: RADIO fuse (front module) CKT 340 (ORN)
2. Interrupted battery voltage source: Ignition switch HVAC I fuse (front module) IGN 3 fuse (rear module)

The ground path is provided to grounds G200/G410.

The ECC modules determines the appropriate blower motor speed and sends a pulse width modulated (PWM) signal to the blower motor. The ECC module may produce the blower motor speed signal in two modes of operation

1. Automatic operation
2. Manual operation

In the manual operation mode 13 speed settings are available and, once selected, the fan speed remains constant until a new speed is selected. In the automatic operation mode the ECC module will determine what blower speed is necessary in order to maintain customer selected temperature.

HVAC Air Delivery/Temperature Control Circuit Description

The electronic climate control (ECC) modules (front or rear) serves as an interface between the passenger and the air delivery system. The ECC modules compares passenger selections with environmental conditions and will adjust air delivery accordingly.

The following air delivery modes are available

1. Defrost
2. Blend
3. Floor
4. Bi-level
5. Panel
6. External air circulation
7. Internal air circulation

The ECC modules requires two voltage sources in order to operate. Voltage is available at all times through the RADIO fuse).

When the ignition switch is in the RUN position, voltage is available through the HVAC I fuse).

A path to ground for the ECC modules is provided to grounds G200/G410.

The ECC modules receives inputs from the following temperature sensors

1. The inside air temperature sensor. A fan located internally to the inside air temperature sensor housing continually draws passenger compartment air over the sensor increasing accuracy of the temperature reading. The HVAC I fuse applies voltage for the fan motor when the ignition switch is in the RUN position. A path to ground for the fan motor is provided to ground G202.
2. The outside air temperature sensor.
3. The sun load temperature sensor.
4. The upper discharge air temperature sensor (front or rear).
5. The lower discharge air temperature sensor (front or rear).

The ECC modules will use the sensor information in order to determine how much airflow to the heater core is required and will adjust the air temperature valve accordingly. The air temperature valve is located internally to the heater core housing. The position of the air temperature valve is reported to the ECC modules by the air temperature valve sensor and motor. The HVAC I fuse applies voltage for the air temperature valve sensor and motor when the ignition switch is in the RUN position.

Operation of the air temperature valve sensor and motor is supported by the following circuits

1. An 5 volt reference circuit.
2. An ground reference circuit.
3. An signal return path circuit.
4. An motor drive signal circuit.

The ECC modules will control the actuation of individual motors of the HVAC assembly in order to direct air to the appropriate outlets. The HVAC I fuse applies voltage for the HVAC assembly when the ignition switch is in the RUN position. With voltage available the ECC modules will complete the current path to ground.