Heating-A/C System Diagnostics
HEATING AND A/C SYSTEMS
ON-BOARD DIAGNOSTICS
CAUTION: Do not exchange A/C-Heater Controls from vehicle to vehicle. Software versions differ between models and model years. Installing an A/C-Heater Control with software that is incompatible for a given vehicle can result in either improper or failed HVAC system operation.
SETTING FAULTS - ATC AND MTC SYSTEMS
Both the automatic temperature control (ATC) and the manual temperature control (MTC) systems are controlled by the A/C-heater control located on the center bezel of the instrument panel. The ATC system communicates on the controller area network (CAN) B bus and is fully addressable with a scan tool. The MTC system is NOT connected to the CAN bus and is NOT addressable with a scan tool.
The A/C-heater control's primary means of fault detection for both heating-A/C systems is through active and stored diagnostic trouble codes (DTCs). The control continuously monitors various internal parameters during normal system operation. If it detects a fault, both an active and a stored diagnostic trouble code (DTC) will set for that parameter. When the offending parameter returns to an acceptable value, the control automatically clears the active DTC. However, the stored DTC remains until cleared (either manually or automatically). Note that DTCs will not set or clear if supply voltage is low.
The A/C-heater control's secondary means of fault detection for both heating-A/C systems is through system tests. For ATC systems, these tests include the HVAC System Test, the Cooldown Test, and Actuator Calibration. For MTC systems, these tests include the HVAC System Test, the Cooldown Test, Actuator Calibration (new MTC installation only), and the Actuator Circuit Test / Door Calibration Function.
DISPLAYING FAULTS AND READING FAULTS - ATC SYSTEM
Use a scan tool to display DTCs stored in the A/C-heater control of the ATC system.
DISPLAYING FAULTS AND READING FAULTS - MTC SYSTEM
Use the A/C-heater control to display DTCs stored in the A/C-heater control of the MTC system. Refer to Entering Display DTC Mode - MTC System; Display Sequencing - MTC System; Reading Diagnostic Trouble Codes - MTC System; Active DTCs - ATC and MTC Systems; Stored DTCs - ATC and MTC Systems; and Clearing Faults - MTC System.
ENTERING DISPLAY DTC MODE - MTC SYSTEM
To enter the MTC system's Display DTC Mode, perform the following:
NOTE: An active DTC 33 will prevent the control from entering diagnostic mode and performing certain diagnostic functions. It will also prevent proper A/C mode switch and status indicator function.
1. Turn the ignition to the On position.
2. Turn the blower motor control (1) to the On position.
3. Press the A/C mode switch (2) down, turn the blower motor control to the Off position, wait until both LEDs illuminate (approximately 5 seconds) and then release the A/C switch. If there are active or stored DTCs to display, the A/C status indicator (3) will begin to flash. If there are no active or stored DTCs to display, the LEDs on the A/C-heater control (4) will turn off and the system will automatically exit the mode of operation.
4. To manually exit Display DTC Mode, either turn the ignition off or disconnect the negative battery cable from the battery.
DISPLAY SEQUENCING - MTC SYSTEM
MTC system DTCs will display in ascending numerical order but not in chronological order. Active DTCs will display before stored DTCs. If there are active and stored DTCs for the same parameter, the A/C-heater control will NOT display the stored DTC for that parameter. The EBL status indicator will illuminate and remain illuminated while the A/C-heater control is displaying stored DTCs. After displaying all active and stored DTCs, the A/C-heater control restarts at the beginning, displaying all DTCs again. To restart the sequence from the beginning while the A/C-heater control is displaying DTCs, press the EBL mode switch down, and then release it. This will cause the A/C-heater control to quit displaying the present code, turn off all LEDs for 2.0 seconds, and then it will begin flashing all DTCs again from the beginning.
READING DIAGNOSTIC TROUBLE CODES - MTC SYSTEM
All active and stored DTCs for the MTC system have an assigned code number. The A/C status indicator flashes in a series of pulses to display the code numbers. Each pulse has an indicator on time of 0.5 seconds. The tens unit value (1) is flashed first followed by the ones unit value (2). A 2.0 second indicator off-time (3) separates the unit values. When displaying multiple DTCs, the A/C status indicator will go off for five seconds (4) before displaying the next DTC (5). After displaying all DTCs (6), the sequence will repeat until exiting Display DTC Mode.
ACTIVE DTCs - ATC AND MTC SYSTEMS
Refer to HVAC - Electrical Diagnostics to diagnose HVAC system DTCs.
STORED DTCs - ATC AND MTC SYSTEMS
The HVAC System Test, found in HVAC - Electrical Diagnostics, provides a means for diagnosing stored DTCs. The HVAC System Test consists of multiple diagnostic procedures which cover:
- A/C System Performance
- System Controls
- Air-door Functionality
Either perform the entire diagnostic procedure for a complete system test or perform an individual procedure if diagnosing a specific symptom, condition, or DTC.
CLEARING FAULTS - ATC SYSTEM
For every fault that sets, the A/C-heater control maintains an independent count of the number of key cycles since the active DTC cleared, the odometer reading when the DTC set, and the elapsed ignition on time that the DTC was active. When the number of key cycles (by parameter) reaches a global number, the system automatically clears all of the information associated with that DTC. A loss of battery voltage will also clear all active DTCs and associated data from memory. However, upon reconnecting the battery and activating the A/C-heater control ("Ignition" line going "HIGH"), the system will again evaluate all parameters and will set active DTCs for parameters outside of acceptable limits.
DTCs can also be cleared manually from the A/C-heater control (which also resets key cycle counters). Use a scan tool to clear DTCs stored in the A/C-heater control of the ATC system.
CLEARING FAULTS - MTC SYSTEM
To clear DTCs stored in the A/C-heater control of the MTC system, perform the following:
NOTE: An active DTC 33 will prevent the control from entering diagnostic mode and performing certain diagnostic functions. It will also prevent proper A/C mode switch and status indicator function.
NOTE: An active DTC 34 will prevent the control from performing certain diagnostic functions and it will prevent proper EBL mode switch and status indicator function. If active, diagnose and repair DTC 34 before proceeding.
5. Turn the ignition to RUN.
6. Turn the blower motor control (1) to ON.
7. Press the A/C mode switch (2) down, turn the blower motor control to OFF, wait until both LEDs illuminate (approximately 5 seconds) and then release the A/C mode switch.
8. When the A/C status indicator begins flashing DTCs, set the Mode switch to the floor position, simultaneously press the A/C mode switch and the EBL mode switch down until both LEDs start flashing (approximately 5 seconds) and then release the mode switches. Stored DTCs will clear from memory in approximately two seconds.
SYSTEM TESTS
HVAC SYSTEM TEST - ATC AND MTC SYSTEMS
The HVAC System Test, found in 24 - HVAC - Electrical Diagnostics, provides a starting point in the diagnostic process by identifying the appropriate diagnostic procedure or system test to perform when diagnosing a given symptom, condition, or DTC. It also provides a means for testing the entire HVAC system by utilizing the A/C-heater control's On-Board System Tests. The On-Board System Tests can also assist in diagnosing stored DTCs.
ACTUATOR CALIBRATION - ATC SYSTEM
The Actuator Calibration function homes and repositions the air-door actuators, removes accumulated positioning errors, and checks for air-door span faults. Once actuated, the entire process takes approximately 90 seconds. Upon completion, all air-door actuators should return to the position that the system is currently requesting. Running the Actuator Calibration function is the only way to detect air-door travel too large faults and air-door travel too small faults. These faults, if present, will display on the scan tool after running the Actuator Calibration function.
STARTING ACTUATOR CALIBRATION - ATC SYSTEM
Use a scan tool to start the Actuator Calibration function.
ACTUATOR CALIBRATION - MTC SYSTEM
NOTE: Diagnose and repair Actuator Circuit Test faults (DTC 61, 62, 63, 64, 65, 66, 67, 68, 69, 71, and 72) before diagnosing Overcurrent faults (DTC 41, 42, 43, and 44) and Calibration faults (DTC 18, 21, 22, 23, 24, 25, 45, 46, 47, 48, 51, 52, 53, 54, 55, and 56). Diagnose and repair Overcurrent faults (DTC 41, 42, 43, and 44) before diagnosing Calibration faults (DTC 18, 21, 22, 23, 24, 25, 45, 46, 47, 48, 51, 52, 53, 54, 55, and 56).
The A/C-Heater Control automatically runs the Actuator Calibration function upon ignition on after installing a new A/C-Heater Control. The Actuator Calibration function homes and repositions the air-door actuators, removes accumulated positioning errors, and checks for air-door span faults. Once actuated, the entire process takes approximately 90 seconds. Upon completion, all air-door actuators should return to the position that the system is currently requesting. Running the Actuator Calibration function is the only way to detect air-door travel too large faults, air-door travel too small faults, air-door bound faults, and air-door broken linkage faults. These faults, if present, will display when checking for DTCs in the A/C-heater control.
STARTING ACTUATOR CALIBRATION MANUALLY - MTC SYSTEM
Refer to Starting The Actuator Circuit Test / Door Calibration Function - MTC System.
ACTUATOR CIRCUIT TEST / DOOR CALIBRATION FUNCTION - MTC SYSTEM
NOTE: Diagnose and repair Actuator Circuit Test faults (DTC 61, 62, 63, 64, 65, 66, 67, 68, 69, 71, and 72) before diagnosing Overcurrent faults (DTC 41, 42, 43, and 44) and Calibration faults (DTC 18, 21, 22, 23, 24, 25, 45, 46, 47, 48, 51, 52, 53, 54, 55, and 56). Diagnose and repair Overcurrent faults (DTC 41, 42, 43, and 44) before diagnosing Calibration faults (DTC 18, 21, 22, 23, 24, 25, 45, 46, 47, 48, 51, 52, 53, 54, 55, and 56).
The Actuator Circuit Test / Door Calibration function first tests the air-door electrical circuits for shorts and then it homes and repositions the air-doors to remove accumulated positioning errors and to check for air-door span faults. This test / function supplements the continuous actuator drive system diagnostics and can provide greater detail about shorted air-door electrical circuits by identifying up to three door driver circuits shorted simultaneously. This test will also detect air-door travel too large faults, air-door travel too small faults, air-door bound faults, and air-door broken linkage faults. The Actuator Circuit Test / Door Calibration function must be manually actuated. Faults detected will display when checking for DTCs in the A/C-heater control.
STARTING THE ACTUATOR CIRCUIT TEST / DOOR CALIBRATION FUNCTION - MTC SYSTEM
NOTE: By running the Actuator Circuit Test / Door Calibration Function, the A/C-Heater Control can identify up to three door driver circuits shorted simultaneously. A DTC 72 will set if more than three door driver circuits are shorted in the same direction (e.g. four door driver circuits all shorted to ground) or if two or more door driver circuits are shorted with at least one door driver circuit shorted to ignition and one door driver circuit shorted to ground. To ensure a proper diagnosis, diagnose and repair Actuator Circuit Test faults in the following order: If present, diagnose and repair DTC 72 first, then DTC 62, then DTC 61, then DTCs 64, 66, 68, or 71, and finally DTCs 63, 65, 67, or 69. In addition, always test the door driver circuits after each repair by clearing DTCs, and then running the Actuator Circuit Test / Door Calibration Function, and then checking for DTCs.
To start the Actuator Circuit Test / Door Calibration function for the MTC system, perform the following:
NOTE: An active DTC 34 will prevent the control from performing certain diagnostic functions and it will prevent proper EBL mode switch and status indicator function. If this occurs, diagnose and repair DTC 34 before running the Actuator Circuit Test / Door Calibration function.
1. Turn the ignition to the On position.
2. Turn the blower motor control (1) to the Off position.
3. Press and hold the EBL mode switch (2) down and then turn the blower motor control on. Continue to hold the EBL mode switch down until the EBL status indicator begins flashing. Then, release the mode switch. While the test / function is running, the EBL status indicator will flash once per second. If the test / function passes, the EBL status indicator will stop flashing. However, if the test / function fails, the A/C and EBL status indicators will flash alternately. Faults detected will display when checking for DTCs in the A/C-heater control.
COOLDOWN TEST - ATC AND MTC SYSTEMS
The Cooldown Test checks A/C system performance by measuring the system's ability to lower the evaporator temperature 11.11° C (20° F) as measured by the evaporator temperature sensor. The following are prerequisites of the Cooldown Test. Verify each of the following before running the Cooldown Test:
- For the ATC system, the Cooldown Test will NOT start if DTC B1031 or B1032 is active. If active, diagnose and repair either DTC before proceeding .
- For the MTC system, the Cooldown Test will NOT start if DTC 31 or 32 is active. If active, diagnose and repair either DTC before proceeding.
- For both systems, verify that the refrigerant system has an adequate charge. Check and repair as necessary before proceeding.
- For both systems, verify that the blower motor operates correctly in all speeds. Diagnose and repair all blower motor related faults before proceeding.
- For both systems, verify that the ambient temperature of the work area is above 18.3° C (65° F) before proceeding. Move the vehicle to a warmer work area if necessary.
- For both systems, verify that the evaporator temperature is above 18.3° C (65° F) before proceeding.
- For both systems, verify that the A/C compressor is NOT running. If the A/C compressor is running, turn the A/C off and allow the A/C evaporator to warm up before proceeding.
STARTING THE COOLDOWN TEST - ATC SYSTEM
Once all of the prerequisites have been met, the Cooldown Test for the ATC system can be actuated by sending a command with a scan tool. Once started, the ATC A/C-heater control automatically sets the blower speed and positions the air-doors for optimal A/C performance. It also sends a request for A/C operation on the CAN B bus. The Cooldown Test can take up to two minutes to run and, will stop running if any of the following occurs:
- The ignition is turned to any position other than RUN.
- The A/C compressor is requested off.
- DTC B1031 or B1032 sets during the Cooldown Test.
While the Cooldown Test is running, the A/C status indicator will flash. During this time the A/C-heater control will ignore most of its inputs. If the ATC system passes the test, the A/C status indicator will stop flashing. However, if the ATC system fails the test, both the A/C and EBL status indicators will flash alternately and an active DTC B10B2 will set. The status indicators will continue to flash until either a successful Cooldown Test is executed or until the vehicle is driven a specified number of miles or after 5 ignition cycles. In addition, DTC B10B2 will remain active until a successful Cooldown Test is executed. Always check for DTCs in the A/C-heater control after running the Cooldown Test.
STARTING THE COOLDOWN TEST - MTC SYSTEM
Once all of the prerequisites have been met, the Cooldown Test for the MTC system can be actuated by performing the following:
NOTE: An active DTC 33 will prevent the control from entering diagnostic mode and performing certain diagnostic functions. It will also prevent proper A/C mode switch and status indicator function.
4. Verify that the ignition is in OFF.
5. Turn the blower motor control (1) to OFF.
6. Start the engine.
7. Press and hold the A/C mode switch (2) down and then turn the blower motor control to high speed. Continue to hold the A/C mode switch down until the A/C status indicator begins flashing, Then, release the mode switch.
Once actuated, the MTC A/C-heater control automatically positions the air-doors for optimal A/C performance and sends a request for A/C operation to the cluster (CCN) via hardwired circuits. The Cooldown Test can take up to two minutes to run. The test will stop running if any of the following occurs:
- The ignition is turned to any position other than RUN.
- The blower motor control is turned to the any position other than high speed.
- DTC 83 or 84 sets during the Cooldown Test.
While the Cooldown Test is running, the A/C status indicator will flash. During this time the A/C-heater control will ignore most of its inputs. If the MTC system passes the test, the A/C status indicator will stop flashing. However, if the MTC system fails the test, both the A/C and EBL status indicators will flash alternately and an active DTC 35 will set. The status indicators will continue to flash until either a successful Cooldown Test is executed or until the vehicle's ignition on time has exceeded a specified value. In addition, DTC 35 will remain active until a successful Cooldown Test is executed. Always check for and diagnose DTCs present in the A/C-heater control after running the Cooldown Test. If the Cooldown Test fails, service the heating-A/C system as required.