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Dick Hussey's Tech Corner
Airflow and Temperature Control
Last issue we covered the refrigeration and heat creation in your HVAC system. The thermostatically controlled system is designed to blend the hot and cold to create the set temperature when the system is set to "AUTO", the air direction control slide is set to "NORMAL", and the fan speed control is set to "NORM". Manual settings are also possible. The system is run by the body computer module (BCM) and the HVAC programmer. The following will describe how the system operates, and also how to use the manual controls to troubleshoot when something is wrong.
There are three sensors which provide information to the BCM: the sun load sensor located up in the center of the dash under the middle of the defroster grille, the in-car temperature sensor which is located behind the Pininfarina script in front of the shifter handle under the DIC panel behind the dash, and the outside air temperature sensor which is located under the hood near the latch behind the grille. The values showing on these sensors can be accessed through the onboard diagnostics in the BCM Data parameters section. The BCM takes input from the sensors and input from the various settings and commands the HVAC programmer how to operate the system.
When the fan speed setting is in NORM, and the airflow slide bar setting is in NORMAL, the fan speed will be controlled according to how close or far away the inside car temperature is from the desired set point. If you get into a very hot car in the summer and turn on the AC, the fan will blow on high and then gradually cut back as the car cools off. The airflow will be directed out the dash vents. In the winter on a cold day the heat will not turn on until the coolant has reached a certain temperature. The system will direct the airflow to the floor vents and run the blower on high and then gradually cut back the blower speed as the inside car temperature gets closer to the set point. If you set your fan speed to LO or HI, you are overriding the automatic system. The fan will stay on low speed or high speed, no matter how close or far away you are from the set point. Especially in the case of low fan speed setting, this can keep the system from heating or cooling the way it should.
There are several components in the system that can fail. When failure occurs, the system manual controls can be used to confirm what is not working. Since these items are not accessible, the only way to show pictures is to remove the dash. With the dash removed, they are readily accessible.
The first condition is no heat or no cooling. There is a small DC motor built into the HVAC programmer. This motor operates the air mix door, which determines how much hot air from the heater core and how much cold air through the evaporator core are needed to reach the temperature set point. Water flows through the heater core constantly. There is no water valve to turn off heat flow. With the compressor on, refrigerant will also flow through the evaporator core. With a no heat condition, possible causes are either a plugged heater core or a bad air mix door motor. With a no cooling condition, possible causes are either a defective compressor not pumping or a bad air mix door motor. In order to check the.air mix door motor, remove the glove box. The air mix door arm is located behind the radio receiver on 1987 through 1992 models as shown. Removing the receiver on 1987 through 1992 models will make it easier to see. 1993 models do not have the receiver in this location so it is in plain view. Start the car, and allow it to warm up. Then change the temperature setting on the DIC panel all the way from 60 degrees to 90 degrees a couple times. Watch the arm to see if it moves back and forth. If the arm moves back and forth and you have no heat, the heater core is plugged. If the arm does not move, the air mix door motor or the control driving it has failed.
Another item that can fail is the link on the defroster door actuator. When this link breaks , all the air flow will go out through the defroster duct up by the windshield, and none will come out through the dash vents. It is also theoretically possible that the vacuum actuator failed, or that there is a vacuum supply problem . But to date all instances of this failure we have seen have proven to be the link. Unfortunately, this link and the AC duct work to which it is attached is one of the first items installed in the interior when the car was built at the factory in Italy. Significant disassembly is required to access it, including removal of the center console between the seats and the dash assembly. It is pressed onto the rod that operates the defroster duct door.
Top picture shows bottom of the up-down valve. The module showing on the left is the airbag module. The other photo shows the pot metal link on the defroster duct door that can break. When that breaks, all air goes up through the defroster, and nothing can go out through the dash vents. The photo to the left shows (on a dash that is removed from a car) the location of the in-car temperature sensor. It is in front of the console behind the black trim piece with the Pininfarina script.
The other airflow control item that can fail it the up-down motor. This is a small DC motor that directs the air flow either up through the dash vents/defroster grille or down through the floor vents. It is located next to the driver's right leg and can be changed without significant disassembly. To check operation, move the air direction slide bar (located just below the electronic climate control window on the DIC panel) back and forth from normal to down to up and see if the location of air discharge from the vents changes according to the settings.
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