Thermostatic Air Cleaner: Description and Operation

Fig. 61 Thermostatic Air Cleaner (TAC):

The Thermostatic Air Cleaner is designed to keep air entering the engine above 100°-120° F, depending on application. Keeping intake air above this temperature reduces exhaust emissions by permitting leaner mixtures and shorter choke on periods. The TAC also improves cold engine driveability and helps prevent carburetor icing.
The thermostatic air cleaner includes a temperature sensor, vacuum motor, control damper and connecting vacuum hoses. The temperature sensor controls the vacuum motor. The vacuum motor then operates the control damper, which regulates the flow of pre-heated and unheated air.
When engine compartment temperatures are below the calibration value of the sensor, the sensor closes, allowing engine vacuum to be directed to the vacuum motor. The vacuum motor then closes the control damper to outside air, allowing heated air from the exhaust manifold to flow up through the heat tube to the air cleaner, Fig. 61.
As the temperature in the air cleaner reaches the calibration value of the sensor, the sensor begins to open, bleeding off vacuum to the vacuum motor. The vacuum motor causes the control damper to partially open, allowing outside air to blend with heated air. This regulated air then enters the air cleaner assembly, Fig. 62.
During periods of wide open throttle or when ambient temperatures are very high, there is not enough vacuum available to the vacuum motor to overcome diaphragm spring tension. The diaphragm then causes the control damper to move downward, closing the hot air duct and allowing only outside air to enter the air cleaner, Fig. 63.
Some General Motors vehicles are equipped with a built-in vacuum trap to hold the control damper door in the hot air mode during acceleration, when ambient temperatures are below 70° F. The length of time that the vacuum will be trapped is dependent upon sensor temperature, varying from several minutes at very cold temperatures to a few seconds at approximately 70° F. Once the sensor reaches its calibration temperature, the trapped vacuum feature no longer applies.
Trapped vacuum is accomplished through the use of a check valve located in the small orifice leg of the temperature sensor.