See Figure 1
A thermostatically controlled air cleaner was used on models through 1990. The system consists of a heat shroud which is integral with the right side exhaust manifold, a hot air hose and a special air cleaner assembly equipped with a thermal sensor and a vacuum motor and air valve assembly.
The thermal sensor incorporates an air bleed valve which regulates the amount of vacuum applied to the vacuum motor, controlling the air valve position to supply either heated air from the exhaust manifold or ambient air from the intake snorkel.
During the warm-up period when underhood temperatures are low, the air bleed valve is closed and sufficient vacuum is applied to the vacuum motor to hold the air valve in the closed (heat on) position.
As the air temperature entering the TAC approaches approximately 115ºF (46ºC), the bleed valve opens to decrease the amount of vacuum applied to the vacuum motor. The diaphragm spring in the vacuum motor then moves the air valve into the open (heat off) position, allowing only air from the intake snorkel only to enter the TAC.
The air valve in the air cleaner will also open, regardless of air temperature, during heavy acceleration (when intake vacuum drops) in order to obtain maximum air flow through the air cleaner.