Jeep CJ/Scrambler 1971-1986 Repair Guide

Thermostatically Controlled Air Cleaner System (TAC)


See Figures 1 and 2


This 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 air from the engine compartment.

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 temperature of the air entering the air cleaner approaches approximately 115°F (46°C), the air 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 underhood air to enter the air cleaner.

The air valve in the air cleaner will also open, regardless of air temperature, during heavy acceleration to obtain maximum air flow through the air cleaner.

Click image to see an enlarged view

Fig. Fig. 1: Vacuum controlled thermostatic air cleaner operation

Click image to see an enlarged view

Fig. Fig. 2: Non-vacuum controlled thermostatic air cleaner


Thermal Vacuum Valve (TVS) Functional Test
  1. Allow the air cleaner to cool to between 40 and 50°F (4-10°C).
  3. Disconnect the vacuum hoses from the TVS and connect an external vacuum source to one nipple and a vacuum gauge to the other.
  5. Apply vacuum to the TVS. Vacuum should not be present when the air temperature is 40-50°F (4-10°C). If vacuum is present, replace the switch.
  7. Start the engine and allow the air cleaner to warm above 50°F (10°C). Vacuum should be present.