Ford Pick-Ups/Bronco 1976-1986 Repair Guide

Thermostatically Controlled Air Cleaner System (TAC)


See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Temperature operated duct and valve assembly

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 vacuum motor and air valve assembly.

The temperature of the carburetor intake air is thermostatically controlled by means of a valve plate and a vacuum override built into a duct assembly attached to the air cleaner. The exhaust manifold shroud tube is attached to the shroud over the exhaust manifold for the source of heated air.

The thermal sensor is attached to the air valve actuating lever, along with the vacuum motor lever, both of which control the position of the air valve to supply either heated air from the exhaust manifold or cooler air from the engine compartment.

During the warm-up period, when the under-the-hood temperatures are low, the thermal sensor doesn't exert enough tension on the air valve actuating lever to close (heat off) the air valve. Thus, the carburetor receives heated air from around the exhaust manifold.

As the temperature of the air entering the air cleaner approaches approximately 110°F (43°C), the thermal sensor begins to push on the air valve actuating lever and overcome the spring tension which holds the air valve in the open (heat on) position. The air valve begins to move to the closed (heat off) position, allowing only under-the-hood air to enter the air cleaner.

The air valve in the air cleaner will also open, regardless of the air temperature, during heavy acceleration to obtain maximum airflow through the air cleaner. The extreme decrease in intake manifold vacuum during heavy acceleration permits the vacuum motor to override the thermostatic control. This opens the system to both heated air and air from the engine compartment.


  1. With the engine completely cold, look inside the cold air duct and make sure that the valve plate is fully in the up position (closing the cold air duct).
  3. Start the engine and bring it to operating temperature.
  5. Stop the engine and look inside the cold air duct again. The valve plate should be down, allowing an opening from the cold air duct into the air cleaner.
  7. If the unit appears to be malfunctioning, remove it and examine it to make sure that the springs are not broken or disconnected, and replace the thermostat if all other parts appear intact and properly connected.