Chevrolet Full-size Cars 1968-1978 Repair Guide

Thermostatic Air Cleaner

Print

OPERATION





Click image to see an enlarged view

Fig. Fig. 1 Schematic of the vacuum motor operation on the THERMAC air cleaner assembly

All engines utilize the THERMAC system (in 1978 it was called TAC, but was the same). This system is designed to warm the air entering the carburetor when underhood temperatures are low, and to maintain a controlled air temperature into the carburetor at all times. By allowing preheated air to enter the carburetor, the amount of time the choke is on is reduced, resulting in better fuel economy and lower emissions. Engine warm-up time is also reduced.

The Thermac system is composed of the air cleaner body, a filter, sensor unit, vacuum diaphragm, damper door, and associated hoses and connections. Heat radiating from the exhaust manifold is trapped by a heat stove and is ducted to the air cleaner to supply heated air to the carburetor. A movable door in the air cleaner case snorkel allows air to be drawn in from the heat stove (cold operation) or from underhood air (warm operation). The door position is controlled by the vacuum motor, which receives intake manifold vacuum as modulated by the temperature sensor.

SYSTEM CHECKS





Click image to see an enlarged view

Fig. Fig. 2 View of the THERMAC air cleaner components

  1. Check the vacuum hoses for leaks, kinks, breaks, or improper connections and correct any defects.
  2.  
  3. With the engine off, check the position of the damper door within the snorkel. A mirror can be used to make this job easier. The damper door should be open to admit outside air.
  4.  
  5. Apply at least 7 in. Hg of vacuum to the damper diaphragm unit. The door should close. If it doesn't, check the diaphragm linkage for binding and correct hookup.
  6.  
  7. With vacuum still applied and the door closed, clamp the tube to trap the vacuum. If the door doesn't remove closed, there is a leak in the diaphragm assembly.
  8.  

REMOVAL & INSTALLATION





Click image to see an enlarged view

Fig. Fig. 3 Make sure the hose is connected securely between the air cleaner and heat stove

Vacuum Motor
  1. Remove the air cleaner.
  2.  
  3. Disconnect the vacuum hose from the motor.
  4.  
  5. Drill out the spot welds with a 1 / 8 " hole, then enlarge as necessary to remove the retaining strap.
  6.  
  7. Remove the retaining strap.
  8.  
  9. Lift up the motor and cock it to one side to unhook the motor linkage at the control damper assembly.
  10.  
  11. To install the new vacuum motor, drill a ;of 7 / 64 ;cf" hole in the snorkel tube as the center of the vacuum motor retaining strap.
  12.  
  13. Insert the vacuum motor linkage into the control damper assembly.
  14.  
  15. Use the motor retaining strap and a sheet metal screw to secure the retaining strap and motor to the snorkel tube.
  16.  

Make sure the screw does not interfere with the operation of the damper assembly. Shorten the screw if necessary.

Temperature Sensor
  1. Remove the air cleaner.
  2.  
  3. Disconnect the hoses at the air cleaner.
  4.  
  5. Pry up the tabs on the sensor retaining clip and remove the clip and sensor from the air cleaner.
  6.  
  7. Installation is the reverse of removal.
  8.  

 
label.common.footer.alt.autozoneLogo