GM Corvette 1963-1982 Repair Guide

Thermostatic Air Cleaner (THERMAC or TAC) System

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OPERATION



See Figures 1, 2 and 3

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Fig. Fig. 1: Cross-sectional view of the THERMAC system



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Fig. Fig. 2: Pre-heated air system operation



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Fig. Fig. 3: Air cleaner assembly mounting-1982 vehicle shown

High performance engines equipped with open-element air cleaners do not employ this system.

This system is designed to improve driveability and exhaust emissions when the engine is cold. Components added to the basic air cleaner assembly include a temperature sensor (connected to a manifold vacuum source), vacuum diaphragm motor (connected to the temperature sensor), and an inlet damper door (installed in the air cleaner inlet snorkel). Additional components of the system include an exhaust manifold-mounted heat stove and a hot air duct running from the heat stove to the underside of the air cleaner snorkel.

When the engine is cold, the temperature sensor allows vacuum to pass through to the vacuum diaphragm motor. The vacuum acting on the vacuum motor causes the motor to close the damper door, which prohibits the introduction of cold, outside air to the air cleaner. The intake vacuum then pulls hot air, generated by the exhaust manifold, through the hot air duct and into the air cleaner. This heated air supply helps to more effectively vaporize the fuel mixture entering the engine. As the engine warms, the temperature sensor bleeds off vacuum to the vacuum motor, allowing the damper door to gradually open.

The usual problems with this system are leaking vacuum lines (which prevent proper operation of the sensor and/or motor); torn or rusted-through hot air ducts and/or rusted-through heat stoves (either condition will allow the introduction of too much cold air to the air cleaner). Visually check and replace these items as necessary. Should the system still fail to operate properly, disconnect the vacuum line from the vacuum motor and apply at least 7 in. Hg. of vacuum directly to the motor from an outside vacuum source; the damper door should close. If the door does not close, either the vacuum motor is defective or the damper door and/or linkage is binding. If the door closes, but then gradually opens (with a steady vacuum source), the vacuum motor is defective.

REMOVAL & INSTALLATION - Vacuum Motor



See Figure 4

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Fig. Fig. 4: THERMAC vacuum motor removal and installation

  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.  

To install:
  1. Drill a 7 / 64 " hole in the snorkel tube as the center of the vacuum motor retaining strap.
  2.  
  3. Insert the vacuum motor linkage into the control damper assembly.
  4.  
  5. Use the motor retaining strap and a sheet metal screw to secure the retaining strap and motor to the snorkel tube.
  6.  

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

Temperature Sensor See Figure 5

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Fig. Fig. 5: Pry the sensor retaining clip tabs with a suitable prytool, then remove the clip and sensor from the air cleaner

  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.  

 
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