GM Caprice 1990-1993 Repair Guide

Thermostatic Air Cleaner (THERMAC)

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See Figures 1, 2 and 3



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Fig. Fig. 1: THERMAC system - 1990-91



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Fig. Fig. 2: Schematic of the vacuum motor operation - 1992-93



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Fig. Fig. 3: Air cleaner operation - 1992-93

OPERATION



All engines use the THERMAC system. This system is designed to warm the air entering the manifold when underhood temperatures are low, and to maintain a controlled air temperature into the manifold at all times. By allowing preheated air to enter the carburetor or throttle body, the amount of time needed to bring the engine up to normal operating temperature is reduced, resulting in better fuel economy and lower emissions.

1990-91

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 or throttle body. A movable door in the air cleaner case snorkel allows air to be drawn in from the heat stove (cold operation). The door position is controlled by the vacuum motor, which receives intake manifold vacuum as modulated by the temperature sensor.

1992-93

This system regulates incoming air temperature without the use of vacuum; instead it uses a wax pellet actuator contained within the air cleaner assembly. Depending upon temperature, the wax pellet actuator changes its state from a solid to liquid state, which forces the actuator piston out and closes off the hot manifold air.

A vacuum door which remains open will cause throttle plate icing and poor cold driveability; a door which remains closed during normal engine operating temperatures can cause sluggishness, engine knocking and overheating.

SYSTEM CHECK



1990-91

Verify there is approximately 16 in. Hg. at the air cleaner temperature sensor at all times.

  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 the vacuum still applied and the door closed, clamp the tube to trap the vacuum. If the door doesn't remain closed, there is a leak in the diaphragm assembly.
  8.  
  9. Verify the temperature sensor within the air cleaner is below 86°F (30°C) and start the engine.
  10.  
  11. The damper door should close to outside air immediately if the sensor is cool enough.
  12.  
  13. When the damper door begins to open (within a few minutes), remove the air cleaner assembly and using a thermometer, verify the temperature of the sensor is 131°F (55°C).
  14.  
  15. If not as specified, the sensor must be replaced.
  16.  

1992-93
  1. Remove the air cleaner assembly and cool to 40°F (4°C).
  2.  
  3. Reinstall air cleaner and ensure the heat stove tube is connected to the air cleaner snorkel and exhaust manifold.
  4.  
  5. Start the engine and watch the door in the air cleaner.
  6.  
  7. As the wax pellet actuator warms up the door should begin to open slowly to outside air.
  8.  
  9. if not as specified, replace the assembly.
  10.  

REMOVAL & INSTALLATION



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 inch 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 1 / 16 inch 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. To install, position sensor into air cleaner.
  8.  
  9. Install retaining clip.
  10.  
  11. Connect the hoses to the air cleaner.
  12.  
  13. Install the air cleaner.
  14.  

 
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