GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Thermostatic Air Cleaner (THERMAC)

Print

OPERATION



See Figures 1, 2 and 3

Most of the carbureted and TBI engines covered by this manual use an air intake system which is designed to draw cool air from the exterior of the engine compartment during most engine operating conditions, but which draws heated air during engine warm-up. On these thermostatic controlled air cleaners, fresh air comes either from the snorkel, which supplies the engine with outside air, or a tube underneath the snorkel, which is connected to a heat stove surrounding the exhaust manifold. The purpose of the heat stove is to supply the engine with warm air during cold running to reduce choke-on time and bring the engine to normal operating temperature as quickly as possible.

An actuator motor in the snorkel operates the door which regulates the source of incoming air. The snorkel door is controlled either by a temperature sensor actuator or a vacuum diaphragm motor. The temperature sensor actuator is controlled by means of a self-contained, wax pellet assembly mounted in the air cleaner. When the engine is cold, the wax material sealed in the actuator is in a solid state, causing the damper to close off the cold air inlet. All air supplied to the engine is routed through the heat stove. As the incoming air warms the wax material, it changes to a liquid state and expands forcing out the piston to reposition the damper, allowing a hot/cold air mix or all cold air to enter the engine.



Click image to see an enlarged view

Fig. Fig. 1: Thermostatic air cleaner assemblies use a snorkel (such as this which runs to the grille)



Click image to see an enlarged view

Fig. Fig. 2: Vacuum controlled THERMAC operation



Click image to see an enlarged view

Fig. Fig. 3: Wax-pellet actuator operation

The vacuum diaphragm operated system uses a vacuum operated motor, mounted to the snorkel, to operate the damper door and a thermostatic bi-metal switch inside the air cleaner. When the engine is cold, the bi-metal switch is cold allowing vacuum to be supplied to the motor. As that the engine warms up, the bi-metal switch opens and bleeds off the vacuum. A spring inside the vacuum motor overcomes the vacuum and opens the damper door to the cold air inlet. Some applications use a delay valve on the hose connecting the vacuum motor to the temperature sensor. When vacuum in this hose drops for any reason, the check valve will bleed off the vacuum to the vacuum motor slowly. On most vehicles, the snorkel is connected to a tube, which is routed to the front of the vehicle in order to take in cooler air from outside the engine compartment.

Incorrect Thermostatic Air Cleaner (THERMAC) operation can result in warm up hesitation, lack of power or sluggishness on a hot engine. Inspect the system for any of the following:

  1. Disconnected heat stove tube.
  2.  
  3. Vacuum diaphragm motor or temperature sensor actuator inoperative.
  4.  
  5. No damper door operation.
  6.  
  7. Absence of manifold vacuum source.
  8.  
  9. Air cleaner not seated properly.
  10.  

TESTING



Vacuum Actuated THERMAC
MOTOR
  1. With the engine OFF , disconnect the hose from the vacuum diaphragm motor.
  2.  
  3. Using a vacuum source (such as hand vacuum pump), apply 7 in. Hg to the vacuum motor; the door should close and block off the outside (cold) air, completely.
  4.  
  5. Bend the vacuum hose (to trap the vacuum in the motor) and make sure that the door stays closed; if not, replace the vacuum motor.
  6.  

Before replacing the vacuum motor (if defective), be sure to check the motor linkage for binding.

  1. If the vacuum motor is OK and the problem still exists, check the temperature sensor.
  2.  

TEMPERATURE SENSOR
  1. Remove the air cleaner cover and place a thermometer near the temperature sensor; the temperature MUST be below 86°F (30°C). When the temperature is below specification, install the air cleaner.
  2.  
  3. Start the engine and allow it to idle. Watch the vacuum motor door, it should close immediately (if the engine is cool enough).
  4.  
  5. When the vacuum motor door starts to open, remove the air cleaner cover and read the thermometer, it should be about 131°F (55°C).
  6.  
  7. If the door does not respond correctly, replace the temperature sensor.
  8.  

Wax Pellet Actuated THERMAC
  1. Remove the air cleaner from the engine and allow it cool to a temperature below 40°F (4°C). The damper door should close fully to block all outside (cold) air.
  2.  
  3. Check the condition of the air cleaner base gasket.
  4.  
  5. Install the air cleaner and connect the heat stove tube at the snorkel and exhaust manifold.
  6.  
  7. Start and run the engine, observing the air cleaner. As the air cleaner warms with the engine the damper door should open slowly to allow outside (cold) air.
  8.  
  9. If the air cleaner fails to operate properly check that the calibrated spring is properly installed and that the damper door is not binding. If this does not remedy the problem, replace the wax pellet assembly and recheck for proper operation.
  10.  

REMOVAL & INSTALLATION



THERMAC Assembly

See Figures 4, 5, 6 and 7

  1. Remove the air cleaner cover and element from the housing assembly.
  2.  
  3. Disconnect the fresh air hose and heat stove pipe from the housing snorkel.
  4.  
  5. Tag and disconnect the hoses and vacuum lines, as necessary, from the air cleaner housing.
  6.  
  7. Remove the THERMAC housing assembly from the engine.
  8.  

To install:
  1. Make sure THERMAC base gasket is in good condition and is properly positioned on the carburetor or throttle body.
  2.  
  3. Install the THERMAC housing assembly to the engine.
  4.  
  5. Connect the hoses and vacuum lines as tagged during removal.
  6.  
  7. Connect the fresh air hose and heat stove pipe to the air cleaner.
  8.  
  9. Install the filter element and cover to the assembly.
  10.  



Click image to see an enlarged view

Fig. Fig. 4: Disconnect the fresh air hose from the snorkel and, if necessary, the vacuum hose from the motor



Click image to see an enlarged view

Fig. Fig. 5: Tag and disconnect the remaining hoses from the THERMAC air cleaner housing



Click image to see an enlarged view

Fig. Fig. 6: On some engine, multiple lines connect to a common fitting, on these remove the fitting



Click image to see an enlarged view

Fig. Fig. 7: If equipped with a vacuum motor, don't forget the temp sensor line (often run underneath the housing)

Vacuum Motor
  1. Remove the air cleaner housing assembly from the engine.
  2.  
  3. Disconnect the vacuum hose from the motor.
  4.  
  5. Drill out the spot welds with a 1 / 8 in. bit, 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 in. hole in the snorkel tube at the center of the vacuum motor retaining strap.
  2.  
  3. Insert and engage 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.

  1. Connect the vacuum hose to the motor.
  2.  
  3. Install the air cleaner assembly to the vehicle making sure the hoses are properly seated. Pay close attention to the brittle heat stove to make sure it is not damaged and it is fully seated on both the exhaust manifold and the snorkel.
  4.  

Temperature Sensor
  1. Remove the air cleaner.
  2.  
  3. Disconnect the hoses from the sensor.
  4.  
  5. Pry up the tabs on the sensor retaining clip, then remove the clip and sensor from the air cleaner.
  6.  

To install:
  1. Install the sensor to the vehicle using a new retaining clip, then secure the tabs to hold the sensor in place.
  2.  
  3. Connect the air cleaner hoses to the temperature sensor.
  4.  
  5. Install the air cleaner assembly to the vehicle making sure the hoses are properly seated. Pay close attention to the brittle heat stove to make sure it is not damaged and it is fully seated on both the exhaust manifold and the snorkel.
  6.  

Wax Pellet Actuator
  1. Remove the THERMAC housing assembly from the engine.
  2.  
  3. If the application uses a plastic heat tube elbow, use a 1 / 8 in. drill bit to drill out the 2 rivets securing the heat tube and elbow.
  4.  
  5. Use a 1 / 8 in. drill bit to drill out the 2 rivets securing the wax pellet carrier assembly.
  6.  
  7. Disconnect the blow down spring, then remove the wax pellet carrier assembly.
  8.  

To install:
  1. Check the spring clip on the hot air damper and replace, as necessary.
  2.  
  3. Install a new blow down spring, then install and secure the wax pellet carrier assembly.
  4.  
  5. If applicable, install the plastic heat tube elbow and secure using new pop rivets.
  6.  
  7. Install the THERMAC assembly to the engine and verify proper operation.
  8.  

 
label.common.footer.alt.autozoneLogo