The thermostatic air cleaner (Thermac) is on all gasoline engines. This system uses a damper assembly in the air cleaner inlet, controlled by a vacuum motor to mix preheated and cold air entering the air cleaner. This is necessary to maintain a controlled air temperature into the carburetor. The vacuum motor is controlled by a temperature sensor in the air cleaner. The preheating of the air cleaner inlet air allows leaner carburetor and choke settings, which result in lower emissions, while maintaining good driveability.
- Either start with a cold engine or remove the air cleaner from the engine and allow to cool for at least 30 min. While cooling the air cleaner, leave the engine compartment hood open.
- Tape a thermometer inside of the air cleaner so that it is near the temperature sensor unit. Install the air cleaner on the engine but do not fasten its securing nut.
- Start the engine. With the engine cold and the outside temperature less than 90ºF, the door should be in the HEAT ON position (closed to outside air).
Due to the position of the air cleaner on some trucks, a mirror may be necessary when observing the position of the air door.
- Operate the throttle lever rapidly to 1 /2- 3 /4 of its opening and release it. The air door should open to allow outside air to enter and then close again.
- Allow the engine to warm up to normal temperature. Watch the door. When it opens to the outside air, remove the cover from the air cleaner. The temperature should be over 90ºF and no more than 130ºF; 115ºF is about normal. If the door does not work within these temperature ranges, or fails to work at all, check for linkage or door binding.
If binding is not present and the air door is not working, proceed with the vacuum tests, given below. If these indicate no faults in the vacuum motor and the door is not working, the temperature sensor is defective and must be replaced.Vacuum Motor Test
Be sure that the vacuum hose which runs between the temperature switch and the vacuum motor is not pinched by the retaining clip under the air cleaner. This could prevent the air door from closing.
- Check all of the vacuum lines and fittings for leaks. Correct any leaks. If none are found, proceed with the test.
- Remove the hose which runs from the sensor to the vacuum motor. Run a hose directly from the manifold vacuum source to the vacuum motor.
- If the motor closes the air door, it is functioning properly and the temperature sensor is defective.
- If the motor does not close the door and no binding is present in its operation, the vacuum motor is defective and must be replaced.
If an alternate vacuum source is applied to the motor, insert a vacuum gauge in the line by using a T-fitting. Apply at least 9 in. Hg of vacuum in order to operate the motor.
REMOVAL & INSTALLATION
Air Cleaner Assembly
See Figures 1, 2, 3 and 4
- Loosen the wing nut and remove the air cleaner cover.
- As applicable, disconnect the snorkel tube and air hoses from the air cleaner housing.
- Lift up slightly on the end of the housing and disconnect the heat stove from either the exhaust manifold riser or from the bottom of the air cleaner snorkel.
- Remove the air cleaner housing assembly from the vehicle.
- Install the air cleaner housing and connect the heat stove.
- Connect the snorkel tube and hoses to the air cleaner housing, as applicable.
- Make sure the air cleaner is properly seated in the housing, then install the cover and tighten the wing nut.
See Figures 5, 6 and 7
- Remove the air cleaner.
- Disconnect the vacuum hose from the motor.
- Drill out the two spot welds with a 1 /16 in. drill bit, then enlarge the hole as required to remove the retaining strap. Be careful not to damage the snorkel tube. Remove the retaining strap.
- Lift up motor, cocking it to one side to unhook the motor linkage at the control damper assembly.
- Drill a 2.8mm ( 7 /64 in.) hole in the snorkel tube at cen