Thermostatic Air Cleaner: Testing and Inspection

1. Apply the parking brake and block the wheels.

2. Remove the air cleaner cover and filter element. Inspect the heated air duct for proper installation and/or damage. Service the duct if needed.

3. Remove components as necessary to ensure that the duct door is in the open to fresh air position.

4. If the door is in the open to heated air position, check the air door and its linkage for binding and/or sticking.
Service or replace the vacuum motor/ air door assembly as needed.

5. Check the vacuum source and condition of the vacuum hoses to the bimetal sensor, Cold Weather Modulator (CWM) or retard delay valve, and vacuum motor.
If any of the vacuum hoses show signs of damage and/or deterioration, replace them.

NOTE: The engine should be cold or completely cool prior to performing the test.

6. Start the engine and let it idle. Note the position of the air duct door.
a. If the door moves to the open to heated air position, proceed to the next step.

System Operation Test:

b. If the door remains in the closed to heated air position, place a finger over the bleed vent of the bimetal sensor.
The air duct door should move immediately to the open to heated air position. If not, stop the engine and test the vacuum motor.


7. Turn OFF the engine and observe the duct door.
a. If the vehicle is equipped with a retard delay valve: the air door will return slowly (10 to 30 seconds) to the closed to heated air position.
b. If the vehicle is equipped with a cold weather modulator: the air door will stay in the open to heated air position for at least two minutes. If less than two minutes, replace the CWM and repeat this test, making sure that the bimetal sensor and CWM are both completely cooled.