Vacuum Operated Thermostatic

This test must be performed with ambient air temperature below 60° F. Do not immerse any part of the vacuum operated duct and valve assembly in water.
1. On systems with Cold Weather Modulator, bypass the modulator by connecting the two vacuum lines from the modulator.
2. Apply parking brake and block wheels, then remove air cleaner cover and element. Check heat riser tube and zip tube for proper installation or damage. Repair or replace as necessary.
3. Remove components necessary to observe position of duct door. On vacuum operated units, disconnect vacuum line at duct and valve. Check that duct door is in the open (fresh air side) position. If not, check for binding or sticking. Repair or replace as necessary.
4. Place a magnetic based thermometer as near as possible to temperature sensor on inlet side. If air cleaner housing is aluminum, use tape to secure thermometer. Do not allow magnet to touch temperature sensor.
5. Start engine. If duct door is in the heat on position, proceed to step 7. If duct door is in the heat off position, turn engine off. Cool sensor temperature by spraying with Refrigerant 12 for 20 seconds. Do not spray refrigerant with engine running, as any refrigerant drawn into the carburetor and burned in the engine will produce poisonous phosgene gas. Conduct this procedure in well ventilated area.
6. Restart engine. If duct door is in the heat off position during idle, check vacuum hoses.
7. Reinstall air cleaner cover without wing nut. With engine running, observe duct door. When door starts to open, remove air cleaner cover and note temperature reading on thermometer. Temperature reading should not exceed the following specifications:
a. For brown color coded sensors, temperature should not exceed 75° F.
b. For black or pink color coded sensors, temperature should not exceed 90° F.
c. For blue or yellow or green color coded sensors, temperature should not exceed 105° F.
d. If temperatures are greater than those specified above, replace sensor.
8. If duct door does not start to open within 5 minutes after performing step, remove air cleaner cover and note temperature. If temperature is less than specified above, the temperature is insufficient to operate the duct door. Reinstall air cleaner cover without wing nut and continue to run engine until specified temperature is reached. If duct door has not started to open by then, replace sensor.