DESCRIPTION
Fresh air or heated air is made available to engine by ducting which directs air into air cleaner assembly. Air temperature is controlled by temperature-sensitive vacuum system that operates duct valve.
Vacuum-operated duct can select cool air from outside through pick-up tube, or warm air from shroud around exhaust manifold. System consists of shroud, air cleaner assembly with vacuum motor, duct and valve assembly, temperature sensor and cold-weather modulator (some models). (Scheme 64)
OPERATION
When engine is cold, air is selected from exhaust manifold shroud. In "open" position, vacuum applied to vacuum motor operates duct valve. (Scheme 65) Duct valve shuts off fresh air supply and opens, allowing heated air to enter air cleaner.
As engine warms up, sensor operates, preventing vacuum from being applied to vacuum motor. In this "closed" position, duct valve closes off supply of heated air, allowing air from outside to flow through pick-up tube into air cleaner. (Scheme 65)
Cold-weather modulator on some models, controls operation of duct valve under certain air temperature conditions for improved emission control.
Vacuum-Operated Air Cleaner Assembly (Typical V8 Assembly Shown; Others Similar) Duct valve switches from heated air to fresh air. Scheme 64
Closed & Open Operation of Duct Valve. Scheme 65
AIR CLEANER TEMPERATURE SENSOR
Temperature sensor should allow vacuum to close duct door to fresh air at ambient temperatures below 75°F (24°C). Sensor should bleed off vacuum, allowing duct door to open to fresh air, at or above 75°F (24°C) on Brown valve, 90°F (32°C) on Pink or Black valve, or 105°F (41°C) on all other colors. If valve does not perform to specifications, replace valve and retest.
AIR CLEANER COLD WEATHER MODULATOR
Vacuum of 16 in. Hg applied to vacuum motor side of modulator should hold or leak as follows
| Valve Color | Holds Vacuum | Leaks Vacuum |
|---|---|---|
| Black | Below 20°F (-7°C) | Above 35°F (2°C) |
| Blue | Below 40°F (4°C) | Above 55°F (13°C) |
| Green | Below 50°F (10°C) | Above 76°F (24°C) |
| Yellow | Above 65°F (18°C) | Below 50°F (10°C) |
MODULATOR SWITCHING TEMPERATURE
VACUUM MOTOR
Disconnect vacuum hose from vacuum motor connector tube. Apply 16 in. Hg vacuum and trap. Vacuum motor should remain closed for 60 seconds. If not, replace duct and valve assembly.