DESCRIPTION
All passenger cars are equipped with a system for preheating the air entering the carburetor or the fuel injection unit during cold engine operation.
This system maintains incoming air temperature to a point where the carburetor or fuel injection system can be kept lean to reduce hydrocarbon (HC) emissions, and reduces carburetor icing.
This system consists of an air cleaner assembly with integral air control door, vacuum control temperature sensor, vacuum motor, heat shroud (on exhaust manifold), heated air tube and vacuum hoses. Some models also use additional controls such as vacuum traps and cold weather modulators.
OPERATION
The air control door temperature sensor closes when the temperature of air entering the air cleaner is less than the calibrated temperature of the temperature sensor. This allows engine vacuum to operate the air control door vacuum motor, and warm manifold air to be routed to the carburetor.
When engine vacuum is applied to the vacuum motor, the air control door closes off the intake of outside air. Air is then drawn into the air cleaner from around the exhaust manifold.
As air inside the air cleaner warms, the temperature sensor begins to open, bleeding off vacuum to the vacuum motor. As vacuum to vacuum motor decreases, the air control door begins to open.
As air control door opens, outside air is allowed to enter air cleaner assembly. When air entering air cleaner reaches a predetermined temperature, the air control door opens completely, and closes off the intake of heated air.
Thermostatic Air Cleaner Assembly With Temperature Sensor & Vacuum Motor. Scheme 49
VACUUM CONTROL TEMPERATURE SENSOR
The vacuum control temperature sensor controls the operation of the air control door. During initial start-up situations, this valve directs engine vacuum to the air control vacuum motor. The motor closes the air intake door, allowing the intake of heated manifold air. When the intake air temperature reaches a precalibrated value, this valve opens, allowing the intake of cooler outside air.
- Tape a thermometer close to vacuum control temperature sensor located inside air cleaner. Leave wing nut(s) off top of air cleaner so that top can be quickly removed to read thermometer during test.
- With engine cold, temperature below vacuum control temperature sensor specifications, check air control door in air cleaner. It should be in fully open position (open to outside air).
- Start engine. As soon as engine starts, door should move to fully heated air position (closed to outside air). Continue running engine and watch air control door. When door reaches fully open position, quickly remove air cleaner top and read thermometer.
- Compare thermometer reading with specifications. If reading is not to specification, perform vacuum motor test. If vacuum motor is okay, replace sensor.
VACUUM MOTOR TEST
- Remove air cleaner from vehicle. Disconnect vacuum hose from vacuum motor. Apply 20 in. Hg vacuum to motor and pinch off hose. Vacuum should not leak down more than 10 in. Hg in 5 minutes. If vacuum motor does leak down, replace it.
- Connect a vacuum pump to vacuum motor. Apply specified amount of vacuum to vacuum motor to close heated air door. See the AIR CONTROL DOOR CLOSING VACUUM table. If door does not close at specified vacuum, replace vacuum motor.
| Application | Vacuum In. Hg |
|---|---|
| General Motors | 7 |
AIR CONTROL DOOR CLOSING VACUUM
SPECIFICATIONS
| Application | Heated Air Temp.: °F (°C) | Fresh Air Temp.: °F (°C) |
|---|---|---|
| All Models | 86 (30) | 131 (55) |
VACUUM CONTROL TEMPERATURE SENSOR SPECIFICATIONS