Description
1. On models with Multec systems, the air temperature is controlled by a controller installed in the air unit. When the engine is cold started, the regulator is closed and intake manifold vacuum acts on the air temperature control valve in the air cleaner assembly. It uses a vacuum booster in the valve assembly to flap off cold inlet air, thus allowing only warm air from the exhaust manifold into the air filter.
2. As the temperature of the inlet air in the air unit rises, the bimetal plate in the thermal switch is distorted. The switch is activated and relieves the vacuum in the air temperature control valve. The flap begins to gradually shut off the warm inlet air. Until the engine is fully warmed up to normal operating temperature, only cold air is supplied to the air filter.
Examination
3. To check the system, allow the engine to cool completely, then remove the air filter cover; the flap must completely cover the hot inlet air. Start the engine. The flap should immediately lift up, blocking the flow of cold inlet air. And then, as the engine warms up, it should gradually lower until it again completely blocks the flow of hot inlet air.
4. To test the thermal switch, disconnect the vacuum control valve pipe from the switch coupler while the engine is running (on the back of the air unit). While the engine is not warmed up, a vacuum from the intake manifold should be felt on the coupling; after the engine is fully warmed up, vacuum should not be felt.
5. To check the air temperature control valve, remove the air filter cover; the flap must completely block the flow of hot inlet air. Disconnect the control valve vacuum tube from the switch connector on the back of the air unit, and suck air out of the tube with your mouth; the flap should lift up, blocking the flow of cold inlet air.
6. If any component is defective, replace it. This means that the lower air filter housing must be replaced if the air temperature control valve needs to be replaced, or the air unit housing must be replaced if the thermal switch needs to be replaced.
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