heating problem
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Jeff,
The HVAC controls in your car are operated by vacuum. Each outlet and bypass door in the duct work has a small vacuum diaphragm motor that operates the door open or closed (except for the temperature control door, which is direct-cabled). Each of these vacuum motors has a spring to return the door to its "home" or default position. Applying vacuum moves the door one way, removing vacuum allows the spring to return the door the opposite way.
The HVAC selector position switch is also a ported vacuum valve that routes the vacuum to the correct door for defrost, heat, A/C, MAX, Bi-Level, etc. The source for the vacuum is the intake manifold or plenum. There is a small (about ¼") hose connected to the intake. If this hose is disconnected or damaged, the HVAC controls will assume their default positions, causing all air to exit the defroster ducts. The defrost position is the default so that if the system fails, driving safety can still be assured since the windows can be kept clear.
Under normal circumstances, when the HVAC controls are moved to a different position, you should be able to hear a slight hissing noise inside the vehicle as the vacuum is vented from one motor and routed to another. If you don't hear any noise when the control is moved, chances are that the vacuum source hose is disconnected, connected incorrectly, or pinched or damaged in the engine compartment. If you hear a constant hissing noise, there is likely a vacuum leak inside the passenger compartment. This can be from a disconnected or damaged hose, or a failing ported vacuum valve in the HVAC controls. In either case, you would need to remove the HVAC control cluster and/or the lower instrument panel trim to access the hoses and controls.
The HVAC controls in your car are operated by vacuum. Each outlet and bypass door in the duct work has a small vacuum diaphragm motor that operates the door open or closed (except for the temperature control door, which is direct-cabled). Each of these vacuum motors has a spring to return the door to its "home" or default position. Applying vacuum moves the door one way, removing vacuum allows the spring to return the door the opposite way.
The HVAC selector position switch is also a ported vacuum valve that routes the vacuum to the correct door for defrost, heat, A/C, MAX, Bi-Level, etc. The source for the vacuum is the intake manifold or plenum. There is a small (about ¼") hose connected to the intake. If this hose is disconnected or damaged, the HVAC controls will assume their default positions, causing all air to exit the defroster ducts. The defrost position is the default so that if the system fails, driving safety can still be assured since the windows can be kept clear.
Under normal circumstances, when the HVAC controls are moved to a different position, you should be able to hear a slight hissing noise inside the vehicle as the vacuum is vented from one motor and routed to another. If you don't hear any noise when the control is moved, chances are that the vacuum source hose is disconnected, connected incorrectly, or pinched or damaged in the engine compartment. If you hear a constant hissing noise, there is likely a vacuum leak inside the passenger compartment. This can be from a disconnected or damaged hose, or a failing ported vacuum valve in the HVAC controls. In either case, you would need to remove the HVAC control cluster and/or the lower instrument panel trim to access the hoses and controls.
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