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Heated Air Comes From Vents Even When Saab A/C Is OnPosted 6/15/2000By Rich Cregar
Vehicle Symptom/Problem Test/Fix Background The customer's complaint is that unheated fresh air, or chilled A/C air, comes from the center vent but the air coming from the side vents remains warm, even when the air conditioning is operating at maximum. Here's what's happening: In the ventilation system of the Saab 900, all air flowing into the passenger compartment actually enters at the hood louvers and flows through the evaporator coil. Once past the coil, some of the air flow is diverted to the center dash vent, while the rest of the air is passed through the heater core and then distributed to the defrost outlets, floor outlets, and the left and right dash vents. If engine coolant is able to flow through the heater core, all air coming through the system will be heated except the air at the center vents. Keep in mind that these cars are designed and built in northern Europe where the summer climate is more moderate and air conditioning is rarely used. In winter, when heat is a necessity, the center vent is used to provide a cool flow of air to the face, which helps prevent driver fatigue. What you will usually discover is that the heater control valve is defective and no longer blocks flow through the core. Unlike most conventional heater valves, the Saab valve allows the coolant to continue to circulate from the head to the water pump by letting coolant flow through a bypass channel that is internal to the valve itself. Before condemning the valve as defective, remove the speaker grille (with defrost tube) on the driver's side and look through the windshield into the opening with a flashlight. Verify that the mechanical linkage from the knob to the valve is intact. The linkage rod is a double-jointed shaft with plastic retainers at both ends. During radio or instrument cluster servicing this rod can easily become disconnected at either end. Also, the plastic retainer at the rod ends can crack and then not drive the valve to its fully closed position. Operate the valve by hand and verify that it is not shutting off. Replacing the valve is not a difficult job technically - but it's not a job for anyone who can't stand to twist their bodies under a dashboard. An added tip: The little rocker controls that regulate airflow at the dash vents do not have any friction “drag” to them, and as a result, keep snapping to the “closed” position as the air flow pushes the air flap shut. This is very annoying. A short piece of matchstick from a matchbook tucked discreetly between the rocker and the vent will solve this problem.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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