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My air conditioner is working but only through the top vents not the front vents. Can not locate the vacumn pump . 2000 GMC Safari Van

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If the heat vents are controlled by vacuum, the problem is most likely a lack of vacuum to the control panel, or possibly a bad control panel. These are usually designed to send all the air to the defroster if there isn't vacuum to the system. This is probably so you can keep the windshield clear so you can see to drive if there is a problem with the system.

The system gets its vacuum from engine manifold vacuum. This often comes from a manifold vacuum block mounted on the firewall in the engine compartment, or on the back of the engine close to the firewall. If you start the engine and listen for a vacuum leak (hissing noise) under the dash or under the hood near the back of the engine, you may find the leak. It might be a hose that fell off a fitting, a split hose, a broken fitting, or a leaking vacuum canister (these are often mounted under the hood on an inner fender, and are typically about the size of a quart jar, and may be molded plastic or a "tin can" that looks like a juice can with a vacuum hose connected to it). If you find a vacuum leak and fix it, that may solve the problem.

If your HVAC (Heating, Ventilation, and Air Conditioning) system is controlled by electric motors or actuators, rather than vacuum, you may have a problem with a sticking or defective motor or actuator. There are typically 4 or 5 of these that operate the doors in the ductwork to direct the airflow to the different zones, to control air flow through the heater core, and to switch to recirculate mode for AC.

Posted on Sep 15, 2009


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