HVAC Mysteries: Air Out the Vents

What in the world goes on inside General Motors S-Series vehicles?
A common question on S-series message boards is why air still
comes out the vents when the HVAC control is set to OFF. Those who
notice such things usually mention that this only happens when the
vehicle is in forward motion. There's a very good reason for this. Let's
start with some diagrams:
This is a diagram of a 1998 and newer S-series HVAC system. It
shows how air flows from the blower fan through the various doors
that lead to the dash vents. In the bottom right corner of the diagram
above, the "recirculation door" is visible. This door is normally open to
the outside of the vehicle, which means the blower fan is pulling in
outside air. The only time the blower fan
doesn't pull outside air is
when the MAX AC setting is selected on the HVAC control. This
setting simply shuts the recirculation door and pulls air from inside
the cab. The idea here is that you'll get better performance out of your
air conditioner if you're not trying to cool hot air from the outside.

So any other time, the door is open to outside air. This is evident in
the next diagram:
The above diagram shows which of the actuators receive vacuum
under various HVAC control settings. The actuator which powers the
recirculation door receives vacuum from Port 5 (Recirc)
only when the
MAX setting is selected. Even in the OFF position, the door is open.
Therefore, when the vehicle is moving forward, outside air is forcing its
way through the ducts. As the diagram shows, Port 4 (Heat) still
receives vacuum in the OFF position. Therefore, expect to feel air at
your feet when the HVAC control is set to OFF and the vehicle is
moving down the road.

As a side note, the temperature control still works when the HVAC
control is set to OFF. So the air forced through the system can still be
heated or cooled (although not cooled by the air conditioner - it does
not run when OFF is selected).

Here are two similar diagrams for 1997 and earlier second-generation
S-series vehicles.