HVAC Mysteries:
Air Out the Vents
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.