HVAC Recirculate Switch
Updated May 15, 2007
This works with the newer vacuum controlled HVAC systems from 1997.5 and newer.
I have a 2000 open top. I just found a way to make my Hummer's front blower blow harder on all the settings including the awful defroster setting. I took the passenger side dash apart and examined the operation of the system and noticed that it blows much harder when set on "max ac". The reason for this is that when set to max ac the system uses cabin air instead of outside air. On most vehicles there is a switch giving the operator an option between using outside air or cabin air. Mine does not have one. I do not know if later years have this option.
I turned the switch to floor heat and then used my hand to manually hold the door directly above the blower motor open ( this is the door that switches you between inside/ outside air). It instantly blew out of the vents much harder. I switched the control through all settings including defrost (which as most know barely works) and it worked much better.
All the doors operate with vacuum diaphragms. I wanted the door to be open to interior air anytime the system was on so that the system blew harder. If I wanted it to blow less I could just use the fan speed switch.
There are two vacuum feed lines going to the system controls on the dash. One is white and one is black. These vacuum lines run right past the blower motor as well. I simply cut the black one and inserted a "T" fitting. I then patched it into the switch. Put a T in the red line and run one side to the door and the other to the switch. Now you can select when you want inside or outside air. In defrost mode dryer outside air is usually better. Thanks to Joe Essert for this Freightliner Part Number ARS-BPV8B.
On my '03 unit, the line going to the fresh air door is pink. So, before you start cutting lines look for the one going to the fresh air door (top right, just behind the air intake stack.) Have an assistant move the controls from normal AC to Max AC and watch for the vacuum motor that moves. That'll be the one.
My defroster actually works fine now and the cab heats up quicker. Another benefit is that you are not sucking in dusty air when following other trucks off road.
In the summer after your truck has been sitting when the outside air is cooler then the inside air you should start the A/C using outside air until it begins to cool off. Then switch to recirc.