WE ALL NEED MORE AIR

[LQQKatJon]

You said you mounted it differerntly then Tom? Can you post a picture?

I have a 75, and I just cant put my thoughts around the max air vent. Mine has a large floor ioening that I would consiter the recirc opening.

Sometime before next spring I am really going to have to test and learn what the vacuum flapper vents are supposed to do, and start there. I still
plan on adding a aux evaporator, but having the oem working better would allow more ease and obviously help heat/defrost side.

I also wonder if maybe there are mouse nests one cannot see. I know in a 91 f-150 they can clog up vents that you never knew existed. I have had
the front of my hvac box opened in the past year and cleaned things up and replaced the gasket on the heat/ac flapper.
--
Jon Roche
75 palm beach
St. Cloud, MN
http://lqqkatjon.blogspot.com/

 

[kab7]

> You said you mounted it differerntly then Tom? Can you post a picture?
>
> I have a 75, and I just cant put my thoughts around the max air vent. Mine has a large floor ioening that I would consiter the recirc opening.
>
>
> Sometime before next spring I am really going to have to test and learn what the vacuum flapper vents are supposed to do, and start there. I
> still plan on adding a aux evaporator, but having the oem working better would allow more ease and obviously help heat/defrost side.
>
> I also wonder if maybe there are mouse nests one cannot see. I know in a 91 f-150 they can clog up vents that you never knew existed. I have
> had the front of my hvac box opened in the past year and cleaned things up and replaced the gasket on the heat/ac flapper.

I believe the "max air" slot change was done sometime in 75, maybe pretty late. There was also a kit available, which I think basically
did the max air modification to existing boxes. If you have no air coming out of that bottom slot with the controls set all the way to the left
on "recirc", either the vac control to the max air door is broken, or you have the earlier style box, which uses the big slot below the dash as a
recirc intake.
If you have the max air version, there will be a flapper door just inside the slot on the left 12" or so, and the old vacuum pot for the recirc door
on the blower side of the box is used to operate the max air door.

Without the max air slot, either moving the old blower to the front of the box as Mark Wall did, or installing the new blower as Tom did, would
improve your airflow somewhat. Unfortunately, the real problem is the ducting.

Anyway, here are pics of my installation directly on the back of the ac box, right in front of the slot below the dash.
In that position, the recirc slot would basically become a fresh air intake for the blower when the outside "recirc" door is open:
http://www.gmcmhphotos.com/photos/hvac-original-and-mods/p63546-new-blower-arrangement.html
http://www.gmcmhphotos.com/photos/hvac-original-and-mods/p63548-new-blower-arrangement.html
http://www.gmcmhphotos.com/photos/hvac-original-and-mods/p63547-new-blower-arrangement.html

Look at the the diagrams in my album as well as the pictures of various boxes and you should get a much better understanding.
One caveat: in Brand's "Separating Hot from Cold", he basically drew the 77/78 duct plenum on all the boxes. For the earlier boxes,
the vacuum controlled doors are spread around the dash, inside and out, and there's no plenum on the cockpit side of the box.

hth,
Karen
1975 26'