Steve Clevenger's
Air Recirculation Modification

 

In the days the GMC's were manufactured auto air-conditioning was still in its infancy. Auto air-conditioning was not near as good then as it is today. High powered fans and re-circulated air were just beginning to be placed in systems. The GMC's were not one of the vehicles that received these changes and therefore it does not perform as well as systems do today. Some of us have made a few changes to help improve the performance of the air-conditioning in our coaches by installing a re-circulating system.

Before considering the air-condition re-circulation modification you should review and determine which of the 4 GMC systems you have. The modification may not give you any improvement. Zay C Brand has a very clear explanation of the different type of heating and air-conditioning systems on the GMC Western States web site. Please click HERE to view it.

This modification will be most beneficial on the 75-76 "Type 2" system.

Below is a list of the parts I used to make this modification and a short explanation of the steps I took to incorporate it. 
 Parts list:  Grentec #  Valtera #
All 3" sewer fittings    
(1) 45* clear elbow w/ quick disconnect

 40791

 T1026
(1) 90* elbow-hose connector

 40926
  
(2) couplings w/quick disconnect

 40740

 T1009
(1) coupling-straight (spigot)

 40723

 T1006
(1) Termination cap

 40727

 T1020
(1) 3' section of flexible sewer hose    
(1) piece of sheet metal cut to close intake hole on air box
(2) hose clamps for hose    
(18) 10-24 screws & nuts    
Parts were about $30. Any RV supply store or dealer will have them.

Remove front metal panel of air box, after removing windshield washer reservoir & any other stuff attached. There are a couple screws on top that are tricky to get out. Use fitting to mark area just below air intake & cut hole (this is TOUGH steel) for new straight intake fitting (I installed the fitting from the back side) & drill 4 holes for retaining screws & nuts. Install screws & nuts before re-installing panel. The sheet metal to cover intake will have to be cut & fitted & bent, then I used screws & nuts to fasten to panel. Some gap is O.K.--it will only let some outside air in. When the front panel is off, you can see how the system works & the air flow pattern changes when the heater is selected--the damper door drops down & allows air to enter from the heater core. Good time to check the vacuum actuators inside the box. On the '76, you can reach the door that opens into the air distribution duct inside the coach, & replace the foam if you wish (I didn't think it was an issue--the air doesn't have anywhere else to go.)

Now, use the other coupling with the quick disconnect to mark the hole in the defroster vent (this is black plastic & much easier to cut <G>.) Then, drill the 4 holes for the mounting screws & install it flush on the front (easy to reach inside & hold the nuts.) You'll be able to look in the hole & see the floorboard.

Cut the flexible hose to proper length (leave a little slack) & attach one end to the clear 45* quick disconnect fitting & the other end to the 90* straight ended fitting & tighten hose clamps. Just push the 90* elbow into the fitting on the air box.

I attached another quick disconnect coupling above the grill on the inside. I drilled 2 holes for the attachment screws & mounted it flat side facing down.

If you want recirculated air, hook the hose to the defroster fitting. If you wish to restore to original, remove the hose & hook it to the grill fitting, where it will pick up fresh air, & put the cover on the defroster quick disconnect--this will force air up the defroster vent.

 

Recirculation Mode

"If you want recirculated air, hook the hose to the defroster fitting. If you wish to restore to original, remove the hose & hook it to the grill fitting, where it will pick up fresh air, & put the cover on the defroster quick disconnect--this will force air up the defroster vent and allow heat for the passenger."

Fresh Air Mode

Fresh Air Mode Detail

"I attached another quick disconnect coupling above the grill on the inside. This connection is used for fresh air when you are running the heater and defroster. I drilled 2 holes for the attachment screws & mounted it flat side facing down."