[77newell]

Having spent a few more hours with the dash HVAC system I now have a pretty good understanding of how it operates. My intention here is simply to share what I have learned.

The system is made up of several pieces (for now I'm leaving out most of the AC system and concentrating on the heating and ventilating systems) I will go into detail about each in a moment:
1) A metal box in front of the cooling fan for the generator.
2) Controls in the cockpit
3) A vacuum source

The metal box in front of the generator fan contains five parts or segments inside it. Starting at the bottom is the air inlet grid on the rear side. Inside this part is also the vacuum controlled damper that controls the inlet of outside air. When the "VENT" button is pushed vacuum causes the damper to open. Return air from the coach comes through a vent under the co-pilot seat and forward through a double floor cavity into the area above the inlet grid to the box.

Just above the inlet compartment of the box is the AC cooling coil.

Just above that is the heating coil that uses hot engine coolant to heat air for either heating or windshield defrosting. The flow of hot water has two controls; the boost pump in the engine compartment that comes on when the temperature control on the dash is pushed toward "hot" a bit and a metering valve in the coolant line controlled by a wire moved by that same lever.

Above the heating coil is a plenum or box containing multispeed electric fans. The two nearest the curb-side are controlled by the fan switch on the heating control panel on the dash. They supply air to either the heating or windshield defroster outlets. The fan furthest from the curb is controlled (as Forest rightly suggested) by the switch marked "AUX VENTS".

Above the fan plenum is are two distribution plenums with flexible ducts arising from their tops. The front row of ducts on the plenum nearest the curb side are for heating and the rear row starting from the curb side are for the defrosters. Air flow is controlled by a vacuum powered set of dampers that operate in either heat or defrost position. The inside plenum above the AUX VENTS blower feeds the vents near the outside arm rests.

The controls in the cockpit are made up of three parts on my 93 coach. The main control on the dash has multiple buttons for selecting heating, cooling and defrosters along with a temperature control lever and a multi-speed fan switch. The push buttons control (for heating and defroster purposes) determine whether vacuum is sent to the outside air damper or the heat/defrost damper. There are three vacuum tubes connected via a plug to the panel. The red tube selects heat or defrost with no vacuum being defrost. The blue tube opens the outside air source with vacuum. And the black tube is the vacuum source. One the top of the control panel behind the dash is the microswitch that through a relay in the driver side engine compartment turns the boost pump on and off. The white control block part of panel control is P/N 3610-501 2262.

There is a rotating multi-speed switch just below the control panel marked "AUX VENTS" which controls a fan in the main heating and cooling box. On console beside the driver is a panel with an on/off switch marked AUX DEFROST that powers the two fans below the dash. They push air to vents below the windshield that aren't fed by the two fans in the main box.

The vacuum source is just above the six-pack below the driver's feet. A solenoid valve controls the flow of compressed air from the chassis system through a venturi that produces the needed vacuum. While I haven't seen these particular parts I understand there is spherical vacuum reservoir behind the dash in front of the driver's left knee along with a vacuum switch in the same location for turning the solenoid on and off. It is easy to test all the vacuum parts of the system with a Mitymite brand or similar hand powered vacuum pump.

Hopefully this will be helpful if you need to work on your system. If what I've written isn't clear please feel free to ask for clarification