This is so neat. I’m in the middle of a heater core replacement and have reassembled the box but have yet to install duct work. It’s a 78 A/C car. So, I decided to verify operation of the vacuum operated heat, A/C and ventilation doors. My view was such that I could watch all doors operate as I moved the temperature control lever…pretty cool stuff. But anyway...

Before proceeding, I’m verifying operations of plenum door, defroster door and A/C door. My understanding of what should happen with control set to MAX…is the A/C door opens (closing airflow to defroster and permitting cool air into the duct assembly) and the plenum door closes. This permits maximum cooling by recycling cabin air and prohibiting the introduction of outside air. However, the plenum door is opening! Also, the water shut-off valve shuts hot water from flowing to the heater core, theoretically (and this works as well).

Testing further, plenum door doesn’t open when control set at VENT. It does get vacuum and the actuator holds vacuum…in fact nicely. The vacuum line (orange) is connected to the green which plugs into the back of the switch...per service manual.

I’ve check connections at the back of the switch that leads to the control levers and they are in the correct order according to the service manual.

So my question is…why is the plenum door opening when it should remain closed? What am I missing here?

 

If you checked everything and no leaks at the actuator, it must work.

 

It does.

 

You've got it right in your mind. You had me going for a while until I realized that you can't see all of the doors operate from one position. One is barely visible at the back of the hood between the fender and the fire wall. I think what you all calling the plenum door is behind the right side kick panel, right? There is another door under the top of the right side fender at the back of the hood that alows fresh air to enter the plenum. This is the door that operates like your thinking it should. At max. aircon. when that door at the kick panel opens the other one closes and cabin air is recirculated through the kick panel door to the blower and out the vents. In the vent position the door at the kick panel closes and the one behind the top of the fender opens so fresh air is blown out the vents.