Defrost Doors not opening on Factory AC 69 RR

[LowBikeMike]

Working on my 69 Roadrunner with factory AC. It was missing the heater core when I bought it, got all of that straighened out and installed. Using the control panel AC door, heater, and fresh air doors open properly. When selecting defrost the defrost actuator moves down, but not enough to fully open the defroster doors. I can pull down on the defrost actuator rod and get the doors to open and they will stay open until a different setting is selected and then they fully close. I don't feel any significant binding in the linkage and have adjusted the actuator bracket lower with no real improvement. I believe since the defroster doors return to close in the AC or Off position that I've them adjusted properly

Off the car I can apply vacuum to either side of the defroster actuator and the rod moves in both directions to what looks like full travel. The close side of actuator will hold vacuum, the open (bottom) will not.

I'm wondering if I have a weak or slightly leaking actuator on the bottom side.

Any thoughts on if I'm on the right track or should I look at something else.

 

[F4R/T]

You can always apply vacuum with a hand pump and see if they hold, also check the incoming vacuum supply amount. You may also consider a another switch for testing purposes, if possible. I do think the factory switches do get weak overtime. I think you are on the right track, just a time consuming job usually.

 

[Thrashingcows]

The plastic in the old switches gets weak and will distort and warp over time from heat/cool cycles. When this happens you can get bleed through from one vacuum circuit into another thus loosing the strong vacuum source you need to keep the actuator(s) open/closed.

 

[lemondana]

How big of a cam do you have? Too big and you will have musical flaps, closing and opening at will.

 

[LowBikeMike]

Camshaft is mild and source vacuum to control is good. I have operated the actuator with a hand pump and it does work, but does not hold vacuum on the bottom (defroster) side. I think my next check will be to put a gauge on the line to the bottom of the actuator. I assumed that since the top side of the same actuator is working properly the bottom was getting the same amount of vacuum when switched, but since they are fed by different slides I guess I could have a problem there. Of all the actuators I have this is the only one that doesn't hold vacuum on both sides. I think that is telling me my actuater is compromised.