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A/C Vacuum Controls

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Happy Friday!  This summer (my first with this car) has shown an odd idiosyncrasy with the a/c controls.  Specifically, when pulling a moderate load at low rpms (auto tranny), the a/c flapper valve flips over from the dash to the  defrost vents.  I know this is caused by low manifold vacuum, but, shouldn't there be a check valve in the system that attempts to hold vacuum in the system during those brief excursions?  Just as soon as either the throttle is lifted slightly, or the stubborn tranny decides to down-shift, everything's back to normal.


A few assumptions on my part (yes, yes; I know...)

1) There is a faulty check valve somewhere between the engine and the heater controls,

B) There is a leak between the hypothetical check valve and heater controls,

4) There is a leak in the flapper motor/diaphragm


This could all be resolved if I knew two things; is there a check valve, and, if so, where is it?  Anyone?  Beuller?  I can't seem to find a proper all-car vacuum system diagram for this critter.


Thanks for your help,


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Hvac is controled by supplied vacuum from the bay...more than likely your vac line cracked or rotted away. Pull the hvac panel and you shall find the vacumm block in the back. The feed line enters the firewall by the heater core lines. 

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the line that opal described is the most likely culprit.  it happens to most of these cars at some point or another.  its a cheap plastic line from the vacuum tree to the other side of the bay and goes through the firewall there.  breaks off and boom, defrost only.

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