My '96 with manual HVAC controls has started to act up. If it'* set on vent, and I go up a hill or step on it hard, the vents switch to defrost.
I know it'* becuase it'* losing the vacuum and it 'defaults' to that, so I went searching under the hood. I found the vacuum cannister under the right front fender, and sure enough, there was a little hole in the tube coming out of it, so I taped it up. That made the controls respond faster, but they still do the same thing.
So where else should I look for leaks? Or is there a check valve somewhere that quit working?
Thanks.

We've seen a bunch of this w/in the last month. Search on HVAC or Vents, both of those should yield stellar results.

Basic premise is you are losing vacuum like you mentioned. And that piece of tape is probably in need of replacement with a piece of rubber tubing from your local parts store.

You may want to also check for other hoses under the hood (vacuum related) that look loose, cracked or dry rotted and replace them. Sounds like your tank is not holding the vacuum for very long once you start up a hill.

I have had alot of problems with mine and finally fixed it. I have one of the electronic dash controls. Im not sure how ours are different. I started a post called " no air coming out the dash vents why" In the 92-99 forum. See if that helps you any. You can pm me if you have any questions.

If our parts are similiar I could help you trouble shoot.. Good luck

Parts will be slightly different locations but most likely the same parts. It is all the same principle.

The cool thing James is his does blow out the vents until he loses vacuum going up a hill. So his programmer is working. He'* just losing vac somewhere.