I hope this is the right place to post this... If not, sorry....
I recently picked up a '89 Firebird (2.8L, 700R4 Trans), and the A/C works GREAT when in Park, but if you put it in Drive or reverse, pick up the R's to over 1500, the air doors switch to the defroster.
I have already fixed all of the vacuum leaks that I can find....
Is there anyone out there that can help me with this? I am trying to get it fixed for my 19 Y/O (college student).....
Feel free to post here, or e mail me direct!
Thanks!
Joe in Arizona

 

There's a check valve near the firewall, that holds vacuum on the controls so they stay in the same place, even when the car is not running (no vacuum). It sounds like when in gear, you're sitting at a lower RPM and your vacuum is dropping too low to keep the vent door from switching, this is where your check valve should be preventing just that.
You can pick them up from advance auto parts I know, and most likely any other decent parts store, for around $5.00. You can find them with the vacuum accessories. I don't know how the stores are around where you're at, but they looked at me funny when I asked at the counter for it, and told me they didn't have it. I found it myself on the shelf with the other vacuum pieces. Sorry about the dirty pics, it's my parts car.

 

Titan,
I have already replaced it.
I tried (at about 2 AM this morning) pputting a 2nd vacuum canister in line, and this looks to fix the problem about 90%. Do you think I may have a faulty check valve?
I will look into it.....
Thanks again!

 

Sounds like low vacuum to me. Do a search on here for "low vacuum" and check out some of the results. There's so many places it could be, I could sit here typing all day and not list them all. You can either use a can of carb cleaner and spray all suspected areas a little, and if the idle drops, you found a leak. Or you can use a propane torch (don't light it) and just poke the tip near suspect areas, and listen for an increase in RPMs. Idle jumps up, you found a leak. If you have access to a testing gauge, or would rather get one, I can tell you that you should be getting at least 17 hg/in (but even thats a little on the low side) You will want to use a fitting directly on the manifold, if you go this route. Plugging in elsewhere you may get ported vacuum, which will not be accurate.
I doubt the check valve is bad, it's such a simple device.

 

There is also a vac operated control on the back of the center stack ventilation control - all the vac lines go into it - this piece unfortunately often leaks ans will cause air direction issues - some of the posts suggest coating with silicone to help - I have seen this part available new on Ebay for around $15, I am sure there are other sources.

 

Titan,
I was thinking Vacuum also, but have never tried the Propane. I will give it a try, as I can not get a hit with the brake clean...
Joe
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Mike,
I already changed the stack out. It did leak real bad!.
Thanks!

 

Titan,
The propane did the trick!
Found a pin leak that the brake clean did not pick up!
Thanks!
It gets REALLY hot here in Phoenix this time of year!
Joe