My heater/AC does not work right it will only blow out the defrost, it will not come out anywhere else. I was wondering if that black thing behind the AC compressor that has a vacuum hose running to it might have anything to do with it?
Thanks, Gary

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1990 Formula 350
13.8 @ 98 MPH 1/4

 

What do you mean by saying "it will only blow out the defrost"?

The heater passages are vacuum operated. These little gates close adn open when the vacuum is appied and when all hoses are attached.

You might have a disconnected hose.
Pull the plastic cover out ( passanger side above the knees) and you should see some vacuum lines. Check if they are connected. If they are it would not hurt to disconnect them and blow in then or suck the air out of them. When you do that you should be able to hear the passages open and close .Maybe one of the little gates is stuck.

That is what i can suggest. Good luck.

 

I am pretty sure you are right. It wont change no matter where i put it. On Vent, Heater, AC it blows out the top of the dash. Sorry should have been more specific.
Thanks, Gary

 

The system will default to defrost if the vacuum supply is removed or falls off. Gm did this for safety reasons so you can still defrost your windows if something goes awry. I would make sure that the vacuum supply line still has vacuum supplied to it at the selector valve, and if that is not the problem, the selector valve itself may be faulty. Loss of vacuum to the system is the usual problem when those symptoms are present.

Good Luck!!

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

Where do you find this "selector valve"? I was told the vacuum was disconnected in the engine bay. Which mine is, thus why i have hte same problem.

Matt

 

The "vacuum selector" is part of the A/C control head. It chooses where to "send" vacuum (there's an oxymoron for you!) to operate all the air control doors via their little diaphragms.

The supply goes into the dash along with the wiring harness for the A/C. This harness is totally separate from the rest of the car's wiring. It goes through the firewall right under the heater hoses. Look for the vacuum line in that harness; follow it out into the engine bay; it should eventually reach the vacuum canister, which is an accumulator with a check valve, to store up vacuum. Then another line should go from that to the intake somewhere.