Main vents not blowing air

Status
Not open for further replies.

Jay520

Not-quite-so-new-guy
Jul 3, 2020
38
26
18
Question: main vents are not blowing air. Where should i check vacuum lines or is it something else?
 

airboatgreg

Comic Book Super Hero
Oct 2, 2016
2,863
3,112
113
Under hood first. Need vacuum to a softball sized black thingy do da do dawl with a black hose from the engine that has vacuum while idling. Purple hose connects to it. If that good replace rotary vacuum switch at HVC controller in dash is a good place to start
 
  • Like
Reactions: 1 user

oldsofb

Royal Smart Person
Supporting Member
Dec 7, 2007
1,368
3,962
113
Maryland
Around 8:35 of the following video, I go through all the Vacuum Servos on my 87 442. If you have a vacuum gauge (hand held vacuum pump is better) you should be able to track down the issue. Start at the servos and work backwards. It could be you have no vacuum coming in from under the hood.


Hutch
 
  • Like
Reactions: 3 users

Jay520

Not-quite-so-new-guy
Jul 3, 2020
38
26
18
Under hood first. Need vacuum to a softball sized black thingy do da do dawl with a black hose from the engine that has vacuum while idling. Purple hose connects to it. If that good replace rotary vacuum switch at HVC controller in dash is a good place to start
Thank you.and is it on top,sides, or bottom?
 

MrSony

Geezer
Nov 15, 2014
6,790
6,673
113
Des Moines, Iowa
Vents go to defrost and floor by default if there's a vacuum leak. It's a safety feature.

Black vacuum ball
Two ports
Small one gets the pink/purple small line
Big port goes to manifold vacuum (vacuum at idle) port. Should work.
 
  • Like
Reactions: 1 user
Oct 14, 2008
8,806
7,746
113
Melville,Saskatchewan
Usually it is the vacuum selector valve on the control head. They fail and you only get the defrost and floor for air flow.
 

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
8,089
17,292
113
Fuse? For what? The modes are not electric.

Although I agree with the best way to check the system is the vacuum pump method Hutch used in his video, to go with Hutch's GREAT VIDEOS on troubleshooting junk, the first few minutes of this one has him using a handheld vacuum pump to test each diaphragm.


If you can change any air flow anywhere, but not get them out the vents, then the purple tube has vacuum. You haven't told us where the air is coming out of when in A/C mode.

The temperature lever controls a cable to the blend door to adjust for hot or cool air (or both if in the middle somewhere). If in cold, it will use the 2 port vacuum valve on the HVAC control unit to shut off the water valve on the back of the intake.

Your mode selector valve (9 port valve on the HVAC control) may be leaking so bad that you won't get any diaphragms to to move. Do you hear a hissing noise from the HVAC control unit when the mode selector is in various positions? That's a rare bird to find if it's the 9 port valve leaking and costs accordingly to find new. You may have better luck trying to fix yours. Usually the plastic center pin wears/loosens up causing leaks.

Assuming you're getting air flow SOMEWHERE, most likely the floor, it may be your lower mode flapper stuck in the up position and possibly the dual action diaphragm isn't working or strong enough to move the flapper. Or the upper mode diaphragm may be hosed, and the defrost door defaults to the defrost mode when there's a lack of vacuum there.

Note: The service manual seems to be wrong on the part where it says when the upper diaphragm for the defrost valve is vented the valve is "closed." The defrost valve defaults to the defrost position when vented. So the GM engineer who approved that figure wasn't paying attention to details. It's confusing for sure, though.
 
  • Like
Reactions: 2 users
Status
Not open for further replies.

GBodyForum is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are trademarks of Amazon.com, Inc. or its affiliates.

Please support GBodyForum Sponsors

Classic Truck Consoles Dixie Restoration Depot UMI Performance

Contact [email protected] for info on becoming a sponsor