I tried searching this for the last few weeks, and took the car apart trying to fix it, but I'm at an impass. The underlying problem is I can't get air to blow inside the cabin at a normal level. I am getting some air out of the vents, but its nowhere near the normal levels, regardless of settings on the switch.

So far I've tried the following:

Replaced the blower motor
removed and cleaned the evaporator core
checked voltages at the blower, all are correct
run the blower with only the battery on, definitely hearing distinct motor pitch changes as I increase the settings
Sealed the blower motor/evaporator core with sealant, to prevent some small noticeable air leaks

At this point, I'm running out of ideas....I'd really appreciate any advice! Thanks so much.

 

Sounds like the physical vents behind the dash are not fully open. I can't remember if they are vacuum or cable controlled on those (mine is the electronic system which is different)

Someone with an 85 or similar system may have more details of how it works on your car behind the dash.

 

Could be a bad vacuum check valve or the vacuum isn't getting to the A/C control head.

The HVAC vacuum source also goes to the cruise control. If that doesn't work, then your problem is most likely vacuum.

 

I do have cruise control, so I think the vacuum is ok. I forgot to mention I did check those lines for any sort of corosion or any obvious signs, but yes, the cruise does work.

I am trying to get to the doors, but it's tough finding out which parts to inspect. I know I do have a cable driven heat/AC selector, however that does work. I obviously can't see inside the shroud to make sure the doors are working, but I do get a difference in temperature when I move it. On another note, that cable driven selector is very difficult to move back and forth from heat to AC.

As for the physical doors, I am also finding a lot of information about the 86-90 C4's, but not a lot for the 85.

 

The HVAC doors are controlled by vacuum. The vacuum source on the engine comes to a vacuum harness on the back of the A/C control head. The control head sends the vacuum to the various doors depending on how the control lever is set (HEAT, AC, VENT, etc.).

The vacuum harness is item 9 in this picture:

http://parts.nalleygmc.com/images/pa...301Y09-010.JPG

I put in 1985 when I went to that site, but the picture also shows the later electronic HVAC controller, so ignore "View A for electrical" on the left and all the parts associated with item 17, which is the HVAC programmer for the electronic HVAC controller. Your HVAC control head is item 5 and you can see how everything is hooked up by looking in that area.

Your heat-cold door cable is item 6 in the picture. The door is inside item 3.

 

Cliff,

That image was hugely helpful. Studying it over, and knowing that if I have a vacuum problem, I'd loose cruise control, I would venture to guess one of the actuators (10-13) is probably the issue.

I went to the main parts page for the website you gave me, and only two actuators are listed, #11 and #13. Unfortunately, they are given a generic name:

ACTUATOR. AIR CONDITIONING (A/C) Control

Do you have any idea which one of those would be stuck to cause an issue such as low output air, both heat and AC? I'm pretty sure that, although door 6, behind item 3, is tough, it is moving because I do get a change in temperature.

I'm leaning towards an actuator, but I just don't know which one it could be right now, since they've all got the same lable. Thanks again!

 

It doesn't sound like a door - or a vacuum solenoid that controls any of them unless you've got air come from everything: heat; defrost and vent (and note that most Vettes bleed a little a/c to the floor for cooling and defrost to keep the windshield clear). GM HVAC sucks the defroster door shut and open with any loss of vacuum causing the defroster to flop open which would be indicated by full time defrost. However, if you want to check the solenoids they're on the left side of the heater box to the right of the accelerator so you'll need to yank the driver's side hush panel. Using a vacuum pump, apply 10 inches of vacuum and you should see the solenoid arm move - or you can just have someone work the controls and see what each one does which is probably a good starting point to verify the Control Panel settings. GM is fairly universal in the vacuum hose colors - red and pink for heat/defrost and blue for the a/c vents and then orange for outside air/recirc and that vacuum solenoid is over on the passenger side kick panel.

I'd take another look at the bottom of the evaporator box where it meets the firewall. Just stick your hand underneath the car to verify there isn't an air stream.

Your ductwork is sealed with foam and it's probably long gone. Remove the passenger hush panel and if there's a stream of air cooling the ECM, you need to seal up the ductwork. That at least involves removing the upper pad. Factory foam isn't available, so use your imagination and/or duct tape where you can. Also note that there is (maybe was) a piece of sealing foam where the a/c vent meets the Display panel trim over on the driver's side. You'll need to remove that trim for better access. Without it, air is cooling the Display and not the interior.

Make sure the squirrel cage is in good shape - pull out the blower motor to check for broken pieces. I think some have gone with a later year cage which supposedly push more air; though I really don't know if that's true or if any of it's interchangeable.

 

SunCr: he has an '85. No HVAC programmer or solenoids.

All the HVAC doors are controlled by the HEAT-VENT-A/C lever. That lever switches the vacuum to the various door actuators. Check the A/C control head to make sure the vacuum is getting switched and that all the vacuum lines are OK.

It's pretty common for the parts vendors to be out of certain parts. They stopped making these parts 25 years ago and have been selling stock off the shelf since then. When they run out (some parts are common to other cars, so we get a break), they're gone forever. Then you have to go with used parts.

 

I know it's Manual Air, but both systems use the same vacuum solenoids - or Actuators it you like. Manual simply feeds the vacuum source to one or more when the ports on the control panel are opened by twisting the ****. Electronic flows vacuum - well electronically - but it goes to the same device. Spring loaded doors are then sucked open by the vacuum to the solenoid or Actuator. Those things rarely fail, but the Control Panels do break - usually a **** in my personal experience - and the only time I've seen a broken door is when someone takes it apart (quite a chore really) and puts it back together wrong.

 

Well I guess I should have used lever or thingamajig but that isn't what you twist on the Controls.

 

Thanks for the suggestions guys. At this point, I'm leaning towards the A/C Control Head vacuum lines. I'm going to finish trouble shooting tomorrow, as I ran the battery down to 8 volts today testing it.

The good news is there are almost no air leaks on the evaporator core shroud, and definitely nothing causing the issue I have. There's a tiny leak, but its definitely not causing the cabin air to be so poor.

Thanks again, I'll keep you updated.

 

I had Pep Boys replace my heater core. Once the job was completed, I was told my AC compressor was out, and needs replacing. I thought that was strange because it worked fine before. I didn't have it replaced at the time because I wanted to check it out for myself. I changed over to the 134 system several years ago and I haven't had any problems with it until now. The weather was still cold at the time, so I recently had some time to check it. I bought a can of 134 freon (the type with the pressure gauge), and began to charge it. It filled fine and everything on the gauge indicated the system was fine. However, when I turned the engine off, I could hear a hissing sound coming from the firewall area. Is there anyone out there that has an idea of what could be leaking in that area? And does the hissing noise have anything to do with the heater core replacement? My e-mail address is vetteman18361@aol.com

Thanks,

 

The A/C evaporator is in that area. It's next to the heater core so I suppose it could have been damaged during the heater core replacement.

 

Thanks Cliff for responding. I have an appointment with the Manager at Pep Boys on Tuesday (04/07/15). At least now, I have a suggestion as to what might have happened.