questions on ac and defrost

[gaboy87chevy]

Man I love these trucks and I really love this forum and glad you guys post here because i am not a mechanic by any means. But i am a guy that likes to try to save a buck!! lol I have A 87 R10 and i have a problem with the a/c or defrost. It blows fine but will not switch over to defrost and blow up on the windsheild.  A friend of mine told me a rod under the glovebox controls this. I took the dash and everything off and when i pull the rod the air blows up on the window like it should but it will not work when i use the control lever on the a/c heater(or whatever its called)?? How can i fix this??

[Grim 82]

Do you have vacuum running to it? The defroster is selected by a vacuum diaphragm.

[gaboy87chevy]

Thanks for replying...Should there be lines running to the rod or each of these plugs?? because i dont see any lines that look like they may have come loose.

[Grim 82]

There should be manifold vacuum running to a reserve cannister on the firewall, and then into one of those plastic tubes that goes to the selector valve. When you select the setting it routes that vacuum through the other hoses and activates the vacuum diaphragms to control the flow direction. When you move the selector between settings do you hear air hissing?

[gaboy87chevy]

Yes I do hear air hissing when i switch between max,norm,vent but when i switch to defrost i dont hear anything. The heat does work. It will blow out of floor vent(strong) and dash on max(not as strong) but nothing out of the top vents on defrost. I work nights and cant really drive it because the window is iced up in the morning when i get back.

[Grim 82]

If I remember correctly, the door with the rod linkage is hooked to the temp lever, which pulls a cable to open or close that door in your pic to allow heat from the plenum. On an AC equipped model, there are two doors that control the routing of the air flow. They are both controlled by vacuum. IIRC the first door lets all of the airflow to the floor (heat). If constant vacuum is applied to the diaphragm on that door, it will switch to make the air flow up. From there the default position would be through the vents. When vacuum is applied to that door it switches it to direct the flow up through the defroster. I may be off on the routing and which door does what, but you get the idea.
I have had limited defroster flow this winter, until about 2 weeks ago when I resealed my intake and found the vacuum hose from the intake to the HVAC cannister on the firewall was very soft and not sealing well at the fitting on the intake. It was enough of a leak to not pull enough vacuum to switch the doors in the ductwork. After replacing the hose, you can only turn the fan on medium or it will blow the contacts right out of your eyes. My advice would be to get under the hood and be sure that you have good manifold vacuum going to the HVAC can. Report your findings...

[blg4351]

How should the lines run from the intake manifold to the vacuum resevoir and the heat/AC lines?  Is there a check valve in between there somewhere?  My resevoir is disconnected and when i get home march 3rd im going to tear into it.  I just want to see how this should be setup so I can get it working again before summer.

[Grim 82]

You don't necessarily need the line to run from the intake manifold. You can run a line directly from a manifold vac port on your carb like I do. They would both accomplish the same thing; you want full vacuum running to it. I'm not sure if they originally had a check valve in that system or not, but I don't use one, and I've never seen one used for that system.
You need manifold vacuum to the reserve can, and from there a harder plastic line goes through the firewall to the control, and from there the switch gives vacuum signal to the diaphragms to control the duct routing. The reserve can is to maintain volume.

[1979C20]

On topic question, I have the manifold line running to the black ball on the pass side  but the tubing that goes to the fire wall is broken. How could I repair, replace, or rig it?

[Grim 82]

I don't know of any place that sells the replacement small plastic tube, but I'm sure somebody does. Maybe grab one at the junkyard or figure out a way to splice it with vacuum tubing or similar. If you get creative let us know what you used.

[1979C20]

Ideas are stirring up. It involves oxygen tubing and heat shrink tube

[SAATR]

Most auto parts stores should have the hard, black plastic line used to route vacuum on everything from the 60's till today.  At my local Advance Auto they sell it in three foot spools for a few dollars, in the same sizes as rubber vacuum line.  Apply a little heat and it bends to your will.

[1979C20]

What about splicing?

[1979C20]

Can anyone get me an inside diameter of the black plastic line? I'm looking on oreillys website trying to find it

[gaboy87chevy]

Update: Thanks for all the help also...
The problem with the air not blowing out of the defrost and vents was the heater box. The door was broken. I went to the junkyard and got a new box($25 ) and a friend help me put it on. It works perfectly now.

pic of the old box