Defrost Issues - Need Pics or Diagram

[bladerunner]

So I am dealing with the age old issue with defrost.  My vents blow hot air just fine, but when I switch to defrost the flow is weak.  I have heard a lot about issues with the flipper getting stuck, clogged or just plain unresponsive.  I also looked for the requisite vacuum hose but did not see it.  My truck has a crate 350 and all the smog equipment is gone, so I figure that is why.  But I still have hot air.  I am basically admitting total ignorance regarding air/comfort control systems, so if someone wants to explain it to me, that would be helpful.

What would be even more helpful would be a diagram or pictures of the the flipper/door that I figure is at the heart of the problem. I won't be able to tackle this until Sunday, but I figure I will look at it then as I will be installing a new engine block heater and fill pipe for the passenger gas tank.

[nucknoel89]

So I am the age old issue with defrost.  My vents blow hot air just fine, but when I switch to defrost the flow is weak.  I have heard a lot about issues with the flipper getting stuck, clogged or just plain unresponsive.  I also looked for the requisite vacuum hose but did not see it.  My truck has a crate 350 and all the smog equipment is gone, so I figure that is why.  But I still have hot air.  I am basically admitting total ignorance regarding air/comfort control systems, so if someone wants to explain it to me, that would be helpful.

What would be even more helpful would be a diagram or pictures of the the flipper/door that I figure is at the heart of the problem. I won't be able to tackle this until Sunday, but I figure I will look at it then as I will be installing a new engine block heater and fill pipe for the passenger gas tank.

Is your truck A/C or non?  I dunno about A/C trucks but on non-A/C trucks, vacuum has nothing to do with it. 

[bladerunner]

A/C truck, but the AC is not hooked up.

[DrDaryl]

Well start removing stuff until you can sort of see inside the heater box.  There is a blend door which is cable activated and is on a vertical metal rod with friction fit.  On mine, the door was reamed out around the rod and was free to move.  I wound up putting a metal pin through the plastic door and the rod to fix it.   There is a flat spot on the rod and a corresponding flat in the plastic door that hopefully will be visible to help get the proper alignment.  There are also some vacuum activated doors and you could check all the vacuum lines to them.  I finally just removed my whole box and went through it and made sure everything was good; also replaced the heater core while it was out.
Hope this helps.

[firefighter]

Yes, please let us know if you have AC or not. If not, I can post a few pics of my heater box since it's off right now.
If non AC, it's really a pretty simple unit.

[ehjorten]

A few things:
1) When you switch to defrost there is a flapper door on the far drivers's side of the heater box.  This flapper door directs air either to the floor or up to the defrost.  the vacuum pod is on the end of the case and is connected by a single vacuum hose.  You can take the small floor deflector off and feel that door up inside there.  Check to see that it is working.  This door also seals via a foam seal that is stapled to the end of the flapper.  This foam can and will deteriorate with time.  For best performance you should R&R this foam seal (the box has to come off for this).
2) The blend and heat door is behind the plastic cover in the middle of the box that directs the air up to the vents.  The heat door and blend door are connected together and work somewhat in harmony depending on the setting you choose.  You can take this plastic vent off of the box with like 3 screws I think.  Then you can check to see that it is operating.  There are two vacuum pods that control the blend door and they are interconnected with a vacuum line.  One short line is jumpered and goes into the box from the top.  This one is usually the first to deteriorate as it is not a plastic hard line, but rather a short piece of vacuum hose.  You have to take the whole box off to replace it.
3) As stated before, the blend door and the heat door are interconnected.  The heat door is controlled by a push/pull cable and depending on the setting of the door it limits the travel of the blend door.  The door has a small plastic piece that is bolted to the heat door and then a flat rod from the blend door and vacuum actuator is connected to it with a small rivet.  This rivet can break and now the blend door does not operate as it should.  You have to take the whole box off to fix it.

Finally...if you have all of these things in proper working order, you seal all of the vent connections with new foam tape, you can get very good air flow and control.  One weak point of these designs is that the defrost molded vent never fits tightly to the heater box.  The design is just an air gap between the two.  You can significantyl improve the defrost performance if you use an aluminized duct tape and seal up that gap.  Last...If you really want some air-flow and you have A/C, go get yourself a replacement blower motor for a '94 Chevy truck!  It comes with a new squirrel cage and has much more power!!

[Grim 82]

Make sure your HVAC reserve can on the firewall has manifold vacuum running to it.

[tylervalo]

Make sure your HVAC reserve can on the firewall has manifold vacuum running to it.

#1 thing, most people tend to remove this because it looks quite unnecessary.  Some times they cut the vacuum hose right after it comes out of the firewall.

[87_Rado]

Does anybody have a picture if this? This post was very helpful and this is my first time hearing about it so I'm not sure if it's in my truck lol

[Captkaos]

It is the round cannister mounted by the wipers.  Far right in this pic

[timthescarrd]

I thought the one by the wipers was for cruise and the one over by the blower motor was for the A/C door?

[87_Rado]

Looking at the pics I don't believe I have either one currently in my truck, it's an 87 350tbi, what problems would this give me?

[timthescarrd]

The one for cruise was only for the early vacuum modulated cruise control systems, probably wouldn't affect your 87.  The one for HVAC controls the door that changes where air flows through.  Without it, you'll probably only get air through the floor, unless someone routed direct manifold vacuum, in which case the selector should work but if you've got a load on the engine it may drop down to the floor.

There should be a small (about 1/8") vacuum hose coming out of the firewall right between the blower motor and the A/C evaporator.  (Just to the pass side of the big black plastic box on the firewall)  see where that hose goes.

[rodeoexpress9]

Yes, please let us know if you have AC or not. If not, I can post a few pics of my heater box since it's off right now.
If non AC, it's really a pretty simple unit.

No.a c

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[rodeoexpress9]

I have a 1979 chevy c10 truck & my defrost doesnt work but i have heat otherwise how do i solve the problem or where would i start at

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