Under dash recon mission

[chengny]

Could someone who has a minute - and access an 81-87 C/K pickup with factory A/C - do me a small favor?

I am installing OEM A/C in my truck. At this point I am assembling the interior air handling system.

I just started last night. The first step was to look around under the dash to see what would have to be removed/moved/rerouted in order to fit the various pieces in place.

I didn't get very far. The first thing I looked at was how the whaletail (the big y-shaped duct that supplies defroster vents) attached to the main distibution housing. These are the only two components that I have currently installed on the cab side of the firewall.

Anyway -to the point - can someone look and see if the lower edges of the defrost duct completely meet the upper surace of the main airhandler. There is a gap in my installation. It is tight at the front edge - where the screw goes. But a gap develops as I look further back. By the time it ends near the firewall the gap is nearly 1/2".

I know I won't like the answer but can someone look under the dash and report back whether their's is like this:

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[89Suburban]

Welcome to my world, I asked this question ages ago

 

[Ole87]

Since I have my dash and glove box out I tried to look for you. I couldn't really see the duct for the defroster vents because the duct for the dash vents is in front of that. With it in front, is hard to tell or see anything behind it. Glad I did look though as the dash vent duct was almost completely off at one of the connections. I will try to get back in there today and look again.

 

[89Suburban]

I don;t understand this manufacturing defect. It fills the dash with heated air to the point it get's hot to the touch. And it cuts down on the CFM flow to the windshield. I tried that silver foiled foam tape to try and seal it but it fell off. I will be modding mine with some sheet metal or vinyl to try and close that gap up.

 

[chengny]

Found a photo of your gap taken a couple of years ago - also went through old files and found one showing the same fit on my old truck (never even noticed it):

You must be registered for see images attach

You must be registered for see images attach

So, I guess we can all agree that;

1. For whatever reason - it is there by design (the gap).

2. The system works fine "as is" and all 81 -91 C/K/R/V trucks have a gap at the defroster duct inlet.

3. No further action is necessary - item closed (my favorite type of repair).



Anyway, here is what I think is the reason for this unusual feature:

1. Trucks equipped as "Heat Only" have no gap at the air handler/defrost duct interface.

2. They also use a blower that has a significantly lower CFM rating than the blower that is spec'd for A/C equipped vehicles. (This because heat exchangers that are used to cool are more efficient at higher flow rates while heat exchangers that are use to heat the medium (especially if used to provide hot air) require a somewhat reduced volume per minute.


3. The cross-sectional area of the defroster vent registers is identical - they use the exact same part.

4. Since the cross-sectional area of the defrost vents is the same for both systems - but the A/C system has a fan capable of delivering a greater quantity of air - the A/C type blower is considered oversized.

5. The above is only true when the system is in the defrost mode. On A/C systems, the ducting and number/size of vents associated with the other modes of operation (Max A/C, Normal, Bi-level & Vent, Heat) are designed to handle the output from the larger fan.


It was probably easy enough to to upgrade the underdash components to accommodate the increased fan output for trucks that received the A/C option. But when it came to the defroster assembly what could they do?

Other than design and specify a totally different dashpad configuration for A/C trucks (too expensive) , the only other option was to find a way to dump some quantity of fan output when the in the defrost mode.

I think they first considered bleeding some off through the floor vents. I say that because if you look at the defrost damper lever, you'll see that there are two holes. Either can be selected to accept the operating arm from the vacuum operated actuator:

You must be registered for see images attach

The damper comes from the factory in the hole furthest from the arm.

NOTE: the damper is set up so that - without any vacuum signal to the diaphragm - it defaults to move downward and close off the floor vent. When heat mode is selected, vacuum is applied to the diaphragm. The diaphragm pulls the actuating arm in. The damper moves upward and the air flow is forced downward - through the floor vents.

Using the most distant hole allows the damper to move all the way down. This completely shuts off the air flow to the floor vents. The total volume of air is then directed up to the defroster vents.

However, if the inner hole is used, when heat is selected or vacuum is lost, the damper can only move about 2/3 of the way down. At that point the actuating arm has reached the end of it's stroke. Since it can't move any further out, it stops the downward motion of the damper at that 2/3 position.

But all that doesn't matter anyway. For whatever reason, that method of dumping excess air volume was abandoned. Maybe people don't like to feel floor air flow when they're in defrost?

So now my theory - some guy says "Why don't we just make a hole in the defroster uptake duct."

Another guy says" Good idea, and lets do it in the back so no one notices."

And finally, some other guy says "Yeah, in the back is good. But, instead of a hole, let's taper the flange face so it doesn't meet the distribution box. It will look real professional. And years from now - after we're all retired or dead - guys who own these trucks will f*ck with it for hours thinking they must've assembled it wrong"

 

[89Suburban]

Now the question is what we gonna do about it...

You must be registered for see images

 

[chengny]

This is my plan:

3. No further action is necessary - item closed (my favorite type of repair).

 

[Bshspherd]

my heat only 81 has a gap, hardly any air comes out of the defrost.

 

[Jims86]

Mine had some 1" thick adhesive backed foam as a seal...available at the hardware store.