Warmer air at A/C ducts

[Stocker]

Got my A/C working again for the first time in many years. Converted to R134a, new parts installed by a local mechanic (don't shoot me). Got a problem we can't figure out, though. I have searched a ton of threads and can't find a solution.

It's blowing lots of air but it's quite a bit warmer at the vents than at the bottom of the HVAC airbox. In my driveway around noon, ambient temp nearly 90 degrees. Had 33 degrees at the bottom of the airbox (great, right?) but 62 at all three A/C vents. Ran it for a good long time, mostly shaded by a tree, no difference. Ducting is new, undamaged, and properly connected.

I'm at the end of my rope and need a miracle. Thanks for any help.

[dmjlambert]

Hmmm, I'm going to be watching this thread to gain knowledge. It sounds like warm air is mixing with cold before it goes through the ducts, but I don't know the air flow of one of these things yet. What parts are new?

[B. W.]

dmjlambert beat me to it, check the blend door, sounds like it's mixing in heated air. Also, check the heater control valve on the pass. fender. It should be off when A/C is on.

[Stocker]

Originally Posted by B. W.

dmjlambert beat me to it, check the blend door, sounds like it's mixing in heated air. Also, check the heater control valve on the pass. fender. It should be off when A/C is on.

I should know this but help me out please, exactly where is the blend door? There is a flapper (for lack of a better word) controlled by the top slider -- moved to the far right side (defroster), it closes off the floor ducts and sends air to the defrost vents. Is that the blend door? [Edit -- I may have figured it out, see post #8 below]

The heater control valve is new, and for extra insurance there is a ball valve in the heater hose going to the control valve. There is no heat coming out of the heater core.

[Andy4639]

Are the vacuum canisters working correctly? It sounds to me the cowl vent is open and warm air is coming inside the cab.

bottom of the HVAC airbox.

What exactly is this your talking about? The outside box on the firewall.

[Stocker]

Originally Posted by Stocker

I should know this but help me out please, exactly where is the blend door? There is a flapper (for lack of a better word) controlled by the top slider -- moved to the far right side (defroster), it closes off the floor ducts and sends air to the defrost vents. Is that the blend door?

I think I figured out what the blend door is (or at least where it is located), thanks to a 10 1/2 year old post I was just reading. Looks like it is inside the fiberglass box under the hood where the evaporator is, and is controlled by the temperature selector slider. Can't see it because of where it is but the cable works okay and the temps out of the ducts go hot or cold depending on the position of the slider.

[Andy4639]

Originally Posted by Stocker

I think I figured out what the blend door is (or at least where it is located), thanks to a 10 1/2 year old post I was just reading. Looks like it is inside the fiberglass box under the hood where the evaporator is, and is controlled by the temperature selector slider. Can't see it because of where it is but the cable works okay and the temps out of the ducts go hot or cold depending on the position of the slider.

Ok here is me trying to remember.
The cable goes outside to the eveperator box and is mounted to a metal rod which goes down inside the box. When you want AC it closes off the path to the heater and forces all the air flow across the eveperator coils. You have to set it to work correctly are as you found out it will let air bypass and come into the cab with no cooling effect.

Here is the flapper you need to close and open. You need to adjust all the cables to operate correcly to get the max out of both AC and heat.

[Stocker]

Originally Posted by dmjlambert

What parts are new?

Compressor, condenser, hoses & muffler, POA valve, expansion valve, receiver/drier, and ducts & outlet vents inside the cab.

[Stocker]

Hi Andy, glad you popped in.

Are you talking about air blowing in through a cowl vent while driving? I'm doing my testing while parked in the driveway & carport so that wouldn't be an issue. If not, give me a bit more specific info so I'll know what to look for. Thanks!

What I called the HVAC airbox (wrong term, maybe?) is inside the cab, under the glove box. Heated or cooled air is routed from there through the ducts to the outlet vents.

[Stocker]

Thanks for the suggestions, Aaron. Lemme answer the easy one first. The techs only worked under the hood, removing & replacing the parts I mentioned in post 4 above. I did the ducts & outlets in the cab. No other major parts were touched, no gaskets were disturbed.... they are all original 46-year-old parts. Which is not to say chunks of those old gaskets might not have disappeared over the years....

I haven't yet wrapped my head around warm air bypassing the evap core, given that I have 33-degree air blasting into the distribution box (or whatever it's called) under the glove box. Isn't that where the cold air is coming from -- the box that the evap core sits in?

At any rate, whether I understand it or not doesn't matter. What matters is that warm air is somehow getting into the system. I'll see if I can pull the fan in order to eyeball the blend door.

[DarkMonohue]

Originally Posted by Stocker

Thanks for the suggestions, Aaron. Lemme answer the easy one first. The techs only worked under the hood, removing & replacing the parts I mentioned in post 4 above. I did the ducts & outlets in the cab. No other major parts were touched, no gaskets were disturbed.... they are all original 46-year-old parts. Which is not to say chunks of those old gaskets might not have disappeared over the years...

Ah. For some reason I thought you'd mentioned replacing the evaporator as well. Darn! Nowhere to point the finger, then.

Originally Posted by Stocker

I haven't yet wrapped my head around warm air bypassing the evap core, given that I have 33-degree air blasting into the distribution box (or whatever it's called) under the glove box. Isn't that where the cold air is coming from -- the box that the evap core sits in?

Yes indeed. But I guess it is just possible that the 33° air you're measuring may be only part of the airflow. Air does weird things.

Originally Posted by Stocker

At any rate, whether I understand it or not doesn't matter. What matters is that warm air is somehow getting into the system. I'll see if I can pull the fan in order to eyeball the blend door.

Aha! This post shows pretty clearly (unless I'm wrong, which I was once) that the door Andy showed us should send air through the evaporator when fully closed, and opens (by swinging forward toward the engine compartment?) to allow air to bypass the evaporator and go through the heater core. Even though the heater core is at ambient temp and not "adding heat", air that goes through that door is at ambient temperature (because it's not being cooled by the evaporator). So ambient air may be diluting the cold stuff.

Anyway, I'm really glad to hear that the air is cold at least somewhere in the system, if not everywhere.

[randy500]

You can get to that door from the inside of the cab by removing the glovebox and duct that pushes the air to the a/c vents, its a job though and have to take out the inner fender I think to get to some of the nuts.

[Stocker]

Thanks Randy, the next thing I will do is check the cable operation of the blend door. Easy stuff first! Hopefully it's that simple..... we will see.

I don't know the correct name for all these parts. Let's try this -- pretty much centered beneath the glove box is a large composite chamber that attaches to the firewall. On the bottom of that chamber is a small (~1 1/2" x 1 1/2") metal trap door. That is where I inserted the thermometer probe.

[randy500]

Originally Posted by Stocker

Thanks Randy, the next thing I will do is check the cable operation of the blend door. Easy stuff first! Hopefully it's that simple..... we will see.

I don't know the correct name for all these parts. Let's try this -- pretty much centered beneath the glove box is a large composite chamber that attaches to the firewall. On the bottom of that chamber is a small (~1 1/2" x 1 1/2") metal trap door. That is where I inserted the thermometer probe.

Be careful putting a thermometer in that door, the door is to access the heater blower motor resistor.
The air past that goes to the a/c diverted valve, visible below he dash and if original is an army green color, the valve is usually broke inside that, it diverts the airflow to either the a/c vents or onto the heater vent which has another valve to divert the air to the defrost vents.
If the a/c diverted valve is broke the air can be directed to both locations which when spread across all e vents is not much airflow to any of them.

Keep in mind the truck is 45 years old and pulls air from the unfiltered cowl, the outside box and the evaporator are probably full of debris. I think you could remove the outside evaporator cover on the engine side to clean that out and then move the evaporator to the side to clean the other side. I think that would work without a recharge, haven't tried it though,

[Stocker]

Originally Posted by randy500

I think you could remove the outside evaporator cover on the engine side to clean that out and then move the evaporator to the side to clean the other side. I think that would work without a recharge, haven't tried it though,

I can verify that does work! I was able to clean out decades worth of old leaf clutter. Mostly lying on the bottom, but some was partially blocking the evaporator. Cover gaskets were harder than.... well, they were hard enough to crumble when the evap cover was opened. Hope I can find new gaskets for it....

Gonna see what I can learn about the blend door next, the remove the cowl and see about the cowl vent. I might try to seal it off like Andy did with his, unless there's a good reason to keep it working.

[DarkMonohue]

Originally Posted by Stocker

Gonna see what I can learn about the blend door next, the remove the cowl and see about the cowl vent. I might try to seal it off like Andy did with his, unless there's a good reason to keep it working.

That's the fresh air inlet for the system. If you seal it off, the only source of air to the blower motor will be the interior of the truck. Effectively, you'll always be recirculating air, and the defogger won't work very well.

[Andy4639]

Even if the truck is sitting and the cowl vent is not shut all the way it will draw hot air into the system.
The point of the cowl vent is to let outside air into the cab when the AC is on.
To blend into the cold air inside the cab for fresh out side air. With it sealed the outside air can't get in weather moving are stopped.


I'm still not sure the flapper door is correct though from what you say.
The heater core is inside the cab under the dash. The evaporator is on the motor side of the fire wall. So why if the flappers are all working is the heater core 33* it shouldn't have that cold of air inside the heater core box inside the cab. The air should be going straight through the vents and out through the vents.

[DarkMonohue]

Originally Posted by Stocker

...the next thing I will do is check the cable operation of the blend door.

Here's a really good illustration of the operation of that door (YouTube clip by F.A.A.benny):
https://youtu.be/PW8T-tXf2wE?t=5m39s

I really hope you don't have to remove and/or dismantle that assembly. It looks like a real endeavor.

edit: Found some more good info from F.A.A.benny in these threads:
http://67-72chevytrucks.com/vboard/s...d.php?t=742484
http://67-72chevytrucks.com/vboard/s...d.php?t=742694

[Stocker]

Randy500 and DarkMonohue: Great information to have, thanks very much! Sounds more & more like I'll be digging into this more than I ever wanted to. I'll do as much as I can and if it needs more attention than I can handle.... it's gonna be in the shop again to replace the noisy (new) compressor.

It may well have some diverter valve issues. A/C sends virtually all the air to the correct vents, but heater and defroster settings allow some air to exit the A/C vents too.

[Andy4639]

I will say this again. The flapper in the cowl works off the vacuum canister. If that flapper under the cowl is opening instead of closing then you are letting hot air inside the cab. If it's not closing all the way you are letting hot air inside the cab.

The vacuum canister that operate the AC vents are different from each other they are not the same. You have to have them in the right spot are they work backwards of what they are suppose to do.

On my 71 I sealed the cowl vent off it is non functional now to keep hot air out of the cab.

[Stocker]

Originally Posted by Andy4639

I will say this again. The flapper in the cowl works off the vacuum canister. If that flapper under the cowl is opening instead of closing then you are letting hot air inside the cab. If it's not closing all the way you are letting hot air inside the cab.

The vacuum canister that operate the AC vents are different from each other they are not the same. You have to have them in the right spot are they work backwards of what they are suppose to do.

I'll check that again today, giving special attention to the cowl vent. I looked last night, mainly just to see if the diaphragms were functioning. The diaphragm on the cowl (under the hood) seems to control the pass. side kick panel vent, opening it to recirculate inside air -- but I need to take a closer look. None of the vacuum lines or diaphragms were moved during the project and all are original to the truck.

[Andy4639]

The cowl vent should have anything to do with the kick panel vent. It has it's own vacuum canister for operation.

On my 71 the cowl is sealed off completely non functional. The inside kick panel I put back to manual open and shut. I leave it open all the time and this cools the truck down very nice. It's like the newer vehicles recirc.
If the cowl vent isn't sealing good it draws hot air into the system which that's what it's for.

[Stocker]

Originally Posted by Andy4639

The cowl vent should have anything to do with the kick panel vent. It has it's own vacuum canister for operation.

On my 71 the cowl is sealed off completely non functional. The inside kick panel I put back to manual open and shut. I leave it open all the time and this cools the truck down very nice. It's like the newer vehicles recirc.
If the cowl vent isn't sealing good it draws hot air into the system which that's what it's for.

That makes sense.... They were both operating and I made an incorrect mental connection, but with your explanation and the diagram you posted, I see they are independent of each other.

Question -- will the cowl vent draw hot air in only if the truck is moving, or does it also draw hot air in when sitting still?

[randy500]

Isn't it clear from the vacuum diagram that there is one vacuum switch that when activated does the following:
1. Shuts off the heater hot water flow to the heater core
2. Opens the vent on the passenger side
3. Closes the cowl vent

All of that happens at once, the cowl closes to allow air to be recirculated from the inside passenger vent that has now opened giving recirculated air to be cooled by the evaporator.

[Stocker]

Originally Posted by randy500

Isn't it clear from the vacuum diagram that there is one vacuum switch that when activated does the following:
1. Shuts off the heater hot water flow to the heater core
2. Opens the vent on the passenger side
3. Closes the cowl vent

All of that happens at once, the cowl closes to allow air to be recirculated from the inside passenger vent that has now opened giving recirculated air to be cooled by the evaporator.

Yes, the diagram makes it crystal clear. My earlier confusion was a result of not having the diagram in front of me.