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77 airconditioning question



OK heres the problem. I bought this car out of storage and wrecked. A lot of parts missing. Most of the AC was missing. Several years later it is finished with extensive front end rebuild. Been driving it for a few years and burning up. The AC is cold but I have very little airflow from the dash AC vents. I seem to get good flow from the floor vents in heater mode. The blower motor seems to be good. Does anyone know if this is just the nature of the 77 model due to the design of the ductwork or am I overlooking something. I can't locate a 77 model in my area so I could compare. Been fighting this for a long time and it prevents me from driving this beast during warm weather. Been over and under several times double checking and can't locate any mechanical problems. I tend to believe this is just poor design of the ducting. Someone with 77 experience could probably answer my question. Again, I get cold air from the vents, but low air volumn. Cold doesn't help much if it doesn't move. Need input. Anyone know if a high performance fan motor is available if that might help. Color me hot in Oklahoma.
Welcome To The Corvette Action Center iwannafly!

Your gonna hate to hear this but I don't doubt that your ductwork has some obstruction gathered from over the years. Also quite possibly the flaps, in order for you to re-direct the flow of air, aren't working either. You'll have to tear into it to find out, but check all of your vacuum lines first.

This is a great place, and we have many, many Sharks here that'll be more than willing to help. Just wait and see. ;)
No debri. All new ductwork. all flaps working with vacuum properly. Been there. Wish I could find something like that. Thanks
We just fixed this problem on another 77. The main vacuum diaphram just opposite the box under the dash was not flipping open.

It was the control head with the problem. It was not diverting the vacuum to the large valve. We figured it out by locating the vacuum line coming from the firewall side of the dash side unit and manually sucked on it while everything was running. It flipped and the a/c came out the proper vents. (engine off with the blower motor on).

I had him buy a new control head. The old one had a leak. He was able to Turned out, if you listened you could hear it making a slight hiss while the engine was running.

Now, ice cold air shoots out the right spot.

If that does not do it, I don't know what will.
control head

Chris what does the control head look like and where is it located in the car. I have been looking for mine as my a/c doesn't work right either, and has the same problem (cold air but low flow). I have a silver looking round disc like looking part with a vac hose going to it on the far right underside of the dash up against the right kick plate panel. Is this the vacuum diaphram ???
If so where is the control head ?
Thanks Roy
77 AC volume

The problem that I have is not concerned with the air coming out of the wrong vents. It is simply that I have a small amount of air flow. I am trying to ascertain if this is simply due to poor design of the ducting in the 77 model and they all suffer from it and if anyone might know of a way to increase volume so it will blow my hair back. I have gone over this car numerous times in the last few years and cannot find any mechanical faults in the a/c but I have not been able to find another working system of the same vintage to compare with. Where are you 77 boys?? Tell me something!
I don't have a '77, but I have a '75 with the same problem. I have been toying with the idea of tearing the fan motor out of the car and replacing it with something else, maybe even with a 'squirrel cage' of my own design. However, the car is pretty much original and I do not like to change things for that reason. Anyone that can shed light on the subject would be a real hero.
anemic ac

According to my info there were two fan motors made. One standard and the other was a little stouter for wagons, convertibles and corvettes. Over time and use these cheap bushed motors will slow down and begin to be less efficent. However this is not my problem as mine is blowing to capacity but doesn't seem to force enough through the ducting to be really great behind the wheel. I tend to believe this is just poor design. I guess what I need is some kind of brainstorm for an upgrade. Even with the extra firewall insulation this thing is a hotbox on a warm day and sure takes the fun out of the drive. Not looking for a hurricane but this thing is a joke.
Control Head

The control head can be found right under the control panel on the console. It can be bought seperate from the entire control panel if desired. If you remove the control panel you will see where the large group of vacuum hoses plug in. That part is where all the action takes place when you move the hot/cold-vent-defrost-etc. lever. Inside there the switching takes place.

Just for FYI, my definition of strong blower air and yours may be different. However, this 77 will blow you out now that all of the air goes where it is suppose to when you want it.

For a quick comparison. I have a 95 3500 Chevy truck. It still has a vacuum operated vent chamber. There is a failure in there right now. It will stop blowing air out of all of the vents even though you can hear the blower fan going like crazy. What is happening, is instead of flipping to the default defrost setting, it is closing off as if the system is shut down. The air goes nowhere. If I shut it off for about 10 seconds and then turn it back on, the vent will flip and the air comes out where I want it as strong as ever. Since I don't use the truck that often, I keep forgetting to look into it.

Good luck.
strange, but similar, problem with 78

Since this problem is intermittent (maybe once every other winter, for 6 years) I also keep forgetting to look into it. I have suffered cold drives with NO HEAT. After I get to where I'm going, or have stopped to gas up and restart the 78, I get warm air like usual. However, I take twice as long to defrost from the initial chilly trip!
You'd think I'd remember the other times and shut the 78 down and restart to get heat...but I never think of it while I'm driving with no heat...
..."Damm, I'm freezing!..How come the heater isn't working?..Is the heater core bad?..The engine is at normal operating temperature, how come I'm not?...Do I smell antifreeze?..I can't feel my fingers!..The lever is on heater, the temperature selector is on hot, why isn't it working?..."
My only excuse is that the cold must have numbed my brain, preventing me from thinking of the solution right away.
...Hey, it's my lie, I'll tell it any way I want to. ;)
ps Good luck getting your system fixed and keeping you cool!
control head

Well my 75 is the same as your 77. Seems like its on low blower when its on high. My blower motor spins like it should and regulates up and down with the switch as it should. The only problem is getting the cold air to blow out into the car. I will be checking on the control head soon. I assume its a GM part but will check to see if the aftermarket has it some where.

Keep the window open for now,,,Roy
anemic ac

My system mechanics are working. Switching from floor, defrost to vents. Multi speed fan blower is OK. My only problem is low volumn thru the ac vents. Let me know if you figure out how to improve yours. When I go check the fan motor it seems to generate a lot of air but the flow at the ac vent outlets are lousy.

Being that you are convinced the mechanics are fine, have you tried to troubleshoot the volume problem a bit differently. What I mean is, have you tried to block the lower vents to see if the amount or volume of cool air increases through the upper vents.

I also checked into the vacuum harness diagram of my 77 Assembly manual, Section B01-B12, sheet #63, its a pretty good functional diagram. Do you have this?

It sure sounds like a circulation problem to me. (divider door has three positions -- upper, lower, and bi-level)

I've got a 77 but no air...sorry can't compare!

Good luck!
If you really believe that the three way door is opening all the way in a/c mode, then it is possible that the actual evaporator is plugged up. That means, that dirt, pine needles, leaves, paper, mold, mildew, and slime may be plugging a large amount of the cooling grilles in the evap cutting the air flow down to nothing.

If this is the case, you will have to pull the entire unit down, flush it out, and start over. I don't know of any way that you can really clean it out without taking everything out.

Let me know, and I can send you some pics of the evap box out of the car so you can see the back side (firewall side). This will help you understand: A. how the door could be sticking in the a/c position causing poor air flow B. how crud can get into the evap box and be stuck there.

Otherwise, there is no reason that the a/c air volume be any less than the heater or straight vent.

Those pics would certainly help. Thanks for your interest. I'm going to keep exploring this till I get something done.
That's what I thought, too...

I thought if the heater and vent modes were blowing good amounts of air, you should at least get that much air when the AC is on.

I know this must be frustrating for you, but there has to be a cause somewhere...keep searching. Good luck.
Link to pictures

Here is a quick link to about 8 pics of the a/c system all apart. The control head in these shots is different from yours, but all the other is basically the same.

As you can see, the blower fan pushes air down past the evap and into the heater box area. It is then diverted at that point. Maybe you can enlarge different shots to see better. Anyway, I retract what I said earlier, because the air has to come down past the evap to get into the main vents for heat or air. That goes back to you having a problem where that vent door is not moving right and it is blocking the a/c air from getting out the dash vents.

Thanks Chris!

Thanks for your interest and help. Was out tonite going over the ac system again. Can't find anything wrong and am beginning to think I am just expecting too much from an old ride. If I can't figure out how to juice up the air flow I'll just continue to live with it.

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