Hi, all.

I'm new to this forum, but have searched without the result I need.

In my '73, no air comes out anywhere when on Defrost. It's like all the air is trapped inside the plenum. I have verified that all of the doors and motors work - except for the defrost diverter. I can't see if it's working or not. I have a shop manual and will perform some more tests on that.

But, while studying the manual, I discovered that there are three vacuum lines coming through the firewall. One is the water valve, one comes off the vac canister, and the third is manifold vac. Both manifold vac and canister vac are connected to the control valve at different ports.

For the life of me, I cannot figure out why manifold vac and canister (constant) vac are routed separately and come to separate ports on the valve. Why does my system require both?

is the fan running at all?
on my 74 Monte, for some reason, the fan wasn't working at all. so i got to investigating, and when i unhook the small 2 wire harness off the AC relay, the fans kick on and i have all the speeds except "High". i fixed it a couple of years ago, but can't remember what i did..

Fan works fine. No air comes out anywhere when control is on DEF. Just makes "wind noise".

My relay was acting up when I got the truck. I disassembled it, cleaned it out, and resealed it. Worked fine ever since. I also had an intermittent bad connection at the junction block on the firewall. I replaced that, cleaned up all the connections, and the fan has worked great ever since.

Connections, connections, connections.

I'm confident I can fix the defroster. I just don't know why they route both manifold and canister vacuum to the controller. I would think that canister vac is all you would ever want. Most systems are plumbed this way.

Hi, all.


For the life of me, I cannot figure out why manifold vac and canister (constant) vac are routed separately and come to separate ports on the valve. Why does my system require both?


The canister is there to keep your heater mode doors in position when you setp on the throttle and lose manifold vacuum. If they were on the same port the canister would be nullified. It would lose vacuum right along with the manifold. Hence the seperate port. I am betting the seperate port has some sort of check valve integral to it that prevents the canister from bleeding over into the manifold port.

On my car 72 Malibu there is no canister. I have a check valve in the vacuum supply line from the manifold to the heater control system. The check valve does the same thing that your canister does. It prevents loss of vacuum signal to the heater control system during acceleration.

The canister is there to keep your heater mode doors in position when you setp on the throttle and lose manifold vacuum. If they were on the same port the canister would be nullified. It would lose vacuum right along with the manifold. Hence the seperate port. I am betting the seperate port has some sort of check valve integral to it that prevents the canister from bellding over into the manifold port.

On my car 72 Malibu there is no canister. I have a check valve in the vacuum supply line from the manifold to the heater control system. The check valve does the same thing that your canister does. It prevents loss of vacuum signal to the heater control system during acceleration.

The canister also has a check valve in it. I's just like your check valve, except you don't have the advantage of stored vacuum on the control valve side.

Without a can, you can hold your doors, but you can't operate them again until you have high man vac.

So, I still don't see why the control valve ever needs manifold vacuum. The line from the canister will always have as much or more vacuum than the manifold line will. Canister vacuum stays relatively high. Manifold vacuum varies wildly.

What good (to the control valve) is a vacuum source that can be zero?

So, I still don't see why the control valve ever needs manifold vacuum. The line from the canister will always have as much or more vacuum than the manifold line will. Canister vacuum stays relatively high. Manifold vacuum varies wildly.

What good (to the control valve) is a vacuum source that can be zero?





I have no vacuum canister on my 72 Chevelle with air conditioning. Unless and until the control valve starts to leak or some other vacuum piece down stream of the check valve on the engine side of the firewall starts to leak, I won't have to worry about the modes of my heater a/c system. So far on my 35 year old car all that stuff still works correctly. At least the modes do. The A/C and the Heater haven't worked for years. The modes still work perfectly regardless of what I am doing with the throttle.

Since I moved to southern cali I haven't cared all that much about heaters or air conditioners. I find it quite comfy here. :)

After some vacuum testing last night, it would appear that my defrost "motor" is blown. It won't hold a vacuum, and is getting a vacuum signal. I can manually operate the door. When the controls are on DEF, all the air comes out the bottom (like in heater mode).

That motor has two vacuum ports on it, though. Hmm... One is full vacuum in DEF and the other gets restricted vacuum in DEF and BI-LEVEL. I don't understand the dual port thing.

Where can I get a new one?

At a guess : The restricted vacuum in DEF probably means that to some extent there is partial signal to open the floor door also, giving heat to both areas. The BI-LEVEL sends conditioned air thru the defroster AND the A/C vents.

After more testing...

I pulled the actuator. I applied vacuum and it did it's thing. It would't budge in the car, but moves just fine out of the car.

I'm thinking I've torn all this stuff out for nothing! I'm thinking I wasted $45 on a new actuator (Old Air Products in Fort Worth carries them - GM does not).

I put it back in - won't work. It's getting vac, but won't pull the door down. WTF?!

I pulled it back out. I applied vac while pulling against it - nothing. It won't pull a load. I let go and applied vac again, it retracted, but I could easily pull it back out. Some valve or something in there is letting go.

So, I'm convinced the actuator's bad, and am waiting on my new one to arrive. I have my fingers crossed, but feel that this will finally fix my "no defrost" problem.