Heater Control Valve Question

[Bill75]

The attached pic is the rotary vacuum valve from the console that changes the function from heat to A/C, to Vent, etc. Each of the places where the lines attach are numbered and each line that attaches has a color code. There's only one way the hoses attach and I'm pretty sure they're correct but I'm trying to solve a heat problem so just to be sure.......does anyone have a drawing or table that tells what number line controls which vacuum actuator. I've looked on Bobchad's site but his 77 is different than my 75.

ALSO......As soon as I start the car my lower right kick panel actuator opens along with the fresh air vent (Console control is Off). This leads to a continual airflow in the car. My Vette shop says the kick panel vent should only open at max A/C to resirc the car air and the valve in the picture is defective. I replaced it with another (used) one and it does the same thing. Is that normal??

 

[JohnZ]

The GM Chassis Service Manual has a diagram of the ports on the vacuum switch and which color hoses go on them and what each one controls in the Air Conditioning section. Your shop is correct - the only time the right kickpad vent is open is when the A/C is in "Max", to recirculate inside air; at the same time, the upper outside air door is closed - this reverses in all other positions.

For complete coverage, you should have the 1974 Chassis Service Manual and the 1975 Chassis Service Manual Supplement - both are available at the NCRS on-line store at www.ncrs.org.

 

[Bill75]

OK John, thanks for the advice. My Assembly manual shows a diagram but it's a little vague. It shows that the right kick panel actuator should have a hose with a green tracer, which is does. Assuming no-one has changed the hose arraingement where they connect to the actuator,(which doesn't look like they have), would you suspect that the actuator valve is faulty? I've rotated the valve with the engine running and the door (and others) change position, but when the selector is put back in the "Off" position to door is open.
Wonder if I used some solvent and sprayed into the actuator ports, the blew them out. I wish it wasen't riveted, I'd take it apart and see what's going on.
I'll check out the ncrs site.

Bill

 

[wishuwerehere82]

The door opens when the switch is off to provide ventilation in the cabin all the time. It's part of the Astro ventilation scheme so there is always some airflow inside.

 

[Bill75]

Correction in my above post, I said green stripe to the kick panel actuator, it's yellow.

wish..82, in my case the kick panel and the upper right actuator (which opens to the outside world) both open when the car starts. That leads to a continual blast of air when going down the road. The faster you go the more air comes in, not a suttle airchange from the blower continually running. There's definately something wrong here.

 

[wishuwerehere82]

Oh, that's different.

Probably something is broken in the valve assembly. You could drill out the rivets to check it out, and reglue or bolt it it back together. I would assume that the individual lines on the control could be in the wrong place, but only if this problem has been there all along. You could try switching lines until it works the way it is supposed to.

 

[Bob Chadwick]

Can you check the vacuum going to the two actuators as you go through the various selections? That will tell you whether the dial is working correctly or not. Sounds like the dial is the problem or the vacuum linea aren't hooked up correctly.

If the two actuators are changing position when you turn the car on they sound like they are okay because the engone vacuum is opening them up and that's what's supposed to happen. Your problem is that they shouldn't be open at that dial position.

They also appear to be working against each other. When the top is open, the bottom should be closed. When the top is closed, the bottom should be open assuming the correct dial position.

 

[Bill75]

Hello Bob,

Yes, I took the dash off and you can see the actuator change position when it's rotated so the vacuum is there but it's like you said they're working against each other. The small manifold that has all the vacuum lines in it that attaches to the roatary switch could have been switched around by someone but I think it's unlikely, they're really bonded in tight I couldn't budge them. It seems like the thing it out of timing, the doors open and close at the wrong positions but it's impossible to assemble it so that could happen. I guess it sounds like the rotary switch is bad but it's pretty suspicious, I put that other USED one on and it did the same exact thing. I wish I knew what #'s on the switch each of the lines were hooked to, that would proove that the switch was bad and not "replumed".
Bill

 

[Bob Chadwick]

Bill,

Your best bet is to get your hands of one of those books John references that has the vacuum lines in it. If not, check out Ebay as well as the NCRS. Various books come up used at good prices all the time. Also, I think the Haynes manual I bought had a good diagram.

I find it hard to believe that the internals of the rotary are bad. I would think that you would get vacuum leaks that would prevent the system from working rather than working in the wrong order.

Is it possible that it is attached with the manifold in the wrong position?

Bob

 

[Bill75]

Bob,
The manifold has an excentric pin araingement so it's impossible to put it on incorrectly, I think it's strange too about the switch going bad. Maybe there's a thin gasket inside that's leaking into another chamber or something. I'm going to take it apart as soon as I verify the hose hookups are OK. I'll post a picture for the hell of it.
Ok on the books John mentioned, I just hated to spend $70 on them just for a picture of the manifold, never thought of Ebay though.
Someone just pm'ed me and is going to forward a diagram, we're getting closer!

Bill

 

[Bill75]

Well, I couldn't stand it any more so I took the valve apart! There are no gaskets in these things, just ports that are exposed to vacuum as the body is rotated. So as far as I can see, unless the valve is machined incorrectly there's absolutely nothing wrong with this valve. There is a light coat of Moly grease on the mating pieces that help seal it an eliminate the need for a gasket.
Thanks to Lucky76 who sent me the plumbing diagram, the hoses are hooked up correctly. So here's todays question:
The vacuum lines for the fresh air actuator and the kick panel actuator are both hooked together, when one gets vacuum they bothe operate. On my car with the engine off the kick panel is closed by a spring, the fresh air vent looks to be open I can see the fan blades. When I start the car the fresh air actuator closes and the kick panel opens. It sure sounds backwards to me.

 

[Bob Chadwick]

When I start the car the fresh air actuator closes and the kick panel opens.

That's how it should work in max a/c mode only. The top actuator closes forcing air to come from elsewhere. By opening the bottom actuator the air is drawn from the cabin. If the bottom actuator was to remain closed you would draw hot air in from the opening at the bottom of the plenum, which would be hot air off your engine.

In all other modes the top should be open and the bottom closed. This draws air from the top of the car down the plenum.

 

[Bill75]

Boy, what you say makes sense, but the car doesn't react that way. With the indicator in the Off position the kick panel opens and the top door closes. If the switch is good and the lines are correctly hooked to it then the adjustment of the linkage to the switch must be loused up to allow vacuum to be presented to the kickpanel valve when it shouldn't be. Can't think of any other reason!
I'll check it out tonight.

Bill

 

[Bob Chadwick]

The actuators don't appear to be an issue because they are functioning according to the vacuum signal they get. If the lines are hooked up correctly and it does the same thing with two valves I would start looking at the connections in the slider that changes the mode. Could the valve not be lining up properly when you move the selector allowing a mix of two selections.

I just bought the whole assembly when I had the dash apart so I didn't have to play with it.

 

[Bill75]

Well, I'm stumped on this one! Counting the origional valve, I've had three of them operate exactly the same, start the engine and the kick panel door opens and the top vent closes, dial indicator is in off position. I even had one of them mounted in the heater control and just plugged in the vacuum manifold. The hoses are in the right places. I even made a model using some clear acetate marked with the orifaces and put it on the switch I had taken apart. When you rotate the marked clear plastic you can see where orifaces line up. There's no way I can see that this can operate the way it's supposed to. I hear that after late 75 there were changes made. Is there anyone out there with a 74 that has a kick panel door that only opens at max A/C? I gotta be missing something!

 

[Bill75]

Conclusion. Hot Air and Valve

After taking the entire dash apart and checking all the actuators and vacuum lines I've found that everything is hooked up according to the manual and works the way it was designed. I've also tried three different actuators and they all work the same way, I've taken one apart to see how it works. On a 75 the heat/air ducts work as follows:

When the console dial is in the off position, vacuum is applied to the top right fresh air vent CLOSING it and simultaniously to the right kick panel OPENING it. This puts the system in a closed loop mode, if the fan were running it would recirculate the air. This is contrary to how later year cars work, normally the kick panel door is closed and only goes in this mode at the Max A/C position. On this year car, when the console dial is in the MAX A/C position the doors remain in the same position.

Additionally I found out why I was getting fumes from the engine in the car.....Someone had worked on the A/C and had removed the gaskets where the pipes enter the heater/AC evaporator coil. The hot air and fumes were entering the box around the pipes and making their way into the car. I sealed them up and NO MORE ENGINE FUMES OR SMELL!

I also believe the excessive heat in these cars also comes from the heater box getting hot from the exhaust manifold. I have a water shutoff but after about an hour driving today, the cab still got hot from the heater ducts being warm even with no airflow. When I got home I touched the box under the hood and almost burnt my hand. It's so close to the exhaust manifold, the thing is like a little furnace. The hoses were warm but not hot like the heater/AC box. Next step is to insulate the box somehow from the exhaust manifold. I'll post how that turns out.

 

[Bob Chadwick]

Great news.

Before you go insulate that box try getting some gasket material and sealing around the area where the heater hoses go up into the car. A lot of hot air is supposed to go up that way.

I haven't had a chance to do it yet and won't because my car is going to get PAINTED next weekend and I won't see it for several months.

 

[lucky76]

Great news.

Before you go insulate that box try getting some gasket material and sealing around the area where the heater hoses go up into the car. A lot of hot air is supposed to go up that way.

I haven't had a chance to do it yet and won't because my car is going to get PAINTED next weekend and I won't see it for several months.

My gasket was completely missing where the heater hoses go up to the heater core. I sealed it with the foam in a can from Home Depot. Worked great.