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Defrost/Heater/Vent Control

Aug 29, 2001
Norcross, Georgia, United Stat
2017 Arctic White Grand Sport
Can anybody give me some advice on how the heater control works? The slider that control the heat is fine and I'm pretty sure the hot water control valve is working.

My problem is that when I move the slide that ontrols airflow, it never changes the direction of the air always coming out of the bottom. My basic understanding is that the slide control vacuum through a vacuum control coming off the back of the slide to a valve on one side of the unit that controls the direction of the air.

The Shop Manual talks of an adjustment on this system but I could not find anything that said how to do it.

My first thought is that I need to check vacuum but can't figure out which line brings the vacuum to the unit, which line should have vaccum in which position, etc.

On a 77

Where are the controls for AC/Vent/Heat/Def
(etc) on the center console top ( by bottom of radio?)

Might have SOME info..but not all ( currently)

Have to go to pick up my 73 this afternoon, I could rummage around and take a peek.

Duh... more info

Ding ...round one

So the 84 works similar to a 77 ( I'm guessing here) You have a VAC supply from the engine
intake manifold & that "T's" to some form of VAC resivior. That 3rd line goes to a VAC switcher in the heater control.When you slide the lever back & forth. There are about 6 or 7 outputs that meter amounts of VAC to the different doors to open & close.

There should be a multi line plug,moulded assy that goes to the back of the heater controls. When you have the car running..do you hear a hiss as you change positions?

I would verify you vac hoses under the engine first, then chase it to the center console.


I have the control on the center console under the radio.

You are right about the 6 - 7 ports.

Any idea where the vacuum line comes through the firewall.

I'll have to check for the hissing. I seem to recall this when I was working on the headlight switch and thought it was the problem with the headlights not going up and down. Turns out it wasn't and just forgot about it.

Even more interesting is that I though that when I put it in Max A/C, it was switching to the center gauge cluster vent but then would go back to the bottom on any non a/c setting and would not change again. I just went out and started the car to check this out and to add this info and to my surprise (well not really, nothing surprises me any more) everything works as it is supposed to. I did nothing. Turned the car off, let it set overnight and just restarted it this afternoon.

I'll try to look at this this weekend but with my son's birthday and the Super Bolw it will likely be next weekend.

Ding....Round 2

Well I was mistaken. The vacuum controlled hot water valve to the heater is not working. I know this because there is no vacuum line hooked to this. What I thought was cool air is just 60 degree air heated to 75 degrees.

Anybody know what it takes to replace this line. I can't see where it goes through the firewall. If it is just a simple matter of pulling the shield over the distributor and a couple of other minor things I might give it a try. If it gets much more compliacted than that I guess I'll have to pay someone.

Any chance this could have an impact on my control problems, which, by the way, are not a problem right this moment as everything is working right this moment.

I had the same problem, and found the line. It was a very small yellow vacuum line. I thought it was a small wire at first, but it was hollow. (couldn't believe a vacuum line could be that small...)
Any info on how to replace it would be great. Mine still works, but it is cracking, and it's so small I don't want to fool around with it and end up cracking it.

I also have the two vents sticking open that are right behind the radio and console. I couldn't figure out why it was so hot...
Didn't go to the yard today

So I couldn't take a peek @ things.

I will venture the following. There is a VAC port multi tap 1/2 way between the back of the carb & the distributor. ( you will probably have to remove the chrome Distrib sheild)

But if you chase the "T's" and filters you will find the line of suction you seek. Just start marking the lines 1 by 1 and inspecting ( old, cracked, soft lines) and replacing as needed.


The vacuum diagram in the shop manual does show a fitting in the line. It has the source coming into the fitting and going out to the heater valve and the vacuum line for the shut off valve coming in to it from the control area and out to the shutoff.

T78 made a milllion dolllar observation for me. I have the same small yellow line, the shop manual says white but after 25 years..., I also thought was a wire as well. When I a closer look at it, I thought "bubba had been here", particularly since there was a small piece of larger black rubber hose on the shutoff that I now surmise is the fitting for the smalll white (yellow) plastic house. Mine, unfortunately, is melted in several areas.

T78 can you confirm this setup and if you get a chance to go digging around where this hooks up? If its a difficult project I will probably splice mine together for the short term. I have other more immediate projects planned, such as air conditioning before the Atlanta Summmer.

Currnetly, I'm still putting the dash back together and only want one project at a time. I can't keep track of it all otherwise.


There is a moulded rubber fitting on the end of the hose to attach to the device ( vac door, valve,solonoid, etc ) So the "fix" when those lines break is to put a short piece of rubber hose on the end.

The hard (white,yellow and after a LONG time..brown) vac line is to prevent collapse. A good fix is to get some oil pressure sender line
( for after market mechanical oil gauges) or just run german fuel/vac line ( the type with cloth on the outside)

Zip sells the replacement VAC switch on the back of the heater controller for 15.00 ( I think )

This WILL be your next failure, the rubber seal gets old and leaks.

So I did some scrounging

And I viewed the following,

Where your throttle cable comes thru the fire wall, move a little to the right ( twds fender) and there is a black normal vac line, that goes to the firewall. There is a rubber grommet for 3 lines ( the 2 others are electrical ) which goes to your heater vac switch control.

Out of a 77.

The source vacuum line for the a/c controls comes off a tee in a vacuum line that comes off your manifold tee behind the carb. The tee should be located next to the brake booster. It is a hard plastic part. The line for the a/c controls is a tiny hard plastic tube that looks like a wire. It should run behind the distributor and into a large rubber grommet in the firewall just to the right of the distributor, up high. There will be two other vacuum lines coming out of this. One of them runs to your water shut-off valve. The other runs to the fresh air vent actuator way down inside the right fender. The grommet is actually a connector, as these vacuum lines don't actually pass through it, but rather terminate within it. You can see this grommet on the inside of the car only if you have ALL of the ductwork, including the large air distribution box, out of the car. Its way up there.

There are two reasons why your air duct controls are not working... 1) no vacuum is being supplied to the vac control switch mounted to the slide lever because the source line is shot or not connected, or 2) the vac control ported switch is shot and leaks all the vacuum out. As wigman said, you can buy this part from Zip or TLD, or even GM, for 10-15 dollars as they are in millions of gm cars. As for the hard plastic vacuum line, I'm not sure where to get that.

If you need a picture from the service manual for routing these lines, just let me know. I'll take my book into the office and scan it for you.

There is an adjustment on the temperature cable that opens the heater core door (cold/hot lever). On top of the distribution box is a vacuum switch that controls the vacuum to the water shut-off valve. You have to adjust the position of this switch and the cable travel so that the switch is opened (pushed in) when the temp lever is moved to the cold position. Then, vacuum will be released to the water shut-off valve, closing it (or vice-versa!).

Hope this helps.

ps... on the inside, its also easy to pinch those hoses when re-installing the distribution box and ductwork... must route carefully.

Thanks for all of this info.

When you say the vacuum line for the water shut off valve terminates within the grommet, I read you to say that the old line can actually be pulled out of it and a new one pushed in and because it is a connector it can be done from either side. Obvioulsy I would want to pull the one from the engine compartment out as that is the side that is currently broken. Is this correct?

I've done some more research on the actual line. It is 3/32 hard plastic line that GM no longer makes but Volkswagon does. GM referred me to VW. The part number is N0201391.

The air duct controls are currently working. I did nothing but let it sit overnight and it has been working ever since.

Thanks for the offer on the diagram from the service manual. I have a copy and found it there.

The adjustment that your talking about Tom, is it just adjusting the travel in the cable using the plastic handle on the cable or is there something else?

Any idea when vacuum should be supplied to the line? When the slide is all the way cold or moved toward hot?

Yes you can

The grommet can silp fit the vac hose..it's not glued in or anything.. you might want to spray some silicone on the hose to start it in.

If you have a mighty vac hand pump.. apply vac to the valve.. that will tell you.. VAC=hot

or visa versa.

I'm curious.. the hard line ( on my 73 ) is the oil pressure sender.

The car's I saw had standard vac line ( not the small stuff) .

and as far as sitting & working..( assuming it's cold outside ) would just mean that the hoses are going bad.. and have either pinholes or very soft rubber.

Thanks Mike.

I don't have a vac pump but guess I will need to get one.

This weekend I'm going to change the coolant so I guess I'll get a thermostat, don't think there is one in there, and a new heater valve and replace both.

Or you can

use that big vac pump under the hood!

Your engine!

Patched the broken vacuum line, replaced the heater valve and voila, no hot air when the slider is set to cold.

Only had 6 psi of vacuum at the valve, where there was 12 at the headlight switch. Would you expect this to be lower?

Well, no, uh kinda, maybe.

It depends on how many check valves are in the line and how gooked they are.

As long as you have enough vacumme to get the job done without it changing states when you punch it.. it SHOULD be ok.

You might have a pinhole leak down stream in one of the VAC motors, measure the vac feed
all the way down starting from the manifold ( don't "T" read direct)

And work your way back.

Someday Mike. Right now they work. I'm going to spend my time and money on new carpet, the seats and door panels, the A/C, some minor body work and a much needed paint job. What's the saying; "if it ain't broke, don't fix it".

Insert crowd cheer here!


Great news

On to the next project du jour

Well I thought I had this problem whipped only to get the first almost 90 degree day in Atlanta. Warm air coming out of the vents with it set on cold. I pulled the vacuum controlled switch out and replaced it with a manual valve. This works better but the heater core still manages to get warm; I guess from either the heat transferring through the brass valve or, perhaps more likely, the hot water from the outlet side of the heater core.

I'm about to put a shutoff valve on this side as well. Anybody have any thoughts?


By the way, I still have 6 pounds of vacuum at the vacuum controlled valve. I tested the valve after I took it out with a vacuum pump and found that it closed at 5 pounds and prevented any flow of air through it so this does not appear to be the problem.

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