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Vacuum troubles?



Well, at first I thought I was just imagining it...but I have put it to the test. Here are my findings:

If the headlights are on, I can have heated air blowing through the vents.
If the headlights are off, I have no heated air blowing through the vents.

I have left the controls alone, both the selector and the temperature pushed all the way to the right. (defroster and hot) When the headlights are on, I get nice warm/hot air blowing through the vents. When the headlights are off, I only get unwarmed (outside temp) air blown through the duct system.

I have not tried:
Headlights on with warm air blowing through vents, then turning headlights off to see if the air temp blowing through the vents changes.
Pulling the lever that makes the headlights stay up when the lights are turned off to see if that lets warmed air blow through the system.

What do I need to do to trouble shoot this? ARE my headlight and heater vacuums connected? AM I going crazy?
Yes, Keith. It is. I only just learned about it recently from Kenny, when I was watching him take the column apart.

I'm thinking that you may have a mis-routed vacuum line connected in error to the controlled line to your headlight operators. The challenge will be to find the line that is connected to your heater control valve and connect it to an uncontrolled vacuum source so that it will work correctly. I don't have a schematic of the vacuum hose routing, that would certainly be of some help to you. s/b=small block, s/bc= small block chevy.


Heidi is correct, the switch is there so that the lights will stay up after the engine is off, for bulb replacement/maintenance. It is often referred to as the headlight vacuum bypass switch.
I think one of the vacuum valves has failed and it bleeding it off when it should be holding constant. You will go to a default "defrost" vent if the vacuum completely fails.

If you smoke (which I hope you don't) or if you know somebody that does, try this.

Have them puff on a cig, then take the main vacuum port off the intake that feeds back into the system and blow as much smoke as possible up the line. While they are doing this work the headlight switch and control switch in some kind of methodical fashion so you can record and monitor results. Have somebody else watch for smoke to escape.

You may start seeing smoke out front at the front set of control valves, or somewhere in the dash. Where there is smoke, there will be a leak.

It has worked for me before (I don't smoke).
No, I don't smoke...

But that is a good idea to try. Need to find a smoker to volunteer!

The long block was installed in 1997, and it has only been this falll season that I am noticing this funny phenomenon. We recently replaced the heater valve...maybe we crossed something up? I have a Chiltons and the 78 Assembly Manual, I'm sure we can find something to follow. I will also check out the L-81 site, as I've been advised there is a great vaccum guide there.

This morning I tried another test:
I turned the headlights on/up, after warmup and while driving I had warm air blowing through the defroster vent. I turned off/down my headlights and after a short bit, I had only cool air coming through the vent.
So. I guess I can safely say it definitely has something to do with the headlight vaccum. Last test to perform is to put headlights up and lock them in the up position with them being off and see if the heater works.

btw, GREAT advice everyone!
Vacuum Problems

The vacuum supply line to your headlight switch is routed from the intake manifold tap (behind the carb) to a plastic filter disc, then to a metalic disc with one intake and two outputs. This metal disc also acts as a "one way" control valve that opens to receive vacuum from the manifold and closes to stop vacuum from escaping in low vacuum or no vacuum situations, such as shuting off the engine.

One of these outputs from the metalic disc connects to a plastic "T" fitting that feeds the heater control vacuum system and the other feeds the vacuum resevoir (long tubular cylinder)located under the headlights. The other vacuum hose from the metalic disc goes to the headlight switch. When the headlight switch is pulled, vacuum is fed to another "T" connection in front of the radiator, which sends it to the two small diaphrams between the headlights, which in turn actuates the two large vacuum supply lines connected directly to the top of the vacuum resevoir, which in turn provides a strong supply of vacuum to the two large diaphrams that raise the headlights.

Your problem with the headlights and heater control valves is that they are competing for vacuum. If your vacuum system is tight and doesn't leak, there is no problem. However, the problem you describe indicates inadequate vacuum to operate both systems at the same time. The headlights will normally win this competetion for vacuum because the vacuum hoses that supply the headlight system are larger in diameter and closer to the vacuum resevoir. The heater control vacuum hoses start out small at the firewall and become very small after they pass through the firewall.

Without sufficient vacuum to keep the heater control diaphrams operating, the heater and vent doors that are kept open with vacuum, close. Simple as that. Turn the headlights off, vacuum in the system increases and there is enough vaccum to operate the heater control diaphrams and open the heat and vent doors.

Recommendation: 1) Check the hose from the metalic disc to the heater controls. Not easy or fun, so skip to 2) Check the hose from the first "T" connector to the vacuum resevoir. Chances are it is loose or cracked and allowing vacuum to leak from the resevoir, leaving only the intake manifold vacuum to operate your system. An easy way to check this is to run your engine for a minute or so, shut the engine off with the heater controls in "off" position, then see how many times you can raise and lower the headlights. Take your time and if you only get 1 or 2 up and down cycles, your vacuum resevoir, or the vacuum lines connected to it are leaking. If you get 3 or 4 cycles, your vacuum system and hoses are fine. 3) Disconnect the rubber hose feeding the metallic disc with vacuum from the manifold. If you can suck air from the single inlet side, it's fine. If you can suck and blow air through the disc, the control valve is shot and needs to be replaced. 4) Check and replace all vacuum hoses in the headlight and heater control system, which would include Item 1 above.

Actually, any leaking vacuum hose anywhere in your vacuum system will cause the other vacuum components to falter and the engine to be less responsive. If your Corvette has 20 years or 100,000 miles under it's belt, it's time to replace all vacuum hoses. Your car will run better and in some cases, much better.

Hope this helps.

Roy in Portland, Oregon
Headlight/Heater Vacuum Follow-Up

It would be nice if I thoroughly read the question before posting such a lenghty response. Please accept my apologies.

To answer the "heat only comes on when the headlights are on" is easy and simple.

Your heater control vacuum hose is incorrectly connected to the headlight vacuum line running from the headlight switch to the "T" fitting that feeds the small diaphrams in front of the radiator. When the headlights are turned on, this hose has vacuum. When the headlights are off, this hose is static and has no vacuum to operate the heater control vacuum system.

To correct the problem, connect the heater control vacuum hose to the "T" connection with the vacuum hose that goes from the metallic control disc to the vacuum resevoir under the headlights. This way, your heater controls will always have vacuum available, with or without the headlights on.

My other posting should be helpfull to anyone having a problem with the vacuum system on their Corvette.

Bad Reader Roy from Portland, Oregon

Awesome job on the vacuum diagnosis. You seem to really have a firm grasp on how this stuff works.

Very nice Roy, something tells me you've spent a fair amount of time restoring/repairing your system. Now I know who to call.

- Eric
My headlamp doors suddenly started opening on their own ,(without me turning the headlamps on ?) sometime back.
I found, I can get them to close back up by pinching one of the two lines that leave the metalic disk to the resevoir you mentioned, and then shut the motor off. But they open back up again as soon as I start the motor? I am just leaving them up until I get this funny problem fixed. I was amazed and laughed outload the first time it happened as I was driving down the road, and I thought, My car now just does what it wants..lol.
Ill print your post, and go see if I can fix this puzzler.
Headlights Stay Open

To Tom C,

Check your manual headlight override valve located under the driver's side dash. For easy access, I tie strap the black plastic valve to the hood release mounting bracket channel. If the manual overide valve is actuated, your headlights will open every time there is enough vacuum generated. Check this valve to make sure it is closed.

The only other possibility is the headlight switch pheumatics are somehow constantly connected, which is highly unlikely.

Double check the hose that you are pinching off in order to close the headlights. You stated it was the vacuum hose to the vacuum resevoir. If this is the case, the vacuum hose from the vacuum resevoir is incorrectly connected at the "T" with the vacuum line to the headlight splitter and two small diaphrams, thereby bypassing the headlight switch.

Let me guess ... when your headlights open without you pulling the headlight switch, the lights are not on, correct? When you pull the headlight switch the lights come on. After shutting the engine off, if you wait long enough for the vacuum to dissapate from the vacuum resvoir, the headlights will close on their own. Correct?

Let me know.

Roy in Oregon
I pinch and "release" this hose and the headlamps close?The other hose coming out of the disk is the one that loops through the firewall to the override.

You said:
>>"Let me guess ... when your headlights open without you pulling the headlight switch, the lights are not on, correct?"<<


>>"When you pull the headlight switch the lights come on. "<<

Correct again!

>>"After shutting the engine off, if you wait long enough for the vacuum to dissapate from the vacuum resvoir, the headlights will close on their own. Correct? "<<

No..The headlights stay open all the time until I pinch and release this hose in the pic.??
and then,
Even after pinching and closing and shutting off the motor, they open back up again as soon as I start the motor??
Many thanks for the help
I've been watching this with great interest. I've learned a lot just by keeping my mouth shut ;) great article and help..

So now I will probably put my foot in my mouth but, It sounds to me like the hose from the light switch and the manual switch (under steering wheel) are not routed correctly.. I'm at work so I don't have my shop manual.. but there is a hose with a Blue(?) tracer if memory is correct that must be connected correctly to both switches or lights come on, but don't come up unless the manual over-ride is pulled, but will go down when the headlight switch is turned off..

Similar to what you said but not exactly I think.

It was working fine and suddenly just started happening and I hadnt done anything related, that I can remember:confused

I may have the info I need from Roys posts, I just need a bit of time to focus and start troubleshooting.
Headlight Vacuum Solution

Tom C,

Thanks for the additional info. It helped to narrow down the cause of your problem. Your problem is caused by:

1. Black plastic headlight overide switch is in the open position (likely), or,

2. Vacuum is leaking through your headlight switch thereby suppling constant vacuum to your headlight diaphrams (highly unlikely), or,

3. The vacuum feed line from your metalic disc is connected directly to the small headlight diaphrams - it should be connected to the vacuum resevoir and the vacuum hose from the headlight switch should be connected to the small headlight diaphrams (very likely - my #1 choice).

By pinching the vacuum hose, you cut off the vacuum to the small headlight diaphrams which allow them to operate the larger diaphrams and close the headlights. If the headlights pop up when you release your pinch with the engine running, you are 99% sure that you have the vacuum resevoir and small headlight diaphram hoses switched.

Note: It appears from your photo that you are using clear vinyl tubing for your vacuum connections. Vinyl tubing, especially clear or white in color, is not the best choice for vacuum applications. Rubber tubing of correct ID dimensions is considered the best.

The reasons for using rubber vacuum tubing vs. vinyl are:

1. Rubber is more flexible than vinyl and shrink-seals itself once it gets warm. Vinyl tubing acts just the opposite, becoming hard and expanding when exposed to heat. I was working on the car today when I tried to remove the new rubber vacuum hoses I installed 2 weeks ago, those ******s just wouldn't let go. I strechted the small ID rubber hose to twice its original lenght and the hose, without clamps, did not budge from the fitting.

This is why pure rubber electrical tape is considered superior to vinyl tape, because once applied and warmed up, the rubber tape bonds to itself and the older it gets, the tighter the bond. Try rubber electrical tape on your next electrical job and you will never use shrink tubing again.

2. Tubing, hoses and tie straps that lack pigment are more susceptable to the sun's UV rays and faster deterioration than colored ties. Black tubing and tie straps will outlast clear or white counterparts by at least 600%.

3. Vinyl can generate static electricity, rubber won't. Static electricity in the engine compartment is not something you want.

Let me know what happens.

Roy in Oregon
Ok Roy,
Im going to order the correct color coded kit and set the vacuum up again.

Cause they are fading and I can hardly see the vacuum anyway as Chris pointed out ..doahh:gap

Seriously Ive been thinking of doing it even before this, because the headlamps were noticibly sluggish and uneven from the get go of my ownership of the car, I hope the kit comes with a schematic, I wouldnt be surprised if the hoses are not completely correct as it is. There were lots of minor little bubba'd situations with my car when I got her.

I tinkered and tested a little bit yesterday with no results except I know this, With the motor running and the headlamps opened(incorrectly), I can pop the hood pinch the hose, the headlamps close (to their correct position) then drive the car, stop and go, and on the road for an hour and they stay closed. Its just when I shut the motor off and start it up again that they open back up and stay open until I pinch again.

Luckily the problem is defaulting in my favor, meaning The lamps stick open not closed ..whew..I think Id consider my car "down" if I had to force the headlamps open by hand!

I see some new simple things to try from your last post, Thanks again.
Ill let you know what happens

You ought to be my first victim. Serious, just send me $450 (the retail cost of the parts and materials) and your lower headlight assemblies. I will rig up my electric lights and get it sent back to you ASAP.

Then, you can take all those vacuum hoses and flush them down the toilet (including the headlight switch with the stupid vacuum module on the back of it).

If this thing works out, future units will cost more, but since you will be helping me test it, you get a nice little break!

Oh, and if you just have to, you will be able to put the vacuum back on later.

Im aware of this, Ive thought about your patent and conversions as you were working out details. Very impressive , the way you find those things fairly simple and interesting. Its a potential goldmine in my opinion.
From my angle, I love the vacuum system, with all its functional inferiority and idiosyncracies. Whenever a person is interested in my car one of the first things I demonstrate is how the vacuum actuation works on the headlamp and wiper doors, Even though it doesnt work all the time :s and has embarrased me a couple times :L , I just think it's a total cool thing, that Chevrolet worked on for awhile with the Corvette.
I also get a huge kick out of the ancient fiberoptic ,head and taillight indicators ...The Babes go wild on this stuff too, their in awe of that old stuff, so thats a big plus with my ornery old bachelor hide:J

Now heres a different twist on your post, I just thought of..
And Perhaps this is not the place for this , but Ill try and slip it Past Rob....
Im a software developer and code programs for companies who market "products on the internet". You get a product(s) togeather and Ill help you promote and sell it.
Keep in mind Marketing on the internet is a long drawn out affair for all companies, regardless of the quality and potential of their products.
See what can of worms you opened with that one Chris..Heh, Heh.
It won't be for everybody.

However, between now and then during the patent pending process I do need to get the R&D up and running. As much as I would love to just give them away to somebody to try, it is not feasable for me. Plus, the benefit is more than mutual for somebody that has just had it with the old vacuum and is ready for something that will work everytime, all the time.



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