PSA
http://vetteworks.tripod.com/techdocuments/Vaccum.html
Thank you to the unknown author.
This article was written in 2000, and it is by far the best explanation of our vacuum control systems that I've found.
Read it all, re-read it, and you'll be enlightened.

 

Really clear write-up. Thanks for the link!

 

With all due respect to the OP on this, and I mean this sincerely, there are far too many errors in the write up to be taken seriously. Please don't take this personally. This link was posted here years ago and if I remember correctly, the same errors pointed out then were never corrected. As stated, the last update on this was posted in the year 2000.

Following the schematic from the vacuum source, the check valve is labeled incorrectly as a filter, which it is not.

The hose labeled #3 connects to a T connection with the #5 hose incorrectly connected to the wrong vacuum port on the headlight switch. The correct port is the one closest to the pull ****. If connected following the schematic, the headlights would not operate.

The same scenario exists for the headlight override switch. The vacuum hoses are reversed. Connected like the schematic, the headlight override switch would not work.

There is a simple rule to follow regarding headlight switches and override switches. The vacuum source hose always connects to the port closest to the pull ****.

The explanations on how the system works gives me concern also. The statements;

"they are fed vacuum pressure through small black lines directly from the manifold"
There is no pressure in an evacuated circuit. The only pressure is located on the other side of whatever is separating the two sides, be it a switch or a diaphragm. It is atmospheric pressure that actuates the components.

"THE ENTIRE CONTROL OF THE VACUUM SYSTEM IS ACCOMPLISHED BY DIRECTING MANIFOLD PRESSURE INDEPENDENT OF THE MAIN VACUUM RESEVOIR OR SYSTEM SUPPLY. MANIFOLD PRESSURE CONTROLS, WHILE RESERVOIR PRESSURE DOES THE WORK"
The author continues to use the premise that it is the force of vacuum that does the work throughout the rest of the article. Maybe it is the engineer in me that has a problem with this kind of explanation. I just feel it is confusing to the layman, or possibly someone just trying to fix their car.

Willcox has an excellent series of vacuum diagrams and helpful articles that are correct.

 

I have to agree with Dave J. This was a valiant attempt at un-masking the vacuum system...but it lacks in many respects. There is no "overview" of the entire system and how it operates. The writer "kinda, sorta" gets how the system works, but doesn't clearly describe anything. If reading it helps folks get their mind around the vacuum system, so be it. I wouldn't recommend it to anyone.

 

Tough crowd. I too am an engineer. Yes, it's atmospheric pressure not vacuum pressure but who cares?
I suppose you condemn the use of the word cold as a common conception of lack of heat.
I thought it was an enlightening article.

 

I started to read it. It's crap. Says when you turn on your lights it sends vacuum to the prop valve.
Noooo. When you turn on your lights it CUTS vacuum to the prop valve and the spring inside pushes the dog bone downward directing vacuum to the lower port and then of course to the front of the actuator. Looks like a very poor overall article to me. Couldn't read it all.

 

FWIW, and I didn't read the article, the author is/was J. Myers
(Just filling in the blanks, not a cent invested, don't care)
M

 

When Dave J chimes in listen!!!!!
He spent allot of time understanding and making improvements on this system.
He invested a bit of time in me sorting out my 68.
He knows these systems.
Marshal

 

I've often thought that this forum could be well served by operating in some part like Wikipedia given that many of the tech problems are rehashed fairly frequently. As an example how this could work:

Someone could start a new thread and post the above linked vacuum article in it's entirety. Others could chime in and point out factual errors or offer worthwhile additions for the OP to use to edit the article. Once a consensus is reached, the thread could become a sticky. I would add one more sub-forum to the C3 section just for "technical" stickies like this. One thing that might need to be addressed would be some type of disclaimer giving the original author of the article credit. Or we could just start from scratch and collectively write a new article on the subject of vacuum. This process could be repeated for many of the common technical issues on our C3s.

DC

 

Good idea and it'll take the heat off me.

 

While this doesn't include the wiper door system here's a truly excellent and very easy to follow explanation of the system. And yes it says that it's actually atmospheric pressure doing the work and not vacuum.

https://www.corvetteforum.com/forums...tem-works.html

Except for the added electric solenoid operated vacuum switch that gets power from the purely electrical wiper switch and the mechanical *single-pole, double-throw" vacuum limit switch that senses door position there's extremely little difference between the wiper and headlamp door systems.

I still appreciate the vacuum systems for their simplicity, light weight, longevity, immunity to damage from being "jammed", inherent ability to modulate (like cruise control) and essentially maintenance free and self-regulating operation in any reasonable conditions of heat/cold/wet/dry without any sort of special protection from those conditions.

After more study of the link provided by the OP I have to agree that it is poor indeed. Beyond the system hookup error the nomenclature is terrible. The one-way valve that allows the system to hold vacuum when not available from the engine is labeled as "filter" when in fact the filter is the component drawn but not identified just after the manifold vacuum source. The important of that one-way valve is not even mentioned.

The switches and relays used in this system are NOT proportioning. They are digital in nature and expose at further from the vacuum source component to either vacuum or atmospheric pressure while preventing the vacuum source from exposure to the atmosphere (e.g. a vacuum "leak") The only proportioning vacuum valve in C3s is in the cruise control modulator.

The so-called "dog bone" part that is the heart of the purely vacuum relays that expose one side or the other of the bi-directional vacuum motors is not illustrated like in the link I provided. Such is critical to understanding how the system really works and utterly required with regard to checking the relay for proper operation.

The purely vacuum limit switch that senses door position is barely mentioned with zero mention of its true place in the system and how to check it.