How Your Headlight Vacuum System Works
12-07-2010, 05:46 PM
#1
Racer
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How Your Headlight Vacuum System Works
There are lots of threads describing specific problems, troubleshooting techniques, and repairs; but I've yet to find anything showing or describing how the system works. So here goes.
The purpose here is to illustrate the NORMAL functions and associated vacuum and atmospheric pressure states of the vacuum operated headlight extension and retraction system in C3 Corvettes.
A brief troubleshooting guide and a hose connections diagram is included.
Please note: All text and diagrams, except for the hose connection diagram from Corvette-101.com, are copyright © 2011 by Business Data Services (my company).
Figure 1: Shows the system state after engine start (and vacuum has accumulated) with headlight switch “OFF” and Manual Override “Pull Down” Switch in the “UP” position. Note that in the relays, vacuum “pulls” the diaphragms and "Dog Bone" valves upward against the springs. This action ports vacuum from the tank to the rear of each actuator. Thus our headlights are held down by vacuum (or pushed down by atmospheric pressure) when our headlights are switched off.
Figure 2: Shows the system state after the headlight switch has been pulled to the “ON” position and atmospheric pressure enters at the headlight switch. Note that in the relays, the springs have pushed the diaphragms and "Dog Bone" valves downward. This action ports vacuum from the tank to the front of each actuator while allowing atmospheric pressure to enter the rear of each actuator. Thus our headlights are pulled up by vacuum (and pushed up by atmospheric pressure) when our headlights are switched on.
Figure 3: Shows the system state with the headlight switch in the “OFF” position and the Manual Override “Pull Down” switch in the “DOWN” position. In this case, atmospheric pressure enters at the “Pull Down” switch. As in Figure 2, note that in the relays, the springs have “pushed” the diaphragm and “Dog Bone” valves downward, vacuum is ported from the tank to the front of each actuator, and atmospheric pressure enters the rear of each actuator. Thus our headlights are pulled up by vacuum (and pushed up by atmospheric pressure) when the “Pull Down” switch is extended and the headlights are switched off.
Troubleshooting: These diagrams can also serve as troubleshooting aids. For example, Figure 1 illustrates that with the engine off, headlights off, and the manual override “pull down” switch in the up position, you can easily test the vacuum integrity of the in-dash components and hoses. Disconnect and plug the single white-striped hose at the “T” immediately above the relays at Circle 1 then disconnect the small hose from the check valve at Circle 2 and connect and operate a vacuum pump to that hose at that point. If that part of the system holds a vacuum, you can conclude that the included hoses and components are functional. If that part of the system does not hold a vacuum, then individual hoses and components need to be tested and replaced as necessary..
Similarly, you can test the vacuum integrity of the vacuum diaphragms in the tops of both relays by connecting a vacuum pump in place of the white-striped hose at Circle 1 just above the relays. If vacuum holds, you can conclude that both relay diaphragms are intact. If not, then test each relay separately and replace the leaker. (There is no repair I know of for leaky relay or actuator diaphragms.)
Figure 2 shows that you can test the vacuum integrity of the “dog bone” valves and the headlight actuators with the engine off by disconnecting each yellow-striped hose from the vacuum tank at Circle 4 and connecting a vacuum pump to each hose (separately) at that point. It takes a fair amount of pumping, but if the components are sealing properly, the headlight will slowly extend. Keep pumping after it has fully extended. If these parts of the system hold a vacuum, you can conclude that the relay “dog bone” valves, the actuators’ diaphragms, the actuators’ front seals, and the yellow-striped and green-striped hoses are intact.
It’ s bit more involved to verify movement of the “dog bone” valves in the relays. With headlights in the UP position, remove the hoses from the “T” to the tops of the relays (see Circle 3 in Figure 1). Attach a short length of vacuum hose and your vacuum pump. Pump until vacuum does not increase. Using a vice grip pliers, pinch off the hose so vacuum is maintained in the top of the relay. Then attach your vacuum pump to the relay’s center (yellow) port. Again, it takes a fair amount of pumping, but if the “dog bone” valve moved correctly and the actuator's diaphragm and is intact, the headlight will slowly retract.
An excellent guide for troubleshooting and testing individual headlight vacuum system components is located on Corvette-101.com. However, Corvette-101.com's "GENERAL FUNCTION TEST" for the system as written is NOT a valid test. So simply ignore it and go on down the page for component descriptions, troubleshooting, and tests. Figure 4 is a copy of the hose connection diagram from Corvette-101.com.
Corvette-101.com includes a relay repair procedure for the “dog bone” valves. Another repair method I used successfully for these valves can be found HERE.
You'll find a good corvettemagazine.com article with lots of photos and narrative showing how to remove the actuators and replace their seals HERE.
From the learned-it-the-hard-way school a final tip: If your headlight vacuum system functions properly EXCEPT that one or both headlights pop up when you are heavy on the throttle for a long period, you should suspect the check valve. Our ‘Vette was doing that as we climbed our local mountain passes. When everything else checked out OK, I finally pulled the check valve. I could easily blow air through it from the single port (manifold vacuum) side, so I knew it was bad. All’s well since I replaced it.
You can download a .pdf file with the above images and narrative HERE.
I learned a lot putting this together. Hope it helps!
Jerry
78IndyPace
The purpose here is to illustrate the NORMAL functions and associated vacuum and atmospheric pressure states of the vacuum operated headlight extension and retraction system in C3 Corvettes.
A brief troubleshooting guide and a hose connections diagram is included.
Please note: All text and diagrams, except for the hose connection diagram from Corvette-101.com, are copyright © 2011 by Business Data Services (my company).
Figure 1: Shows the system state after engine start (and vacuum has accumulated) with headlight switch “OFF” and Manual Override “Pull Down” Switch in the “UP” position. Note that in the relays, vacuum “pulls” the diaphragms and "Dog Bone" valves upward against the springs. This action ports vacuum from the tank to the rear of each actuator. Thus our headlights are held down by vacuum (or pushed down by atmospheric pressure) when our headlights are switched off.
Figure 2: Shows the system state after the headlight switch has been pulled to the “ON” position and atmospheric pressure enters at the headlight switch. Note that in the relays, the springs have pushed the diaphragms and "Dog Bone" valves downward. This action ports vacuum from the tank to the front of each actuator while allowing atmospheric pressure to enter the rear of each actuator. Thus our headlights are pulled up by vacuum (and pushed up by atmospheric pressure) when our headlights are switched on.
Figure 3: Shows the system state with the headlight switch in the “OFF” position and the Manual Override “Pull Down” switch in the “DOWN” position. In this case, atmospheric pressure enters at the “Pull Down” switch. As in Figure 2, note that in the relays, the springs have “pushed” the diaphragm and “Dog Bone” valves downward, vacuum is ported from the tank to the front of each actuator, and atmospheric pressure enters the rear of each actuator. Thus our headlights are pulled up by vacuum (and pushed up by atmospheric pressure) when the “Pull Down” switch is extended and the headlights are switched off.
Troubleshooting: These diagrams can also serve as troubleshooting aids. For example, Figure 1 illustrates that with the engine off, headlights off, and the manual override “pull down” switch in the up position, you can easily test the vacuum integrity of the in-dash components and hoses. Disconnect and plug the single white-striped hose at the “T” immediately above the relays at Circle 1 then disconnect the small hose from the check valve at Circle 2 and connect and operate a vacuum pump to that hose at that point. If that part of the system holds a vacuum, you can conclude that the included hoses and components are functional. If that part of the system does not hold a vacuum, then individual hoses and components need to be tested and replaced as necessary..
Similarly, you can test the vacuum integrity of the vacuum diaphragms in the tops of both relays by connecting a vacuum pump in place of the white-striped hose at Circle 1 just above the relays. If vacuum holds, you can conclude that both relay diaphragms are intact. If not, then test each relay separately and replace the leaker. (There is no repair I know of for leaky relay or actuator diaphragms.)
Figure 2 shows that you can test the vacuum integrity of the “dog bone” valves and the headlight actuators with the engine off by disconnecting each yellow-striped hose from the vacuum tank at Circle 4 and connecting a vacuum pump to each hose (separately) at that point. It takes a fair amount of pumping, but if the components are sealing properly, the headlight will slowly extend. Keep pumping after it has fully extended. If these parts of the system hold a vacuum, you can conclude that the relay “dog bone” valves, the actuators’ diaphragms, the actuators’ front seals, and the yellow-striped and green-striped hoses are intact.
It’ s bit more involved to verify movement of the “dog bone” valves in the relays. With headlights in the UP position, remove the hoses from the “T” to the tops of the relays (see Circle 3 in Figure 1). Attach a short length of vacuum hose and your vacuum pump. Pump until vacuum does not increase. Using a vice grip pliers, pinch off the hose so vacuum is maintained in the top of the relay. Then attach your vacuum pump to the relay’s center (yellow) port. Again, it takes a fair amount of pumping, but if the “dog bone” valve moved correctly and the actuator's diaphragm and is intact, the headlight will slowly retract.
An excellent guide for troubleshooting and testing individual headlight vacuum system components is located on Corvette-101.com. However, Corvette-101.com's "GENERAL FUNCTION TEST" for the system as written is NOT a valid test. So simply ignore it and go on down the page for component descriptions, troubleshooting, and tests. Figure 4 is a copy of the hose connection diagram from Corvette-101.com.
Corvette-101.com includes a relay repair procedure for the “dog bone” valves. Another repair method I used successfully for these valves can be found HERE.
You'll find a good corvettemagazine.com article with lots of photos and narrative showing how to remove the actuators and replace their seals HERE.
From the learned-it-the-hard-way school a final tip: If your headlight vacuum system functions properly EXCEPT that one or both headlights pop up when you are heavy on the throttle for a long period, you should suspect the check valve. Our ‘Vette was doing that as we climbed our local mountain passes. When everything else checked out OK, I finally pulled the check valve. I could easily blow air through it from the single port (manifold vacuum) side, so I knew it was bad. All’s well since I replaced it.
You can download a .pdf file with the above images and narrative HERE.
I learned a lot putting this together. Hope it helps!
Jerry
78IndyPace
Last edited by 78IndyPace; 08-09-2011 at 06:26 PM. Reason: Noted invalid "GENERAL FUNCTION TEST" on Corvette-101.com
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12-07-2010, 07:04 PM
#3
It's obvious that an incredible amount of work and thought went into this. Congratulations. 
I'll suggest that this be made a sticky by the mods, hopefully people will point to it when the all too frequent headlight posts come up.
I'll suggest that this be made a sticky by the mods, hopefully people will point to it when the all too frequent headlight posts come up.
The following users liked this post:
mtydings (12-12-2020)
12-07-2010, 07:51 PM
#5
Thank you so much for all your hard work.
I'm quite sure many many C3 owners will benifit from your information.
Well done!!!
Favorites...Add to Favorites....C3 service notes. .....Done!!!
I'm quite sure many many C3 owners will benifit from your information.
Well done!!!
Favorites...Add to Favorites....C3 service notes. .....Done!!!
12-07-2010, 10:59 PM
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An excellent guide for troubleshooting and testing individual headlight vacuum system components is located on Corvette-101. However, the system hose connection diagram on that web site is WRONG as it has the headlight switch hose connections REVERSED. A corrected version of that diagram is shown in Figure 4.
Very nice guide, thanks for posting it!
12-08-2010, 01:31 PM
#11
Racer
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Thanks, guys, for your kind comments.
Excellent question. Here are a couple of suggestions:
1. Engine off. Disconnect the yellow-striped hoses from the vacuum tank and plug both of those ports on the tank. Disconnect the larger vacuum hose from the double-ported side of the check valve and connect your vacuum pump into that hose. If you pump like mad for 1-2 minutes you should be able to attain somewhere around 10 in. Hg (inches of mercury) vacuum, and it should continue to hold steady or at least very nearly steady when you cease pumping.
2. Engine running. Before starting the engine, disconnect both yellow-striped hoses from the vacuum tank and plug one of those tank ports. Attach a vacuum gauge to the other tank port. On the double-ported side of the check valve, remove the smaller hose and plug that port on the check valve. (Make sure the check valve is functioning properly: You should not be able to blow through that valve at all from the single-ported side.) Start your engine, and while it's running check the vacuum gauge and note the reading. Stop the engine. The vacuum gauge reading should continue to hold steady or very nearly steady.
How do you test the tank for leaks?
1. Engine off. Disconnect the yellow-striped hoses from the vacuum tank and plug both of those ports on the tank. Disconnect the larger vacuum hose from the double-ported side of the check valve and connect your vacuum pump into that hose. If you pump like mad for 1-2 minutes you should be able to attain somewhere around 10 in. Hg (inches of mercury) vacuum, and it should continue to hold steady or at least very nearly steady when you cease pumping.
2. Engine running. Before starting the engine, disconnect both yellow-striped hoses from the vacuum tank and plug one of those tank ports. Attach a vacuum gauge to the other tank port. On the double-ported side of the check valve, remove the smaller hose and plug that port on the check valve. (Make sure the check valve is functioning properly: You should not be able to blow through that valve at all from the single-ported side.) Start your engine, and while it's running check the vacuum gauge and note the reading. Stop the engine. The vacuum gauge reading should continue to hold steady or very nearly steady.
12-08-2010, 02:38 PM
#12
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Hi Jerry,
Very nice diagrams and desriptions. As others have said, the effort you put in this is very evident.
Regards,
Alan
Very nice diagrams and desriptions. As others have said, the effort you put in this is very evident.
Regards,
Alan
12-08-2010, 03:30 PM
#13
Burning Brakes
This should be a STICKY........Great job!
12-08-2010, 06:50 PM
#18
Racer
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If your Vette is really a '59 as your public profile on this forum says, there is no vacuum tank.
According to the Mid America Motorworks (a forum sponsor) web site, vacuum reservoir tanks on 68-72 cars are located in left front fender behind the wheel, 73-79 cars use the front crossmember behind the bumper cover, and the 80-82 use a "coffee can" type located in the front end attached to the radiator support rod by a clamp.
On my '78, I can easily access the vacuum tank's ports through the opening between the hood and the headlights when the hood is fully open.
According to the Mid America Motorworks (a forum sponsor) web site, vacuum reservoir tanks on 68-72 cars are located in left front fender behind the wheel, 73-79 cars use the front crossmember behind the bumper cover, and the 80-82 use a "coffee can" type located in the front end attached to the radiator support rod by a clamp.
On my '78, I can easily access the vacuum tank's ports through the opening between the hood and the headlights when the hood is fully open.
12-08-2010, 09:47 PM
#19
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headlights wont go down.
ok my headlights wont go down. if i remove one of the yellow stripe hoses from the tank and put my finger over the tank port the opposite headlight goes down. then do the other and they are both down. when lights are switched on they both come up very slowly. what could be my problem? thanks oh its a 1978.
Last edited by marcmullins; 12-08-2010 at 09:47 PM. Reason: forgot year.
