How Your Headlight Vacuum System Works
10-02-2015, 07:14 AM
#181
Instructor
Drivers headlight won't go down
I guess that it is my turn with the headlight problem. I found this thread and is a lot of good information. I have to work this week-end, but I am hoping to do some troubleshooting inbetween if I have time.
1981 corvette: Lights worked fine until the other night when I came home and the drivers light will not go down.
Electrically, both sets of light work correctly, and vacuum wise, the right side assembly goes up and down correctly. The left side stay up no matter what.
I did not get under the hood yet, but when I pull the small override under the steering wheel, both lights assemblies stay up when I pull the light switch in to the "off" position. When I push the override back up into the "normal" position, the right light goes down as normal, but the left stays up.
I will try to do some troubleshooting, but if someone has had this exact problem before and save me some time.....all the better.
1981 corvette: Lights worked fine until the other night when I came home and the drivers light will not go down.
Electrically, both sets of light work correctly, and vacuum wise, the right side assembly goes up and down correctly. The left side stay up no matter what.
I did not get under the hood yet, but when I pull the small override under the steering wheel, both lights assemblies stay up when I pull the light switch in to the "off" position. When I push the override back up into the "normal" position, the right light goes down as normal, but the left stays up.
I will try to do some troubleshooting, but if someone has had this exact problem before and save me some time.....all the better.
Last edited by idcwmidi; 10-02-2015 at 07:42 AM.
10-02-2015, 12:20 PM
#183
Instructor
I guess that it is my turn with the headlight problem. I found this thread and is a lot of good information. I have to work this week-end, but I am hoping to do some troubleshooting inbetween if I have time.
1981 corvette: Lights worked fine until the other night when I came home and the drivers light will not go down.
Electrically, both sets of light work correctly, and vacuum wise, the right side assembly goes up and down correctly. The left side stay up no matter what.
I did not get under the hood yet, but when I pull the small override under the steering wheel, both lights assemblies stay up when I pull the light switch in to the "off" position. When I push the override back up into the "normal" position, the right light goes down as normal, but the left stays up.
I will try to do some troubleshooting, but if someone has had this exact problem before and save me some time.....all the better.
1981 corvette: Lights worked fine until the other night when I came home and the drivers light will not go down.
Electrically, both sets of light work correctly, and vacuum wise, the right side assembly goes up and down correctly. The left side stay up no matter what.
I did not get under the hood yet, but when I pull the small override under the steering wheel, both lights assemblies stay up when I pull the light switch in to the "off" position. When I push the override back up into the "normal" position, the right light goes down as normal, but the left stays up.
I will try to do some troubleshooting, but if someone has had this exact problem before and save me some time.....all the better.
Could be the pivot link is stuck or the relay valve stuck.
10-03-2015, 09:52 AM
#184
Race Director
Member Since: Apr 2007
Location: South Western Ontario
Posts: 11,061
Received 845 Likes
on
721 Posts
I guess that it is my turn with the headlight problem. I found this thread and is a lot of good information. I have to work this week-end, but I am hoping to do some troubleshooting inbetween if I have time.
1981 corvette: Lights worked fine until the other night when I came home and the drivers light will not go down.
Electrically, both sets of light work correctly, and vacuum wise, the right side assembly goes up and down correctly. The left side stay up no matter what.
I did not get under the hood yet, but when I pull the small override under the steering wheel, both lights assemblies stay up when I pull the light switch in to the "off" position. When I push the override back up into the "normal" position, the right light goes down as normal, but the left stays up.
I will try to do some troubleshooting, but if someone has had this exact problem before and save me some time.....all the better.
1981 corvette: Lights worked fine until the other night when I came home and the drivers light will not go down.
Electrically, both sets of light work correctly, and vacuum wise, the right side assembly goes up and down correctly. The left side stay up no matter what.
I did not get under the hood yet, but when I pull the small override under the steering wheel, both lights assemblies stay up when I pull the light switch in to the "off" position. When I push the override back up into the "normal" position, the right light goes down as normal, but the left stays up.
I will try to do some troubleshooting, but if someone has had this exact problem before and save me some time.....all the better.
Swap the 2 lines going to the headlight vacuum pot at the control valve beside the headlight and see if it lowers. It's probably the valve that is stuck.
You can also check the control valve by taking the hoses to the headlight pot off the valve and then remove and replace the little vacuum control hose and see if the vacuum switches between the 2 ports. Do it with the lights off and override off. You'll have to hold your fingers over the ports or use a gauge to see if it builds vacuum.
10-09-2015, 07:48 AM
#185
Instructor
Tonight, with the headlight switch in the "off" position and the override in the normal UP position, I changed hose 1 and 2. This should have changed the vacuum and atmospheric pressures in the actuator so the drivers light drop......but there was no change.
Would this mean that the actuator is bad and not the relay / valve common to where I switched the vacuum lines?
10-09-2015, 08:15 AM
#186
Instructor
I did some additional checking and noticed that the passenger side, although it worked, it was slow. When I pulled the small vacuum line that goes from the override to the relays (splits just before the relays in the previous post picture) on the drivers side and plug it, the right side goes down way quicker. This would reinforce that I have a leak on the drivers side.
10-17-2015, 03:30 AM
#187
Instructor
I would like to thank the person who made up this troubleshooting as well as the people who replied to my query.
I went through the troubleshooting the other day, narrowed it down to the valve, ordered two new ones (one a precautionary change) from Wilcox and installed them tonight.
I can not remember when my light assemblies came up and went down so quickly.
Saved me a bunch of cash as with the troubleshooting guide and about a 20 minute reading up on the system, I was ready to take on the job myself and not go to a shop.
I went through the troubleshooting the other day, narrowed it down to the valve, ordered two new ones (one a precautionary change) from Wilcox and installed them tonight.
I can not remember when my light assemblies came up and went down so quickly.
Saved me a bunch of cash as with the troubleshooting guide and about a 20 minute reading up on the system, I was ready to take on the job myself and not go to a shop.
11-06-2015, 10:13 PM
#188
I have a 79 I bought last month. Its in the shop this winter to fix/find whatever it needs.
When pulling my headlight switch, the headlights turn on but they do not come up.
Pulling down the manual over ride, the headlights go up and down as they should.
Does this indicate a bad headlight switch ?
Thanks
8Valve
When pulling my headlight switch, the headlights turn on but they do not come up.
Pulling down the manual over ride, the headlights go up and down as they should.
Does this indicate a bad headlight switch ?
Thanks
8Valve
11-07-2015, 03:01 PM
#190
1. The headlight switch on the dash will turn on the headlights. It will not cause the doors to open.
2. The Headlight door override will on command open and close the headlight doors. and if the dash headlight switch is pulled the headlights will also be on
Does this not point to the dash headlight switch having a problem in the vacuum portion.
I have tried to follow the prints given in this thread, thats whats pointed me at the headlight switch. Maybe I am all wrong.
Thank You
8Valve
11-07-2015, 03:43 PM
#191
Instructor
Could be the headlight switch. Inside the switch, there is a piece of plastic that run on two copper runners with some small springs. I have seen the plastic break or the spring pop out and when this happens, the valve or plastic won't move to let vacuum out.
11-22-2015, 03:53 PM
#192
Burning Brakes
Question....how much vacuum should the system hold in order to operate properly? Should it be the same psi from the start of the system to the end when the car is running?
For example, I have 16-18 psi right off the manifold and after the check valve it drops to about 5-6.
Does that mean my check valve is shot? It's brand new.
What should I expect the psi to be at the headlight actuators?
For example, I have 16-18 psi right off the manifold and after the check valve it drops to about 5-6.
Does that mean my check valve is shot? It's brand new.
What should I expect the psi to be at the headlight actuators?
11-27-2015, 06:13 PM
#193
First of all, thanks to the OP for taking the time, effort, and energy to put together the highly detailed and informative diagrams and description of how our headlights work!
Since purchasing the car a month ago, I have spent considerable time under the hood tracing lines, hoses, wiring, etc, to get a better understanding of how it all works, and this tutorial really helped me trace down the operation of my lights.
It's helpful threads like this one that whet my appetite to get on the forum and search for the answers to my questions about my beautiful car.
-Ray
Since purchasing the car a month ago, I have spent considerable time under the hood tracing lines, hoses, wiring, etc, to get a better understanding of how it all works, and this tutorial really helped me trace down the operation of my lights.
It's helpful threads like this one that whet my appetite to get on the forum and search for the answers to my questions about my beautiful car.
-Ray
03-26-2016, 07:34 PM
#194
Thanks
Thanks a million, very helpful !
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
04-30-2016, 11:21 PM
#195
1st Gear
Member Since: Apr 2016
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
I need help!
I have 1978 i bought from a guy that had no clue what he was doing and frankly I don't myself. I am learning as I go. I cannot figure out these headlights. I have vacuum and have tested the accuators and they both hold vac but the lights will not lift. I do not have the manual pull down switch....its just hosed directly to the headlight switch. I apologize because I am no no mechanic..I am trying to learn this stuff and it is getting frustrating. Any help would be very appreciated!
05-01-2016, 05:45 AM
#196
Instructor
I have 1978 i bought from a guy that had no clue what he was doing and frankly I don't myself. I am learning as I go. I cannot figure out these headlights. I have vacuum and have tested the accuators and they both hold vac but the lights will not lift. I do not have the manual pull down switch....its just hosed directly to the headlight switch. I apologize because I am no no mechanic..I am trying to learn this stuff and it is getting frustrating. Any help would be very appreciated!
08-13-2017, 03:11 PM
#197
Thank you!
Thank you! This is VERY helpful.
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
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Sxrxrnr (08-14-2017)
01-27-2018, 06:58 PM
#198
Intermediate
Sorry to dredge up an old one, This is an excellent thread.
I'm working on my 77, (my 1st c3 or newer). I'm only tackling the dash now, the outside of the cockpit is later.
I'm not sure I understand the purpose of the manual switch if someone can enlighten me?
Right now I've got hoses going to the physical headlight switch, the line closest to the **** is pulling a vacuum with the engine running.
The other line, with a blue strip, goes from the back of HL switch to the bottom of the manual pull down switch.
The other line I'm assuming goes to the business end of the headlight system.
The manual pull down switch has the double "boot" attached to it, the headlight switch just has hose ends.
Is this correct? Just want to make sure as I am reassembling the dash and console. Had lotsa gremlins under the dash.
Brakes are next and then on to amenities like Headlights.
Thanks, Mike
I'm working on my 77, (my 1st c3 or newer). I'm only tackling the dash now, the outside of the cockpit is later.
I'm not sure I understand the purpose of the manual switch if someone can enlighten me?
Right now I've got hoses going to the physical headlight switch, the line closest to the **** is pulling a vacuum with the engine running.
The other line, with a blue strip, goes from the back of HL switch to the bottom of the manual pull down switch.
The other line I'm assuming goes to the business end of the headlight system.
The manual pull down switch has the double "boot" attached to it, the headlight switch just has hose ends.
Is this correct? Just want to make sure as I am reassembling the dash and console. Had lotsa gremlins under the dash.
Brakes are next and then on to amenities like Headlights.
Thanks, Mike
01-27-2018, 08:11 PM
#199
Instructor
Sorry to dredge up an old one, This is an excellent thread.
I'm working on my 77, (my 1st c3 or newer). I'm only tackling the dash now, the outside of the cockpit is later.
I'm not sure I understand the purpose of the manual switch if someone can enlighten me?
Right now I've got hoses going to the physical headlight switch, the line closest to the **** is pulling a vacuum with the engine running.
The other line, with a blue strip, goes from the back of HL switch to the bottom of the manual pull down switch.
The other line I'm assuming goes to the business end of the headlight system.
The manual pull down switch has the double "boot" attached to it, the headlight switch just has hose ends.
Is this correct? Just want to make sure as I am reassembling the dash and console. Had lotsa gremlins under the dash.
Brakes are next and then on to amenities like Headlights.
Thanks, Mike
I'm working on my 77, (my 1st c3 or newer). I'm only tackling the dash now, the outside of the cockpit is later.
I'm not sure I understand the purpose of the manual switch if someone can enlighten me?
Right now I've got hoses going to the physical headlight switch, the line closest to the **** is pulling a vacuum with the engine running.
The other line, with a blue strip, goes from the back of HL switch to the bottom of the manual pull down switch.
The other line I'm assuming goes to the business end of the headlight system.
The manual pull down switch has the double "boot" attached to it, the headlight switch just has hose ends.
Is this correct? Just want to make sure as I am reassembling the dash and console. Had lotsa gremlins under the dash.
Brakes are next and then on to amenities like Headlights.
Thanks, Mike
The manual override is in case your headlight switch takes a dump. Inside the switch is a slide that moves in or out when you pull the lights on or off. Should this fail to open the light doors, you use the over ride.
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mikejana (01-28-2018)
01-28-2018, 07:36 AM
#200
Intermediate
Originally Posted by 78 silver 78
The manual override is in case your headlight switch takes a dump. Inside the switch is a slide that moves in or out when you pull the lights on or off. Should this fail to open the light doors, you use the over ride.
The manual override is in case your headlight switch takes a dump. Inside the switch is a slide that moves in or out when you pull the lights on or off. Should this fail to open the light doors, you use the over ride.
Originally Posted by mikejana 
Right now I've got hoses going to the physical headlight switch, the line closest to the **** is pulling a vacuum with the engine running.
The other line, with a blue strip, goes from the back of HL switch to the bottom of the manual pull down switch.
The other line I'm assuming goes to the business end of the headlight system.
The manual pull down switch has the double "boot" attached to it, the headlight switch just has hose ends.
Right now I've got hoses going to the physical headlight switch, the line closest to the **** is pulling a vacuum with the engine running.
The other line, with a blue strip, goes from the back of HL switch to the bottom of the manual pull down switch.
The other line I'm assuming goes to the business end of the headlight system.
The manual pull down switch has the double "boot" attached to it, the headlight switch just has hose ends.