(I know theres about a billion of these threads but I swear I do use the search button.)

My headlights on my 74' do nothing. I pull the switch for the lights..the dash lights up... but the headlights do nothing.. Well one bulb lights up still. So I know 3 bulbs are burned out, but as for the rest I'm at a loss. I know that there's a green wire coming from either the horn or a relay that's unplug and laying on the vaccum canister by the radiator but I don't know where it goes too. I also know that there's no springs to bring them back down because they rusted off. The actuators work because I detatched their hosing and appleid air directly into them and they went up and down. I have now clue other then that. Any help is greatly appreciated, especially since there's so many topics about this kind of problem.

On a side note to atleast add some originality I'll throw in another problem. The seat warning sensor has been disconnected under the driver's seat. I have the 3 slotted plug but I don't know where to connect it too. Can someone tell/show me where to plug it into under the seat?

 

Hi 21,
If I may, can I suggest you get a:
Vacuum Hose Diagram
Vacuum System Trouble Shooting Guide
74 Assembly Instruction Manual
GM Chassis Service Manual

We're all working on a 30 + year old cars. Everyone needs all the informaton we can get.

Regards,
Alan

 

I have 76.
Get the manuals Alan suggests they do help. Believe me. You may also want to get the big 3X2 ft. electric diagram.
The green wire on my 76 (by vac. relay) is for the horn not sure if you have 2 horns, mine has 1. But, the connection is there for the 2nd.
On my 76 the healight switch has a little vac. actuator mounted on the switch; believe me you can't repair it. When you pull the switch it re-routes the vacuum and opens the doors, much like the manual override works. This is not an easy switch to get at.
If your door actuators are working then I would look to the realy's just because they are easy to get at. The start working backwards.
Does 74 have manual override valve? If so will this open the doors? If so you've narrowed it down. Now the hard part, getting to it.
the vac. trouble guide is good info. & will guide you thru the process of checking the system.
Good luck

 

do you hear any hissing behind or under the dash? could be just a simple line unplugged from the switch or if it has one the override pull down valve directly under the steering column dash area.

 

One of the first things you can do is determine if your vacuum reservoir is holding vacuum to operate the headlights. The reservoir is a long round cylinder in the nose of the car in front of the radiator support. It has a vacuum hose that goes to the engine where it derives it's vacuum. It also has hoses that go to the headlight relays that operate your headlight actuators. This is what they look like...

Stand in front of the car with the hood up and reach (look) under the edge of the front body and you will see them attached to the support that the headlight assemblies attach to. There should be two of them, side by side on your car, one for each headlight actuator. Some of the later C3's make do with only one. You can test the relay's one at a time by disconnecting the large center hose (to the reservoir) and plugging it while you test the other relay.

The relays operate by moving a slide in the center that ports vacuum to one side or the other of the headlight actuators. They will have three large ports (on the black plastic portion) and one smaller one on the top (metal part). The center large hose will go to the reservoir and the two outer ports will have large hoses that attach to either side of the headlight actuator. The small hose, on the top, goes back to the headlight switch and controls the position of the slide.

The slide in the relay does have O-rings on it that will harden over time and not hold vacuum or at least leak. There is also a small foam filter in the bottom of the slide that prevents dirt and debris from being drawn up into the relay and damaging the O-rings. These filters often fall out over time and contribute to relay failure.

Since you've tested the actuators and they appear to be functional, and assuming you have vacuum at the reservoir, I would look at them next.

Good luck and let us know how it progresses... GUSTO

 

As soon as I have time to get out there I'll be sure to check for leaks out of those parts. I've been eliminating problems so I figure the part(s) that are failing will show up eventually. Thanks a lot though guys.

 

Ok, so part of the problem was that reservoir wasn't holding any pressure because..there was a hole in it. Patched it up until it can be welded/replaced and tried again.. Something else isn't work. When I had my finger over the whole and was applying pressure 1 opened. When I tried after the patch nothing. The actuators are extremely rusty but they seem to be holding air.. Still seems to be a problem because they still aren't opening or closing if I raise them..

 

wow!! a hole in it? isnt that fairly thick metal? was the car outside a long time? i would think its just a thick as a regular air tank would be.

 

18 years of water > thick metal
I need a new tank but does this sound like an actuator problem? They seem pretty bad too.

 

Are you applying air pressure to the system or pulling a vacuum?

 

Air pressure.. probably not the best thing to do..

 

You still have a number of issues to deal with; I recommend you try dealing with them one at a time.

You mentioned that when you connected vacuum directly to the actuators they operated. I would move on to the next component.

Since the reservoir is leaking, I recommend you simply remove it (figuratively) from the equation. Since the reservoir clearly has rust inside it would not be impossible to draw rust into the vacuum line and add one more problem you will have to resolve. Eliminate it now and move on.

With the engine idling, you can operate the headlights directly off the vacuum line from the engine. You may have to adapt the small line from the engine compartment to the large one going to the vacuum relays, but it will eliminate one more variable and allow you to troubleshoot the relays and their operation individually.

Simply connect the engine vacuum line to the center (large) port on the relay and, if everything is working, the headlight will operate. If it operates, move to the other relay and check it out.

The vacuum system for the headlights has a lot of places to leak vacuum. Cumulatively they can leak enough to keep the system from operating properly. Therefore there is good reason to break the system down and test its most basic components.

You mentioned earlier that there are no springs on the headlights as they had rusted off. Don't think the headlights are going to function normally without these springs installed. You can manually assist the headlights to overcome this, but you will still need to rectify this problem to get it to work normally. Also the reservoir itself can be replaced with almost any reservoir from another vehicle. Some of the other C3's use a large "tin" can instead of the one you have. These are smaller, lighter and easier to mount, so you might consider one of them. Reservoirs are pretty easy to spot as they are a sealed canister with a fitting to "T" into the vacuum system. They will also usually have some sort of bracket to mount them with.

I do think you will end up finding at least one if not both of the actuators to be bad. Even if they just leak a little they will affect the operation of the headlights. You can shoot a little silicone spray into the relay (from the bottom where the small filter is, or was located) and it will improve the functioning of the relay's, at least temporarily.

The vacuum headlight system is not nearly as complex as many believe. You just need to break it down into its basic components/circuits and deal with them one at a time. The next thing you know it's working and it doesn't seem nearly so frightening.

Good luck... GUSTO

 

Try here for some help, http://www.corvette-101.com/vacuum.htm the diagram is wrong at the headlight and pull down connection, but the rest is good and the text is good. PG.

 

That seems as basic as it gets Gusto, It doesn't seem nearly as bad as before and I'm sure it'll get easier if I slow down. Thanks again guys.

 

This is one of my to do's that still are on my list. This vacuum system is PITA.... Is have to lift my hl manually !!!! actually have all the parts to tackle this problem, but don't know were to start.

 

Sebastian,

Go to the link that Pete posted above. It's a terrific resource and should help you to understand the vacuum system on the C3 much better. It does seem much more intimidating than it really is. Once you understand how it works, dive in. When you hit a wall, come back and ask questions... then you'll become a headlight expert in your part of the world!

Good luck... GUSTO

 

Not trying to pirate this thread, but it seems that there are a few on here now that know a thing or two...
I've been meaning to post one of these myself, but have been dealing with brake issues.

I had no headlight function a week ago, but after reading posts & viewing similar diagrams like the one posted here, realized that my 1st problem was a disconnected override switch ( tell-tale was hissing in the car & will go away when you turn lights on). I now can get the lights to come up & go down, but have two issues.

1) the driver's side gets stuck on the way up (alignment issue?). have to give it just a little help. Goes down just fine on its own.

2) And this is the concerning issue... it takes a long amount of time when lights are turned on, for the doors to come up. I cannot get them to come up at all with the engine off. I also get a big pressure release when the engine is turned off. Is this a resevoir tank issue? I also thought is could be a relay that has a leak & is not completely gone. Any thoughts would be appreciated...

 

#1...If you are able to manually assist the headlight assemblies up and down, alignment is probably not the problem. There are a few moving parts that do restrict movement, HEADLAMP FLANGE BEARINGS, not very typical...

and HEADLAMP HOUSING ACTUATOR LINK SPRINGS, of which there are a bunch, and they do frequently rust and break...


#2...As mentioned in an above thread, there are multiple places for the system to typically leak. Headlight actuators, relays and even the headlight/over ride switches. If you add this up, it can be difficult to maintain enough vacuum to get the system to function properly.

One thing I do is separate the system by removing one relay/headlight actuator from the circuit at a time. Simply disconnect one relay (large center hose from reservoir) and cap it off tightly. Test the other side for functioning. If it works fine, do the same with the other and retest the first side. If each functions, but weakly, you have a few leaks and need to run them down. The major sources of leaks in an unmolested system are the HEADLAMP ACTUATOR SHAFT SEAL, easy and inexpensive fix... (when these are the culprit, the headlights will go up slowly, if at all, but close properly)

and the VACUUM RELAY VALVE... a bit more expensive to address, but there are some tips above to help them work before replacing them.


Some of the vendors carry a rebuild kit with most of the hardware that typically result in movement problems...

This kit doesn't address the vacuum issues, so you'll have to address them separately.

Good luck... GUSTO

 

Gusto,

Thanks so much for the input!

Based on your comments, I am thinking the HEADLAMP ACTUATOR SHAFT SEAL is my first order of business, then the VACUUM RELAY VALVE. I will keep you updated on what i find... Thanks again!

 

These headlight vacuum systems really are simple to fix. I manually raised and lowered mine for nearly a year, then finally decided to fix them. Once I got going, it really wasn't much to it. You can test this and that to see what's leaking, but honestly, if it's a '74 and any of those actuators and relays are the original ones, I would just replace them all. There should be two actuators and two relays up front. There should be a vacuum check valve and a small inline filter between the brake booster and distributor. If you replace those six parts, and everything else is hooked up properly, your headlights should work like new. I just reread your original post and see where you stated the springs are missing for the lights. That will have to be fixed also. Good luck.
KC