Headlight Vacuum System on 1977 vette has stopped working

read the service manual>? at GM ask or


https://eautorepair.net/Marketing/Default.asp

The first thing , this system works on vacuum there are vacuum lines that run from intake to storage can or cans then to light switch inside car then back out to headlights,

i would first start by starting the car and listen for vacuum leaks, you may want to follow the vacuum line form the motor checking for splits or cracks in the lines, listen around can, this can sometimes rusts and leaks vacuum, this can or cans are intended to store vacuum and use it when needed, if they are leaking then the lights will work slow until there is enough vacuum to open them, this goes the same for a bad vacuum line and light switch, the light switch is a two part switch, one part supplies power to lights and the other part is vacuum to open lights.

you will need to listen for a vacuum leak inside the car at the switch, (headlight switch) .

Make sure all lines are good and have no leaks, you will need to make sure the hinges to lights are free and not binding up.

There are three round plastic pellets that wear out and turn to powder causing the headlight gearbox to malfunction. They cost about 4 dollars per side, pretty easy fix if you take your time. I just replaced them in my 89 vette. Of course I bought the best brass gear thinking I needed them. The plastic gear was still in great shape, don't waste your money, you don't need the brass gear, They cost $58.00 each. All you need is the pellets, $4.00 on ebay. There is no vacuum on the headlight system

First, check that the vacuum line from the engine going into the dash is connected and not leaking. Then, check for vacuum at the control head. If there is vacuum there, check the vacuum "motors" on the heater box. If they have vacuum and fail to operate then the diaphragm inside the non-moving one has failed and the motor needs replacement.If motors are not getting a vacuum signal back up to the control head. The control is bad.

ignition timing, cam grind of a performance cam shaft, can lower engine vacuum at idle, also check for a bad diaphragm or vacuum storage container in the system, some of the old vettes had a vacuum storage container to store vacuum because of the solid lifter motors and the cam grind they were noted for having low vacuum , so GM would use often a container that almost looked just like a 1 lb. coffee can painted black and put in in-line in the vacuum system to store vacuum so when you suddenly accelerated the headlight doors would not slam shut, it would provide enough lag time in the vacuum loss, that the storage would recover.... hope this helps...

Raise the headlights, either with the switch or manually by turning the knob on the headlight motor counter-clockwise.
Unplug the lights NOW to avoid an accidental closing on your fingers,
Remove the 3 philips screws holding the black plastic surrounding the headlights, and slide it forward, there is a small clip on the top that helps hold it to a corresponding clip at the top of the headlight,
Look inside the cavity and you can see the 5 nuts/bolts ( 6 on later years ) that need to be removed,
If you are on the driver's side you can now remove the whole assembly, on the passenger's side you must first unplug the headlight door control module.

bad vacuum lines or need replacement fuel filter could even be a leaky line faulty fuel sending unit or check air filter ..start there first

if they both stoped workin at the same time i'd start by checking all the vacuum lines going to the headlights

I believe, the best way to track down a headlight vacuum problem with a Corvette, is to make yourself a checklist, and go through it one by one.

Start at the vacuum source.

1.Is the main hose coming out of the intake manifold getting vacuum?
Pull the hose loose from the Vacuum Filter. (White round plastic object with a molded fitting on each side) Is there vacuum present?
No? Remove the hose from the intake manifold fitting, remove the fitting. Take a stiff piece of wire, (Open wire clothes hangar?), and see if the opening is blocked. Perhaps the carbon has built up here.
Vacuum present? Go on.

2.Remove the hose from the other side of the Vacuum filter. See if you can blow through the filter. This filter is to prevent particles from going down into the engine. The filter may fill up, and be blocked over time.
You can blow through it? Go on. No? Replace it. (Replace items that you have disconnected, and checked to be good, as you go forward in your checklist)

3.Is the Vacuum Check Valve working correctly? The check valve has two ports on one side, and one port on the other. You should be able to blow air through either of the two ports, but not be able to blow air through the single port.
Checks okay? Go on. It's bad? Replace it.

4.All devices connected and hoses. Set the parking brake. Start the car. Disconnect the vacuum supply hose under the dash, to the headlight switch. This hose connects at the back of the headlight switch.
Is there vacuum present? Should be. If not, it's the hose coming up to it. If so, disconnect the hose that is at the back of the 'Pull Down' switch.
(Manual Override Pull Down Switch)
Vacuum coming out of the fitting on the 'Pull Down' switch? Is the knob pushed up, or pulled down? Pull it down to open it. Vacuum coming through? Yes? Go on. No? Replace it. (Should be in the Up position to keep the headlight doors open. You are just testing the switch itself)

5.Car off, not running. Things get a little tougher here. You now need to access all of the front vacuum components, and hoses. The best way to do this, is to remove the hood. I would mark around where the hinges attach to the hood, so that realigning it will be easier. If you have a concours show car, I leave the method of marking it up to. Or, you just may want to remove it, and reset it without marks on your baby. I Fully understand! ('70 and '76 'Vette owner)

You should still have vacuum built up. The hose from the 'Pull Down' switch comes from under the dash, through the firewall, down the left fender well, and ends in a 'T' fitting in the Front/Middle, of the car. Remove the hose from the 'T' fitting. Use care, as the fitting may be plastic. (Or you may have replaced it with a metal one. Smart person you, if you did!)

There should be a residual amount of vacuum. If not, reattach the hose, fire the car up. Disconnect this hose again. You should be well away from the fan, and your parking brake should be set.
(I realize that the radiator, and radiator shroud, blocks you from the fan, but I just had to say that)

Vacuum coming from the hose? Okay, reattach it.

6.Your Actuators have a rod coming out of the middle of them. This rod operates the headlight mechanisms. Down where the rod first comes out of the Actuator, there is a Rod Dust Shield. Under this dust shield is a Rod Seal. The seal goes around the rod, and is attached to the middle of the Actuator. Press your finger/thumb against this seal. Hear a vacuum leak? Time to replace that seal.
Go to the other side. Is it leaking? Time to replace that seal. New Actuators are about a $100 bucks a piece. Seals are a WHOLE lot cheaper!

The Green hose opens the headlight door. The Red hose closes it.
If the headlight switch is in the On position, and the 'Pull Down' switch is in the Up position, go from underneath the car, and push up on the front of the headlight door. The headlight should come up.
No? Go to the door mechanism. There is a L shaped link. Push down on it. Now the door should come up. Suspect a bad Actuator.
Depends on if those seals are bad. If they leak enough it may not be the Actuator.

7.Your Vacuum Reservoir is up on the fender well. If it's the original one it's steel. Check it for holes. These tanks rust through.

8.Check the Vacuum Relays. There are two. The middle fitting on the body of the relay, is where the hose from the Vacuum Reservoir attaches to. If memory serves there is a single hose coming from the Vacuum Reservoir, and comes to a 'T' fitting, then splits off to those Vacuum Relays, for the '69 'Vette.

There is a diaphragm at the end of each relay. The diaphragm enclosure, looks like two pie pans put together. These enclosures can rust over time, and develop a hole for a vacuum leak. The hoses from the 'T' fitting, that goes up to the 'Pull Down' switch, comes to this diaphragm. If this diaphragm is bad, the headlight doors won't go up, or come down.

All the diagrams for your corvette can be found here:

http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c1528008370b

Headlight wiring is towards the bottom of the page.

Hope this helps, have fun wrenching on that vette.