My corvette is a 1978, when I pull the headlight switch all the way out, the headlights will turn on and I can switch between hi and low beams, but they won't open. I've tried with the car running and not running, same result. I've also tried pulling the coloum switch that makes them go up and down without them being on, same result. I've checked some of the hoses and they seem to be in good shape and hooked up, none hanging loose. I pulled the hose from the tank to check for vacuum pressure, and it is sucking. Any ideas??

also, not sure if it is related or not but my guage lights don't work either!

 

I'll take the easy stuff first. When you were pulling your headlight switch k n o b you likely turned it about 1/4 turn and dimmed your gauge lights all the way. So try turning your headlight switch k n o b the other way and see if that cures that problem.

Your headlights UP/DOWN are operated by vacuum and the usual cause of them SUDDENLY not opening is one of the large hoses have been pulled off the relay valve that is located under the nose of your '78 right in front of your radiator OR the hose has split where it attaches to the vacuum tank.

 

I've tried turning the ****, definitely not it. I'll recheck the hoses.

 

There is also a fuse for GAUGES on your fuse block so check that too.

 

https://goo.gl/images/s9Ixna, you can review this vacuum diagram.

 

Go to the over-ride switch under the dash, pull this down and see if the lamps come up... If they don't remove the vacuum connector from the switch and see if you have vacuum on the hose marked H in the diagram below.

If you don't have vacuum there then I'd take a look at the headlamp switch.

If you do have vacuum there then I'd move outside the car and see if you have vacuum on hose D at the relay...

The system by default should open when no vacuum is present on the small hose side so with this in mind, go to the relay's that operate the headlamps and test. I've posted a picture below to help you understand how the relay works and how it switches vacuum from up to down.





And then there is this video that will walk you through how the headlamp items work and how to test them.

 

The most likely fault is with the headlamp switch. Vacuum is actually passed thru it to send the vacuum signal to the headlamp [vacuum] relays so that they will open.

But, you might also have a problem in the vacuum system that prevents that vacuum from reaching the headlamp circuit. Open your hood and find the location of the vacuum relays for the headlamps. There are 3 hoses connected to each relay. The CENTER hose (of those 3) is the vacuum SOURCE. You can pull that line off the relay and start the engine to see if that line has good vacuum on it (check with vacuum gauge, if you have one; it should have 15"Hg or more on it.) If there is no vacuum on that line, the headlamp switch is not passing vacuum OR you have a vacuum line disconnected, MIS-connected, blocked or missing.

If vacuum is good on that hose, turn engine off and pull one of the hoses off the vacuum servo for that headlamp (the large vacuum can which opens the headlamp), and connect the relay vacuum line directly to the servo. Now start the engine and see what happens to that headlamp. If it opens, your problem is that the vacuum relay is defective (or frozen in place). If it does not open, turn engine off, remove relay vacuum line from the servo fitting and move it to the OTHER fitting on that servo can. [Also replace the loose servo line to its proper fitting.]

Start engine again. If no action, your servo can may be defective or frozen in place.

You can do this same test on the other headlamp circuit. Do not expect that the cause of the problem with one headlamp is the SAME as with the other headlamp.

 

If you want to tackle this yourself do yourself a favor and buy a Mity-Vac hand vacuum pump to help troubleshoot.
Start at the actuators and put the pump on them individually to see if they are good then just work your way back with the diagram Wilcox posted.
You said that the override switch under the steering column doesn't work either so I think one of your under dash lines has come loose, possibly A or J. Also make sure the check valve G at the vacuum supply is working correctly.

 

The small 1/8" hose on the relay valve should have vacuum with the headlight switch turned OFF and NO vacuum with the headlight switch turned ON. Turning the headlight switch ON dumps the vacuum in that hose which allows the spool in the relay valve to drop which sends vacuum to the FRONT of the headlight door actuators.

 

As Willcox drawing indicates, Operations Vacuum is connected at all times to the CENTER fitting on the relays. The SIGNAL vacuum is sent to one of the other fittings to turn that relay ON (or OFF) via the headlamp switch.

It would be good to understand the system before attempting to advise others.

 

Quick question. My headlights will not open when the switch is pulled out. However, they do open when the override switch is pulled. I’m thinking the switch is bad and is no longer blocking the vacuum at the switch. Should the switch be replaced?

 

Yes. And please don’t use old threads. Just start thread and ask question. Before replacing the switch, make sure the vac hoses are hooked up to it.

 

Thanks Derek.

 

Please start a new thread rather than asking a question in a thread from 4 years ago.