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how to fix the vacuum for the headlights?

S

Stifler

Guest
I just got a 1981 and Im trying to fix the vacuum tubes so that the lights will come up. Im not sure how the system works. Can anyone tell me how I should troubleshoot the system to determine why my lights will intermittently come up. Sometimes they come up, sometimes they dont and sometimes they dont go down. What do you think is happening???
 

jdp6000

Well-known member
Joined
Aug 23, 2004
Messages
646
Location
Aurora, Ontario, Canada
Corvette
1982 CROSSFIRE
I just got a 1981 and Im trying to fix the vacuum tubes so that the lights will come up. Im not sure how the system works. Can anyone tell me how I should troubleshoot the system to determine why my lights will intermittently come up. Sometimes they come up, sometimes they dont and sometimes they dont go down. What do you think is happening???


Thats odd that they work sometimes. Trace the hoses. They start from the intake driverside rear. Look for leaks? Do you hear any hissing?

I just saw a drawing...I'll see if I can paste it.


Jim
 
B

bmotojoe

Guest
Because your vacuum system seems to act erratically try this. Using the diagram above pull the vacuum line on the round metal splitter valve that is going through the firewall to headlamp switch. This will send vacuum directly to the headlamp relays through the main vacuum reservoir and lights should go up, plug in the hose and they should go down. Do these two steps over and over and see if the problem still happens. If the problem stops using this method your problem is not on the outside of the firewall. I would suspect headlamp switch or manual override switch if provided.
Brian
 

Evolution1980

Well-known member
Administrator
Joined
Feb 25, 2002
Messages
4,292
Location
Cleveland, Ohio
Corvette
ZZ4, 700R4, Steeroids rack & pinion, VB&P Brakes
This will send vacuum directly to the headlamp relays through the main vacuum reservoir and lights should go up
For my own knowledge here... I'm pretty sure it's vaccum that keeps the lights down. Loss of vacuum allows the lights to go up. The reason for this is safety: should you ever lose vacuum while driving, especially at night, the lights will be stuck up rather than locked down.
Also, this seemed to be the case one time when I was doing a sustained WOT run. I was drawing so much vacuum that one of my headlights popped up. Got off the throttle, vacuum was restored, headlight went back down.

I agree though that it sounds like a bad switch rather than a vacuum line/resevoir issue. But...you can never rule out vacuum lines on these cars. (On newer cars, computers are the curse. With ours, it's vacuum.) :D
 
B

bmotojoe

Guest
The vacuum lines attached to the top of the relay valves is signal vacuum ported through the headlamp switch, and by itself is not enough to open the headlamps. When the headlamp switch is pulled it sends signal vacuum that pulls up the diaphragms of the relays thus the internal plunger changes state and sends main reservoir vacuum directly to actuators. If any of the diaphragms of the relay or actuators are defective you will see headlamp doors not function correctly.
So by doing the troubleshooting step above this should save you time and money by isolating the problem.
 
R

RedChev

Guest
vac oper Headlights

Hi Stifler, let me intro myself first since I don't post to this forum very much.
i own a '70 vette and have had it for about 22 years. Do all the work myself including body work and paint. Maybe someday I'll take time to figure out how to put a picture of my car on the forum, but I guess not yet.
I haven't had to fix a headlight vac problem in along time but maybe I can help. If both your headlights do what ever they do in unison then I suspect that you either have a vac leak or poor vac from the engine. look at all your hose connects, put alittle spit on each joint to see if it is drawn into the line. look at your lines for cracks. The thingys that are down under the front valance near the middle of the valance between the headlights are the vacuum relays. These relays have two different sources of vac coming to them. the white hose is from the headlight switch via the manual override switch. I call this the control vac. just like an electrical control circuit which pulls in a relay to control a larger circuit which carries a heavier load. Then along the body of the relay you see three other hoses. the middle hose is from the vac tank. this is just a resouvoir so that you have vac all the time provided the check valve isn't leaking. When the white control vac from the switch pull the relay in one direction the headlights are pulled down an held there. When the vac on the relay is released the relay pulls the other way an the headlights go up. Now at this point i honestly don't remember, but it was suggested that the vac holds the headlights down and I think that is right because if you look at the drawing provided here, the white hose controling the relays is coming from the headlight switch via the overide switch. The overide switch is shutting off the vac when pulled so as to raise the head lights. In any case , if your lights are doing things in unison, then I think everything under the hood at least at the front end near the head lights are ok. The headlight switch and manual overide switch are a real bear to get to, but you may have to check them for leaks. As was said earlier, connect good vac directly to the white hose going down the side of the engine bay to the headlights. if the lights work up and down as you apply and remove vac then that stuff is ok. Another reason your lights maybe doing everything irradically is if the check valve is leaking back. Its purpose is to hold vac stable in the system while the engine will have varing vac based on load and RPM. I believe the check valve is the first little thingy after the vac port connection at the manifold. I think the drawing is labled wrong. Again your system could very well be leaking. The tank can leak too. Just find a way to put vaccum on each component and see if it holds it. Your lights could be mechanically binding at the pivot points of the buckets. You can check to see if the pivots are loose by trying to wiggle them. Disconnect the linkage and move the lights up and down to see if they are tight. And one more thing, if you have changed to a radical camshaft, overlap on the valve timing will reduse your vac to a point that vaccum components just don't work.
 
B

bmotojoe

Guest
The drawing is correct. The first valve in the drawing is a filter. The metal splitter valve is the check in the system and operates correctly as shown in drawing.
Brian
 

Vettehead Mikey

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Joined
Aug 26, 2003
Messages
3,487
Location
Not that far from Ottawa
Corvette
1973 Coupe
You need vacuum to move the lights in either direction- up or down. The only fail safe in the system is the knee joint that 'over centers' to lock the lights in the up position. There's minimal spring pressure that keeps the lights shut when the engine is turned off and the vacuum is slowly bleeding off.

There's probably a hundred different spots where a leak could develop, any one of which could cause partial/complete loss of system function. You just have to be patient in examining evey bit of hardware to find it (or them).

A good trick is run a long vacuum hose to a source outside your garage- another car or a vacuum pump. Makes it much easier to find the leak by ear.
 

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