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C3 Headlight Door Help Needed

twamd80driver

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Sep 29, 2004
Messages
102
Location
Cleveland, OH
Corvette
1976 L-82 4-Speed
Can anyone suggest a methode to trouble shoot the door operation on a 76. When I started the car for the first time this year, the lids flipped right up, and the lights were off. I tried cycling the switch, and they would not go down. I dont have a diagram in the maint manual nor do they give a troubleshooting guide.
Thanks!!!


Chris
 
B

bmotojoe

Guest
For some reason did the manual override switch get pulled out?
Here is what I have used in the past to troubleshoot if the problem is located on the interior side or the engine compartment side. There should be a metal splitter check valve located towards the back of the drivers side valve cover. There will be one vacuum line into it and two exiting. The smaller line exiting goes directly to the headlight switch and then to the manual override switch. The larger line from splitter valve travels along left fender well and connects to the large round air tank behind front bumper.
At the metal splitter valve you disconnect the small vacuum line there it will send vacuum directly to the relay valves and then to actuators. The headlights should go up once vacuum line is pulled, and then down when reconnected. If the headlamps do not go up then your problem is in the engine compartment side. If they do go up then the problem is in the interior side.
Hope this helps,
Brian
 

Evolution1980

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You've lost vacuum to the headlights. Gotta find where/why you are losing vacuum.

Vacuum keeps the lights down. It's a safety measure. That way if you are ever driving, particularly at night, and you lose vacuum, the lights will at least be in the "up" position and not "down" in the car.

One time I was doing a top speed run and I had the the throttle mashed long enough that my driver side headlight popped up due to loss of vacuum. Kinda funny. I slowed down and it tucked itself back in. :L

Our cars are pretty simple troubleshoot. If the problem isn't electrical, you've got a vacuum problem. :D We don't need no stinkin' computers!
 

twamd80driver

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Joined
Sep 29, 2004
Messages
102
Location
Cleveland, OH
Corvette
1976 L-82 4-Speed
Headlight Lids

I think it was pulled down, I pushed it up with out the engine running and nothing happened. I will start it tomorrow when I move it out to wash it and let you know! Thanks for your offer of help....I hope I dont need it!

By the way do you have a good Mechanic in the Cleveland Area?

Thanks everyone, I will Let you know how it goes!
 

Evolution1980

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Messages
4,302
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Cleveland, Ohio
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ZZ4, 700R4, Steeroids rack & pinion, VB&P Brakes
I think it was pulled down, I pushed it up with out the engine running and nothing happened. I will start it tomorrow when I move it out to wash it and let you know! Thanks for your offer of help....I hope I dont need it!

By the way do you have a good Mechanic in the Cleveland Area?

Thanks everyone, I will Let you know how it goes!
IMHO, I have the best mechanic in the Cleveland area. He's the only person (besides myself) to turn a wrench on my car in the past 10+ years. Believe it or not, it's one of the reasons I'm not too excited about leaving Ohio. I don't want to have to hunt down another mechanic! :L
Check your Private Messages.
 

Vettehead Mikey

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Not that far from Ottawa
Corvette
1973 Coupe
Vacuum keeps the lights down. It's a safety measure. That way if you are ever driving, particularly at night, and you lose vacuum, the lights will at least be in the "up" position and not "down" in the car.

This is a common misconception. The system was not designed to open the lights in case of vacuum loss, nor was it designed to keep them open if the same thing happens.

A loss of vacuum can move the lights in either direction, or have no effect on their present position. It all depends where the fault is in the system. :beer
 

Evolution1980

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ZZ4, 700R4, Steeroids rack & pinion, VB&P Brakes
A loss of vacuum can move the lights in either direction, or have no effect on their present position. It all depends where the fault is in the system.
How would a loss of vacuum cause the lights to go down? :confused
 
B

bmotojoe

Guest
As soon as the car starts there is vacuum to and through the headlight switch and the manual override switch, both of these switches are normally open in the in position.
The headlight door opens from the vacuum signal being cutoff from either the headlight switch or the manual override switch, let not be confused about the two different sources or supplies of vacuum to the headlight door system. Pulling either switch will cutoff signal vacuum to the headlight relay valves relaxing the internal diaphragm of the relay. The headlight relay has an internal spool that is connected to the diaphragm. Once the internal spool has shifted state the main vacuum (From big pipe behind front bumper) is then redirected through the internal spool to the headlight actuators. Once the headlight door is in the up position it is then locked into the up position until the relay valves signal vacuum has again been reapplied, changing the internal spool of the headlight relay valve to exhaust through the middle of the three ports of the headlight relay valve. Now, because the vacuum is being exhausted does not mean that the exhaust vacuum will be enough to pull the headlight doors down, that’s where the 4 large springs on each headlight door comes into play.
When you mashed the throttle to the floor your vacuum was basically depleted from the signal side of the system causing one or both headlight doors to open, but there was still enough in the (big pipe behind front bumper) to move one actuator because the headlight relays internal spool had changed state.
Now, if you were ever to lose vacuum and need headlight turn your motor off and pull up each headlight door into the up-lock position.

Brian
 

Vettehead Mikey

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How would a loss of vacuum cause the lights to go down? :confused

If the system is in good operating state, then the lights won't close, but neither will they open. They'll just sit at the last commanded position.

If the relay(s) have an internal leak/jam where the vacuum is bypassing the spool and (a big and) the tube moving the doors open (the big one at the front of the actuator) is open, the doors can unlock and hang half closed.

Only my car could develop two faults. Wanna see a picture? :beer

As bmotojoe explained your doors opened not because of a safety feature, but because of vacuum depletion and imbalance in the system. Probably your check valve needs attention.

If loss of vacuum were a safety feature that opened the doors, they would pop up and stay up every time you stopped the engine.
 

Evolution1980

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ZZ4, 700R4, Steeroids rack & pinion, VB&P Brakes
If loss of vacuum were a safety feature that opened the doors, they would pop up and stay up every time you stopped the engine.
I thought that the vacuum cans kept the vacuum until it was released by either the headlight switch or bypass/release switch. Oh well... whatever! :)
 

Vettehead Mikey

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That would be true if the system stayed 100% perfect throughout the life of the vehicle. I've seen restored cars (really restored cars, not rattle can restored) that still had enough vacuum to open AND close the lights one full cycle after being parked and untouched for 48 hours. That's how they were supposed to operate from the factory. Pretty impressive.

Most cars today (including mine) are lucky to hold vacuum for 48 seconds after engine shutdown. This means that there's a moderate/serious leak somewhere/anywhere. Even a leak at the rear deck vent actuator can cause a malfunction of the headlight doors.

What these leaks also mean is that the system never reaches a state of true balance, leading to lights that open and close at weird times or take forever to respond.

Tracking the leaks down is a true pain, but I'll take that any day over a short in an electrical harness.:beer
 

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