Hi guys,

Since last saturday something funny happens on my 69 bb 427 baby.

When engine is running the headlights work perfectly. However, when I shut down the engine, the left side headlight goes up.

Any idea?

The car is an early 69, it has the vaccum system of a 68.
All the hoses are tightly connected and are not porous.

Thanks in advance for your help.

Rod.

 

Maybe a check valve or actuator? If one of those are bad then vacuum may be leaking through backwards when the engine vacuum is no longer being produced.

 

Check the vacuum line to the top of the left side relay. Engine vacuum to the top of the relay pulls the dogbone UP , keeping the headlights closed. it sounds like it's losing vacuum control signal to the relay but there's still plenty in the reservoir to raise the headlight.

 

Thank you guys but I forgot to mention something:

The car is a very early 69 (chassis # 16). It has the headlight system of a '68 meaning only one relay between the headlights so if I follow your thinking and knowing that the right side headlight work just fine it should mean that the problem comes from a leak in the actuator of the left side headlight itself.

Do you agree?

 

2 issues, one of them being a problem:

1) One headlight always goes up before the other. No biggie, it just happens. One actuator requires less vacuum, is more efficient, has less friction, doesn't leak, whatever. One of them will always open before the other, or at least start to open before the other. So no big deal on one vs two opening (though you may have a leaky actuator on the other side)

2) The control side of the system has an immediate leak upon shutdown. The actuation side of the system draws from the vacuum canister which takes a while to bleed down. The control side (switches & valves that send the open/close) signal doesn't draw directly from the canister.

When you shut your car off, the control side is losing pressure immediately. That causes the spring inside the relay to force the relay into the "headlights up" position. There is enough vacuum in the canister for the headlight(s) to follow that command long enough for one of them to open.

So what's going on? Sounds like a check valve leak at least initially. You can check that by finding the check valve under the hood. While the car is running and the headlights are down, pinch the small tube coming off the check valve (heads from the check valve through the firewall). Then shut the car off. Does the headlight stay closed? Yes? Replace the check valve. Does it still open? Yes? You have a problem somewhere else along the "small" vacuum tubes. The small tube routes through the firewall to the headlight switch and the underdash switch before heading up front to the headlight relay. Most likely, if the check valve is ok, the leak is in one of those valves, or the relay itself (as opposed to a hose leak).

 

I had the exact same problem. It was a loose vacuum hose.
See pic of hose with chrome cover at valve.

 

Thank you for the help, I'll look into this. I have the same problem with my passinger side head light, I have to restart the engine to get it down.

 

I have to do the same thing once in a while for my driver side light. Must be a little gremlin causing that to happen

 

I had the same problem. about a year back and it was the check valve.

What happens is when the engine stops, the vacuum source is gone and the check valve holds the vacuum in the resevoir.

When the check valve goes, the vacuum bleeds off. Basically when the vacuum bleeds off it does it unevenly and can actually cause your relay to switch states making the headlight come up. Some people have a headlight pop up and some have the wiper door pop up. Just depends on how the vacuum bleeds off for your car.

You can test the check valve. Have the car running and pinch off the vacuum hose on the engine side of the check valve with some vise grips. Turn off the car and watch the headlights. They should do nothing. Wait a minute and release the vise grips. If you hear a hissing, then the check valve is bad.

EDIT: OK, I just went back and read ddecarts post entirely. Sorry for the double post.

 

Not really a double post, you gave some additional information - thanks.