Why do my lights flip open when I turn my engine off?

[leeboy66]

I just bought a 1977 and my headlights only stay down when the car is running. When I turn it off one or both flip open. Does anyone have any advice as to why this is happening?

 

[tscott9330]

you have a vacuum leak. The headlights are held down by vacuum actuators. The leak is probably small enough that when the engine runs it pulls enough vacuum to hold them down, as soon as it is shut off, the vacuum signal is lost and the lights pop up. The only good fix is to start changing hose until you replace the right one.

Tom

 

[68Roadster]

I just bought a 1977 and my headlights only stay down when the car is running. When I turn it off one or both flip open. Does anyone have any advice as to why this is happening?

Buy a handheld vacuum tester and start plugging lines and testing them. Could be a bad actuator/seal, check valve, hose....

Good luck!

 

[SR'71]

Buy a handheld vacuum tester and start plugging lines and testing them. Could be a bad actuator/seal, check valve, hose....

Good luck!

What is a handheld vacuum tester. What do you use?

 

[68Roadster]

What is a handheld vacuum tester. What do you use?

Made by Mity-Vac....probably around 35-40.00
Good for things like this...

 

[70 LS1]

I had the exact problem and it was the check valve between the engine and vacuum canister.

 

[jdp6000]

Listen for a vac leak. If both come up opposed to one or the other it should be easier to find the problem. Your leak under these circumstances is in the supply line for both so it should be a little easier to find. It could be the diverter valve or the canister behind the front bumper (thats where mine is)Check up under your steering wheel. Hoses go to a bypass and the headlight switch. Check the hoses they come from the rear driverside of the intake manifold...follow them...they should run to the diverter valve in the front between the headlights. Use some water...spray it on the hoses and fittings...it will bubble in the area of the leak. If the leak is big you'll hear it.

Do some tests...pull hoses...plug hoses....you can't hurt the system.

Jim

 

[srs244]

not to be contrary, but when i had a vacuum leak (initially it was a bad actuator seal, then it became a bad actuator itself. of course it wasn't the one i fixed the seal on, lol), my headlights wouldn't work at all. i could get the driver's side to work (both up and down) if i manually locked the passenger side into the open position, but it didn't work in the reverse. (it became obvious at that point that the passenger actuator was defective. the main diaphram had gone bad).

just my convoluted thinking, but i would think that your condition is an indication that the line or lines from your vacuum relay(s) may be hooked up backwards, or the connection at the headlight switch is backwards. you didn't indicate if this was all the time or just whenever you turned the lights on, etc. it might also help to know what happens when the headlight switch is turned off (if it only happens when you turn the lights on).

since you have a 77, you have 2 vacuum relays (mounted under the center of the nose between the headlight assemblies). each one controls one headlamp assembly through one actuator.

by the way, it could also be a bad headlight switch (the vacuum portion of it causing the leak of vacuum), but i would check everything else first as changing the switch itself is a real pain in the rear.

 

[70 LS1]

I guarantee it is the check valve. To test it, run the car and pinch the vacuum line running to the engine. I used vise grips. Then shut the car off. The headlights will stay down. Now release the line and if it hisses, it is a bad check valve.

What happens with a bad check valve is it won't hold vacuum in the tank when the car is shut off. When this happens the vacuum bleeds off and as it does the relays that direct the vacuum to hold the lights shut switch states. There is enough residual vacuum in the tank to open the lights before it bleeds off as well.