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1969 Mark III Headlight Door Issues

Bill -

Good Morning and thank you for this site. I have been troubleshooting the vacuum system for a headlight door problem. I can hear vacuum hissing when the headlight switch is in the ON position. Is this designed to dump vacuum when the car is running in order to keep the lights open?

Greg

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Greetings Greg -

Several threads have been posted previously on this blog that are related to this subject. They can be located and read also. The short answer to your question is yes, there will be a short vacuum sound when the headlamp covers are repositioning to the open or closed mode. However, the system is not designed to constantly "dump" vacuum.

In a proper operating vacuum headlamp system on your Mark III, vacuum flow should be heard for a few seconds only when the Headlight switch is moved to the H/lamps On or Off position. When the switch is moved to either position (with the vacuum system charged and maintaining engine vacuum) vacuum is reversed to the opposite side of the dual port Headlamp Vacuum Servos located at the the H/L doors. During this vacuum reversal and subsequent repositioning of the headlamp doors a vacuum sound will be detected from the vacuum portion of the switch for only approximately 3-5 seconds as the vacuum is reversing . If you are still hearing a vacuum "leaking" sound from this system after the doors are repositioned, a leak must exist somewhere in the vacuum system. If this is so, the leak will continue when he engine is turned Off and the h/l doors will open in a short time period until the engine is started again.

To diagnose and locate a leak, a good vacuum diagram and some knowledge of how the system operates will be a great help to you. Remember though that the h/l switch can be cracked and leaking BUT with only a leaking sound at the switch does not prove for sure that the switch is faulty. With a large leak elsewhere, the sound that you are hearing could possibly be the vacuum rushing through the switch towards the large leak. If a leak is strongly suspected at the switch, it should be removed and the vacuum portion bench tested. The above are only possibilities that we have encountered in the past. The system consists of many connections and hoses that can cause leaks. Many times we find multiple leaks in some vehicles. Accurate diagnosis and patience is required to pinpoint the culprit. At Lincoln Land we stock many parts to repair these systems.

Sincerely,

Bill

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Bill -

Thank you. I did find a cracked hose going to the firewall. One of the vacuum cans has a "built in" check valve. Do you know the best way to test that? I have been from the bumper to firewall, checked vac motors for leaks, lines for leaks, check valves for operation, cans for leaks, and I am now inside the dash looking at the vac motors on the climate control system. My climate system is not follwing the troubleshooting flow chart (in the black repair manual) by changing air flow through the dash outlets when it should. I think I can hear hissing in that area, but my years of race cars and loud music is hurting the diagnosis. Your information is very helpful and I thank you! This year I am determined to find the problem.

Greg

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Greg -

The vacuum check valves which are located in the rubber hose lines or integral with a reservoir are a simple device. They allow engine vacuum to flow into the vacuum system that they serve but when a period of low engine vacuum occurs or the engine is shut off, the vacuum is sealed in the system at the valve so that it is not lost back to the engine. When vacuum resumes at the check valve from the engine, the vacuum will now flow again as necessary through the valve as designed.

We like to use a hand held vacuum pump to diagnose and verify the integrity and operation of any suspect components such as the check valves. Good luck with your diagnosis and repair.

Sincerely,

Bill