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1978 Mark V Headlamp Vacuum Issues

Bill -

I have a 1978 Lincoln Mark 5 with the 6.6 engine. I have replaced the major components of the headlight vacuum system and still can't make them work. It seems when I start the car they close, and when I shut the car off they open again..



Hey Tim -

Your headlamp doors open because they are designed to open as a safety feature if the system cannot hold vacuum. Vacuum can leak anywhere that the vacuum lines are routed. I'm not sure what you did replace, but if they open immediately as you describe you have a large leak.

The most likely components to leak are as follows.....Vacuum check valve, Headlamp door vacuum motors, Vacuum Reservoir, Headlamp switch vacuum valve, Autolamp Vacuum. module (if equipped), or any cracked or disconnected vacuum lines. Some vehicles have more than one leak. If you are not familiar with this system and its installation you will benefit greatly by consulting a manual and vacuum diagram. If you are keeping the car and doing your own repairs I strongly advise that you order the proper Ford manual. Diagnosing these components correctly is a must otherwise you will replace unnecessary parts. If you do wish a set of manuals or need any parts, we have these available for you. Good Luck on a speedy repair.