'78 headlight vacuum question
Thread Starter
Heel & Toe
Joined: Dec 2007
Posts: 23
Likes: 5
From: Orange County, NY
'78 headlight vacuum question
Driver headlight goes up/down fine. Passenger headlight very slow up and doesn't like to go down.
With my mity vac at the actuator and relay putting vacumm on the different up/down hoses it goes up/down fine as it should. With car running, and using vacuum gauge to check vacuum from the tank - the hose coming from the pass side (the one that doesn't work good) is pulling 15in hg, the drivers side hose (the one that works good) coming off the same tank is pulling only 5in hg. Don't the two hoses come from the same tank? Are there separate tanks inside the tank? Seems strange - also strange that the side that doesn't work as well has more vacuum.
Anybody have any ideas what might be up?
With my mity vac at the actuator and relay putting vacumm on the different up/down hoses it goes up/down fine as it should. With car running, and using vacuum gauge to check vacuum from the tank - the hose coming from the pass side (the one that doesn't work good) is pulling 15in hg, the drivers side hose (the one that works good) coming off the same tank is pulling only 5in hg. Don't the two hoses come from the same tank? Are there separate tanks inside the tank? Seems strange - also strange that the side that doesn't work as well has more vacuum.
Anybody have any ideas what might be up?
Last edited by gtwhoa; Jun 25, 2020 at 06:05 PM. Reason: typo
Melting Slicks
Joined: Apr 2001
Posts: 3,399
Likes: 278
From: Olmsted Falls, Ohio
I had a very similar problem a few years ago. The disk filter in the vacuum line near the firewall in the line coming from the port on the rear of the intake manifold had become very dirty inside. I bought a new one and the lights went up and down quickly.
Try removing the filter and connecting the lines to see if there is a difference.
Try removing the filter and connecting the lines to see if there is a difference.
Thread Starter
Heel & Toe
Joined: Dec 2007
Posts: 23
Likes: 5
From: Orange County, NY
I had a very similar problem a few years ago. The disk filter in the vacuum line near the firewall in the line coming from the port on the rear of the intake manifold had become very dirty inside. I bought a new one and the lights went up and down quickly.
Try removing the filter and connecting the lines to see if there is a difference.
Try removing the filter and connecting the lines to see if there is a difference.
Le Mans Master
Joined: Aug 2006
Posts: 7,419
Likes: 1,555
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Thread Starter
Heel & Toe
Joined: Dec 2007
Posts: 23
Likes: 5
From: Orange County, NY
Melting Slicks
Joined: May 2015
Posts: 2,365
Likes: 428
From: Cape Girardeau Missouri
Be sure to check the vacuum relay for proper operation BEFORE installation! The currently available (Chinese) vacuum parts are notorious for having problems when new! With just a little searching you'll find instructions on how to test the relay. It should hold vacuum indefinitely.
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