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Headlight vacuum actuators activated when starting engine
Hi fellow Forum members,
I am sure this has been answered before somewhere on this forum, but I just could not find it.
I am working on my vacuum system on a 1972 C3 coupe and have spent a lot of time checking and replacing hoses,
check valves etc. and I also have a handivac to check individual parts of the system.
Today I had a little win, because my headlights actually came up, but the issue is that they
came up by themselves when starting the engine. The light **** in the car was not out and
also the override switch under the steering column was also not activated. Moving either of them
did nothing and they stayed up.
I am suspecting that I either have something incorrectly connected somewhere (which would
not surprise me because I have found some strange fixes on this car by the previous owner(s))
or the light switch is faulty. Any suggestions where to start?
Btw, I have looked at diagrams for the hoses, but I can't tell what is wrong from them. It appears
right to me. Any input would be greatly appreciated.
No vacuum to the small line on the top of the relays is headlights up.
pulling the headlight switch or the over ride switch cuts vacuum off.
So.
You are not getting vacuum to the top of the relay. You do have vacuum on the large center line of the relay.
The small vacuum line starts by the check valve, goes through the firewall, to the over ride switch, then to the headlight switch and then back through the firewall and a long hose all the way to the front to the relays.
Follow that line. It's broken or fallen off somewhere.
Best of luck.
Here is a drawing of the vacuum system. It was made for a 69, but the differences are small. On a 72 you should have two headlight relay valves as they added another one sometime during the 69 year. The vacuum system always causes confusion because of the possibilities of leaks.
No vacuum to the small line on the top of the relays is headlights up.
pulling the headlight switch or the over ride switch cuts vacuum off.
So.
You are not getting vacuum to the top of the relay. You do have vacuum on the large center line of the relay.
The small vacuum line starts by the check valve, goes through the firewall, to the over ride switch, then to the headlight switch and then back through the firewall and a long hose all the way to the front to the relays.
Follow that line. It's broken or fallen off somewhere.
Best of luck.
I respectfully disagree...
The main vacuum feed (Black Line) lands at port #2 of the headlight switch.
Headlight Switch Off position vacuum passes through port #3 (Lt. Blue) then lands on the upper port of the override switch...
White line to the relays...
Last edited by bmotojoe; May 31, 2024 at 09:23 AM.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
okay so lets make this really confusing...
there are 2 different vacuum systems in your car. one side is the switching side, the other side is the work side. Your work side is working because your headlights moved. IF they go up on their own their are multiple issues that could be at hand, all of which are leaks in the control side. The control side also is part of the wiper system and is tee'd just before the wiper switch. their is not a one way valve at this point so if the wipers have a leak it will effect the headlights. IF your headlight switch vaccum fails it fails open like you have. Also iF the actuator relays leak the headlights will pop up too. IF the bypass switches leak they will pop up as well.
Easiest ways to find this is isolating the wiper system from the headlight system, then isolating the actuators from the relays and using only one relay then the other. YOu need some caps to plug male vacuum ports and golf tees to plug hoses. I bought a hvac vacuum pump and a remote switch so I can turn it on and off from a different room, plug in a long hose from it into my manifold vacuum port line and run it until it builds about 15 hg then shut off the pump and listen for leaks....faster, easier and less likely for you to lose feeling in your hand using those stupid mighty vacs
I respectfully disagree...
The main vacuum feed (Black Line) lands at port #2 of the headlight switch.
Headlight Switch Off position vacuum passes through port #3 (Lt. Blue) then lands on the upper port of the override switch...
White line to the relays...
OK, but your spliting hairs. Bottom line, with both switches on off, vacuum flows through to the top of the headlight relays. No vacuum in the switching side and headlights rise.
Headlight vacuum actuators activated when starting engine
