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On my 77 vette what would make my drivers side headlight go up to slow. When I turn on the lights the passenger side goes up first then about 5 seconds later the drivers side goes up. Is it normal also for one headlight at a time to go up or are both headlights suppose to go up at the same time?
this is your vacuum relay. the round hole is the vacuum valve, this is in the headlights up position. the vacuum is connected from the center hose to the lower hose to pull the headlight up. BUT, vacuum doesn't suck. it is a low pressure area. so that the atmospheric pressure can blow on the other side. that smile shaped hole is the fresh air supply to raise the headlight. it had an air filter of sorts in that square spot 50 years ago. not really an air filter. really a bug filter. that hole is a very tempting spot for a mud wasp to make a nest. and then, even though the vacuum system is working, the air flow system is blocked. your headlight is rising slowly because there is a bug nest that it has to blow-suck atmospheric pressure through to push the diaphragm that pushes on your light. a 3/16ths drill bit spun by hand will open that passage up. also, all the big diameter lines can get bug nests in them. unhook both ends and blow them out. don't suck. blow. inhaling hunks of bug nest and dead bug is ...unpleasant. don't ask how i know. that is what the 5th amendment is for.
Last edited by derekderek; Apr 25, 2022 at 07:22 AM.
I've had my 77 for 35 years. one side goes up, then the other. I've rebuilt or replaced everything in the system. Still, just as when I bought it. passenger side goes up. a couple seconds later drivers side goes up. And, they go down the same way! If they go up and down. They work!
Don't over think it. they run off vacuum, they are not electric.
I sprayed all the hinge points and spring ends with the newer, plastic-safe wd40 and the slower light now is almost as fast the other one. The dry plastic bushings are lube-for-life items but they get dirty and rough over time, so new ones would be better. The wd40 penetrant spray hasn’t been catching extra dirt in eleven years of use. I spray a little in there whenever the hood and door hinges get lubed.
Dads car was slow to open. There was a small rust hole on the bottom of the actuator. JB weld and they work good again. Even still, one side moves a little ahead of the other side.
We did notice the car has to idle a bit to build full vacuum in the tank before they operate fast, otherwise, they move pretty slowly and one goes ahead of the other. There is a restriction in the hose feeding the light system on his car, the engine will be at 18in-hg as soon as it starts but the tank takes time to reach that vacuum level.
A vacuum gauge you can put on the various hoses to check the vacuum level is the best way to troubleshoot. Start on the tank with both relays disconnected and plugged and see that it build vacuum then try one headlight at a time. We ended with the gauge on the hose that sucks the passenger actuator closed and finding that was all good vacuum wise, but once that actuator was connected the vacuum went away because it was constantly leaking.
I also suspect a faulty relay.
My 77 also did the same thing, one side really struggled to lift.
If you pull a vacuum on the signal port (top port on the relay actuator) and it holds a vacuum Sometimes you will have success rebuilding them.
I say sometimes as the Spool (dog bone) has to have NO defects.
One of my relays spool had a few cracks and was no good, the other is a good used spare.
Here is a photo of the cracks and the parts I used for a successful repair of one.
slow headlight
