1970 Headlights pop up when driving
Thread Starter
Intermediate
Joined: Oct 2023
Posts: 33
Likes: 1
1970 Headlights pop up when driving
Evening,
Still having trouble hunting this one down...headlights and wiper door come up at approx. 2700-3000 rpm while driving, then go back down as RPM drops to around 2000.
I have replaced the headlight switch behind the dash, the vacuum filter and the headlight/wiper door vacuum check valve.
Lights actuate normally at idle.....
...anyone had this, or have a fix?
thanks!
Still having trouble hunting this one down...headlights and wiper door come up at approx. 2700-3000 rpm while driving, then go back down as RPM drops to around 2000.
I have replaced the headlight switch behind the dash, the vacuum filter and the headlight/wiper door vacuum check valve.
Lights actuate normally at idle.....
...anyone had this, or have a fix?
thanks!
Race Director
Joined: Nov 2006
Posts: 13,248
Likes: 7,840
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
I tryed several of those replacement check valves. Even the Gen. GM replacement parts one. ALL JUNK! none of them held vacuum. I still have one on the car because it looks the part and provides a spliter. But I run a generic vacuum check valve inline before that one, hidden under my distributor so my system actually works!
So, just cause you purchased a new vacuum check valve doesn't mean it works! Headlight doors open when switching side of the system has no vacuum.
Symptoms say that at open throttle when it is normal to have little if any manifold vacuum, your doors are opening.
Bad check valve, leak in vacuum tank or vacuum leaks in system. that's it. Start testing.
So, just cause you purchased a new vacuum check valve doesn't mean it works! Headlight doors open when switching side of the system has no vacuum.
Symptoms say that at open throttle when it is normal to have little if any manifold vacuum, your doors are opening.
Bad check valve, leak in vacuum tank or vacuum leaks in system. that's it. Start testing.
Instructor
Joined: Oct 2021
Posts: 209
Likes: 33
The head lights should never go down with loss of vacuum. If they do the mechanism is not locking. Usually a vacuum line arrangement issue. To put headlights down the headlight switch reverses the vacuum flow on the lock retracting the lock allowing the headlights to drop. This issue is vacuum line related, or the mechanism is frozen or broken
Melting Slicks
Joined: Mar 2005
Posts: 2,095
Likes: 299
From: Midlothian VA
Evening,
Still having trouble hunting this one down...headlights and wiper door come up at approx. 2700-3000 rpm while driving, then go back down as RPM drops to around 2000.
I have replaced the headlight switch behind the dash, the vacuum filter and the headlight/wiper door vacuum check valve.
Lights actuate normally at idle.....
...anyone had this, or have a fix?
thanks!
Still having trouble hunting this one down...headlights and wiper door come up at approx. 2700-3000 rpm while driving, then go back down as RPM drops to around 2000.
I have replaced the headlight switch behind the dash, the vacuum filter and the headlight/wiper door vacuum check valve.
Lights actuate normally at idle.....
...anyone had this, or have a fix?
thanks!
Simply suck/blow through it - It should only pass air one way.
You should be able to suck air through the engine (single port) side but not blow air into that side.
The reverse is true for the system (dual port) side.
Race Director
Joined: Nov 2006
Posts: 13,248
Likes: 7,840
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
The head lights should never go down with loss of vacuum. If they do the mechanism is not locking. Usually a vacuum line arrangement issue. To put headlights down the headlight switch reverses the vacuum flow on the lock retracting the lock allowing the headlights to drop. This issue is vacuum line related, or the mechanism is frozen or broken
The vacuum system for the headlight doors is 2 vacuum systems. One system is the switching system, the other is the work system.
The small vacuum line that goes to the top of the vacuum relays is the switching line. When it has vacuum, it pulls the dog bone inside the relay up directing the vacuum to the close the door side of the actuator. When the small switching line has no vacuum. As in, it's actually shut off when you turn on your lights. With no vacuum to the top of the relays the spring inside the relays push the dog bone down directing the vacuum to the open side of the actuators and the doors open! Then vacuum returns to the small line when engine RPM drops and throttle closes. As soon as vacuum returns to the small switching line, doors close.
The large work side vacuum lines are connected to the vacuum tank and there large diameter size helps them hold more vacuum than the rather tiny switching side vacuum system with it very small diameter lines.
nothing to do with broken linkages.
Instructor
Joined: Oct 2021
Posts: 209
Likes: 33
I believe that you need to learn how these systems work. Popping up on hard acceleration and then yes, going back down is a common issue on these cars. And has nothing to do with a broken locking mechanism.
The vacuum system for the headlight doors is 2 vacuum systems. One system is the switching system, the other is the work system.
The small vacuum line that goes to the top of the vacuum relays is the switching line. When it has vacuum, it pulls the dog bone inside the relay up directing the vacuum to the close the door side of the actuator. When the small switching line has no vacuum. As in, it's actually shut off when you turn on your lights. With no vacuum to the top of the relays the spring inside the relays push the dog bone down directing the vacuum to the open side of the actuators and the doors open! Then vacuum returns to the small line when engine RPM drops and throttle closes. As soon as vacuum returns to the small switching line, doors close.
The large work side vacuum lines are connected to the vacuum tank and there large diameter size helps them hold more vacuum than the rather tiny switching side vacuum system with it very small diameter lines.
nothing to do with broken linkages.
The vacuum system for the headlight doors is 2 vacuum systems. One system is the switching system, the other is the work system.
The small vacuum line that goes to the top of the vacuum relays is the switching line. When it has vacuum, it pulls the dog bone inside the relay up directing the vacuum to the close the door side of the actuator. When the small switching line has no vacuum. As in, it's actually shut off when you turn on your lights. With no vacuum to the top of the relays the spring inside the relays push the dog bone down directing the vacuum to the open side of the actuators and the doors open! Then vacuum returns to the small line when engine RPM drops and throttle closes. As soon as vacuum returns to the small switching line, doors close.
The large work side vacuum lines are connected to the vacuum tank and there large diameter size helps them hold more vacuum than the rather tiny switching side vacuum system with it very small diameter lines.
nothing to do with broken linkages.
Melting Slicks
Joined: May 2021
Posts: 2,274
Likes: 1,321
From: Fleming Island, FL
I believe that you need to learn how these systems work. Popping up on hard acceleration and then yes, going back down is a common issue on these cars. And has nothing to do with a broken locking mechanism.
The vacuum system for the headlight doors is 2 vacuum systems. One system is the switching system, the other is the work system.
The small vacuum line that goes to the top of the vacuum relays is the switching line. When it has vacuum, it pulls the dog bone inside the relay up directing the vacuum to the close the door side of the actuator. When the small switching line has no vacuum. As in, it's actually shut off when you turn on your lights. With no vacuum to the top of the relays the spring inside the relays push the dog bone down directing the vacuum to the open side of the actuators and the doors open! Then vacuum returns to the small line when engine RPM drops and throttle closes. As soon as vacuum returns to the small switching line, doors close.
The large work side vacuum lines are connected to the vacuum tank and there large diameter size helps them hold more vacuum than the rather tiny switching side vacuum system with it very small diameter lines.
nothing to do with broken linkages.
The vacuum system for the headlight doors is 2 vacuum systems. One system is the switching system, the other is the work system.
The small vacuum line that goes to the top of the vacuum relays is the switching line. When it has vacuum, it pulls the dog bone inside the relay up directing the vacuum to the close the door side of the actuator. When the small switching line has no vacuum. As in, it's actually shut off when you turn on your lights. With no vacuum to the top of the relays the spring inside the relays push the dog bone down directing the vacuum to the open side of the actuators and the doors open! Then vacuum returns to the small line when engine RPM drops and throttle closes. As soon as vacuum returns to the small switching line, doors close.
The large work side vacuum lines are connected to the vacuum tank and there large diameter size helps them hold more vacuum than the rather tiny switching side vacuum system with it very small diameter lines.
nothing to do with broken linkages.
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Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,094
Likes: 4,449
From: Marlton NJ
And all the vacuum lines need to be solid & leak-free. Leaks abound on these cars.
Mine had a melted vacuum line behind the cylinder head.
You need to get a vacuum troubleshooting chart, IIRC Wilcox made a good one.
And a mity-vac, and then start testing.
Mine had a melted vacuum line behind the cylinder head.
You need to get a vacuum troubleshooting chart, IIRC Wilcox made a good one.
And a mity-vac, and then start testing.