69 wiper door opens when engine is shut off
Thread Starter
Heel & Toe
Joined: Apr 2015
Posts: 16
Likes: 2
From: Williamsburg, Ohio
69 wiper door opens when engine is shut off
Just replaced the headlight/wiper Vacuum actuator relay. Everything works fine when engine is running. As soon as engine is shut off the wiper door opens. Headlights remain closed so I don't believe I am bleeding off vacuum anywhere. I'm thinking the new actuator may be defective. Any thoughts from anyone before I return it?
Burning Brakes
Joined: Jul 2018
Posts: 979
Likes: 258
From: Maryland
Just replaced the headlight/wiper Vacuum actuator relay. Everything works fine when engine is running. As soon as engine is shut off the wiper door opens. Headlights remain closed so I don't believe I am bleeding off vacuum anywhere. I'm thinking the new actuator may be defective. Any thoughts from anyone before I return it?
1. Apply vacuum with a vac pump on the red line (from the relay) to the wiper door actuator itself. The door should close and stay closed as long as you keep vacuum on the line - does your vacuum stay there or does it
go down?
2. Do you have vacuum on your yellow line (going into the actuator relay) when the engine is running?
Red = closes door
Green = opens door
3. If the above works as designed, check the black (control line). Do you have vacuum on this line (going into the relay valve) when the engine is running? You could try running the engine and applying vacuum to the control port
on the relay valve - it should close the door if all is OK.
Melting Slicks
Joined: May 2002
Posts: 2,475
Likes: 756
From: Laporte, IN
The way the system works is as follows:
The small control hoses to the relays and control items like headlight switch etc. control whether items like headlights/wiper door opens or closes.
With vacuum in these hoses all items will be closed.
The relay is porting muscle vacuum to the large red hoses closing those items through the relays.
When you select your headlight switch to ON vacuum is bleeding off from the small hose.
The relays have springs in them with reduced or no vacuum the spring reposition porting and now vacuum is applied to the large green hose opening the item.
So when you are shutting down your engine vacuum is leaking from your small hose causing the relay to reposition and muscle vacuum to open your headlights.
This leak could be your hoses, connections, relays or control items like headlight switch or the associated override control.
The small control hoses to the relays and control items like headlight switch etc. control whether items like headlights/wiper door opens or closes.
With vacuum in these hoses all items will be closed.
The relay is porting muscle vacuum to the large red hoses closing those items through the relays.
When you select your headlight switch to ON vacuum is bleeding off from the small hose.
The relays have springs in them with reduced or no vacuum the spring reposition porting and now vacuum is applied to the large green hose opening the item.
So when you are shutting down your engine vacuum is leaking from your small hose causing the relay to reposition and muscle vacuum to open your headlights.
This leak could be your hoses, connections, relays or control items like headlight switch or the associated override control.
Melting Slicks
Joined: May 2002
Posts: 2,475
Likes: 756
From: Laporte, IN
Since your 69 should have 1 relay for the wiperdoor this is what you can do. Do you have small set of vise-grips? If you do clamp the small hose just before the relay after you start your engine then stop your engine. If that stops your problem your relay is leaking or the hose at the relay. If no you have to work your way back on the small hose to your control items switches etc.
Last edited by PJO; Oct 1, 2019 at 02:17 PM.
Thread Starter
Heel & Toe
Joined: Apr 2015
Posts: 16
Likes: 2
From: Williamsburg, Ohio
The way the system works is as follows:
The small control hoses to the relays and control items like headlight switch etc. control whether items like headlights/wiper door opens or closes.
With vacuum in these hoses all items will be closed.
The relay is porting muscle vacuum to the large red hoses closing those items through the relays.
When you select your headlight switch to ON vacuum is bleeding off from the small hose.
The relays have springs in them with reduced or no vacuum the spring reposition porting and now vacuum is applied to the large green hose opening the item.
So when you are shutting down your engine vacuum is leaking from your small hose causing the relay to reposition and muscle vacuum to open your headlights.
This leak could be your hoses, connections, relays or control items like headlight switch or the associated override control.
The small control hoses to the relays and control items like headlight switch etc. control whether items like headlights/wiper door opens or closes.
With vacuum in these hoses all items will be closed.
The relay is porting muscle vacuum to the large red hoses closing those items through the relays.
When you select your headlight switch to ON vacuum is bleeding off from the small hose.
The relays have springs in them with reduced or no vacuum the spring reposition porting and now vacuum is applied to the large green hose opening the item.
So when you are shutting down your engine vacuum is leaking from your small hose causing the relay to reposition and muscle vacuum to open your headlights.
This leak could be your hoses, connections, relays or control items like headlight switch or the associated override control.
Thread Starter
Heel & Toe
Joined: Apr 2015
Posts: 16
Likes: 2
From: Williamsburg, Ohio
Yes, as long as vacuum is maintained in the canister assy the door stays closed. I removed the check valve in the vacuum line and it appears not to be working and letting the vacuum bleed back as soon as the engine is stopped. Will install new one and let you know what happens.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Burning Brakes
Joined: Jul 2018
Posts: 979
Likes: 258
From: Maryland
Once you have the mityvac, you can test the relay as I and others have suggested. It's really a great tool. especially for a vacuum driven accessory car such as the C3s were. Just remember that the red lines close while the green lines open. The Yellow lines supply relay vacuum, while the black lines supply control vacuum. So, for a test, if you want to open a wiper door or a headlight, apply vacuum from the mityvac to the green line that goes to the corresponding actuator. To close the headlight or wiper door, apply mityvac vacuum to the red line that goes to the actuator. To test a relay, start the car and put the mityvac on the black (control) line going into the relay. Pump up the mityvac and the relay should then port vacuum (from the engine, yellow line) to the red (close) line which will then close the wiper door or headlight. Remove vacuum from the control line and the relay will port vacuum to the green (open) line. This behavior makes it a fail safe system. i.e. if there is no vacuum on the control lines the system (headlight or wiper door) will fail open. Take a look at the Willcox site - he has fantastic videos that cover all of this.
Thread Starter
Heel & Toe
Joined: Apr 2015
Posts: 16
Likes: 2
From: Williamsburg, Ohio
Thanks for the info. I believe I have isolated the problem. Defective vacuum check valve. ordered one yesterday. Hopefully this fixes the problem. As I mentioned before. No problem with the head lights. Wiper door opens when engine is shut off. When I clamped the hose behind the check valve and turned off engine then the door stayed closed.
Obsessed
Joined: Apr 2019
Posts: 627
Likes: 394
From: Valencia, Spain
2025 C3 ('68-'73) of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
2019 C3 of Year Finalist (stock)
Also fixed mine... I have a 71 coupe, you will find that vacuum switch on the passenger side under the windshield whipper cover grill.... It’s kind of hard to see but if you turn on your whippers and stop then when the’re open up on the windshield you will see it and be able to get at it better... if that switch loses its ability to hold vacuum the whippers will not close when you turn them off and the door when you shut off the engine without touching the whippers... When you turn the whippers they retract and rest on top of that switch which depresses the plunger and actuates the switch and closes the whipper door. Not sure why a faulty switch causes the door to open when you turn off the car but it does...
Safety Car
Joined: Oct 2004
Posts: 3,926
Likes: 738
From: Youngstown Ohio
Thread Starter
Heel & Toe
Joined: Apr 2015
Posts: 16
Likes: 2
From: Williamsburg, Ohio
I replaced this a few months ago. I'm sure this is not the problem as the door works just fine when the engine is running and the only leak that occurs is behind the vacuum check valve when the engine is shut off.
Burning Brakes
Joined: Jul 2018
Posts: 979
Likes: 258
From: Maryland
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
scummins06 --
You have a lot more faith in the reliability of aftermarket parts than most of the rest of us. These "new" parts are NOT the equals of the original pieces. The rubber [?] seals are the most suspect, as it seems foreign manufacturers will use anything that compresses, rather than the proper elastomer that was specified by GM for that part.
If you purchase the Mighty-Vac, you can use it to evaluate the sealability of any component in the C3's vacuum systems. You CANNOT evaluate very much of the system at one time, as the pump doesn't have enough capacity to do that. If you isolate each component separately for testing, you will be successful at diagnosing your problems. My .02 is that 'vacuum lines are rarely the problem; [cracked] fittings and worn/bad seals in components are almost always the cause of your vacuum-related problems'. In the headlamp system, the relays are the most likely cause of slow or non-functioning lamp operation.
You have a lot more faith in the reliability of aftermarket parts than most of the rest of us. These "new" parts are NOT the equals of the original pieces. The rubber [?] seals are the most suspect, as it seems foreign manufacturers will use anything that compresses, rather than the proper elastomer that was specified by GM for that part.
If you purchase the Mighty-Vac, you can use it to evaluate the sealability of any component in the C3's vacuum systems. You CANNOT evaluate very much of the system at one time, as the pump doesn't have enough capacity to do that. If you isolate each component separately for testing, you will be successful at diagnosing your problems. My .02 is that 'vacuum lines are rarely the problem; [cracked] fittings and worn/bad seals in components are almost always the cause of your vacuum-related problems'. In the headlamp system, the relays are the most likely cause of slow or non-functioning lamp operation.