I've got a 1968 convertible that for the past two months I've been going through the headlight and wiper systems. Neither were working when I bought the car a little over a year ago. After testing all of the components and replacing most of them, both systems are now work properly. However, about 30-60 seconds after shutting off the engine I hear a “psst” sound and then the wiper door lifts about .25 inches. I’ve not been able to identify where the sound is coming from, but I’m sure it’s a bit of vacuum leaking from somewhere. What I don’t understand is why just a bit of a leak? I would think if a component is failing it would, over time, allow the wiper door to open completely. I've let it sit for a couple days and the door is still up .25 inches. When I start the engine back up it closes perfectly. But then again it will open about .25 inches.

Any ideas what I should be looking for?

 

does the underdash override work after you hear psst?

 

You have an imbalance in between the 2 vacuum circuits. What usually happens in this scenario is the small hose circuit (control) leaks down enough to allow the internal piston in the actuator relay to switch the vacuum ports. The large (actuating) vacuum hose then tries to open the wiper door, but there is not enough residual vacuum to do that. The door will stay in that position until more vacuum is applied to the actuator.

The solution is to test components till you find the leaking culprit.

Try this test; Start the car, and clamp the control vacuum hose closed leading to the top of the actuator relay. Turn the car off and observe the door's reaction.

 

the psst is the leak, its on the closing side, as it leaks down the door starts to open but there isn tenough vacuum left to raise it fully.

On the left side down in the fender is the vacuum door relay that is also vacuum operated. Its the relay that controls where the vacuum goes on the door actuator.

You'll need someone to help you. Take a length of hose and snake it down to the relay and stick the other end in your ear, then have your assistant shut off the car. you'll have to keep doing it and moving the hose around until you hear it.

MIne did the same and the relay itself was leaking.


keep us updated
scott

 

Gentlemen, thanks for the responses! The explanations regarding the relay causing the problem makes perfect sense to me now.

Scott, you indicated that the vacuum door relay is under the "left" fender. However, mine is under the right fender, or is there another relay under the left fender as well? If so, I was not aware of one being there.

BTW, that wiper door relay was one of the many "new" parts I bought, and I tested it with my vacuum gun prior to putting it in because it's location is such a pain to get to!

 

No, youre correct, its the right side. I get my left and right screwed up all the time looking at the car. I usually say passenger/driver because I screw it up so often.

Did you replace the hoses or just trim thte ends so they fit snug?

 

The hoses appeared new when I bought the car so I just cut .5" from each end when putting it all back together.

I'll check the relay(s) today and let you know the outcome.

 

For each of the following tests I ran the engine to build vacuum and then shut it down:
1) Wiper door was still closed after 10 minutes.
2) Wiper door opened .25 inch after 12 minutes, using a stethoscope I could hear what sounded like it may have been the relay valve moving but no “psst” sound.
3) Wiper door opened .25 inch after 11 minutes, no noticeable “psst” sound.
4) Prior to shutting down the engine I clamped the black/white vacuum hose going the top of the wiper relay valve. The wiper door remained closed but the headlight doors opened about .75 inch after 11 minutes. Then in approximately 4-5 minutes the headlight doors closed. I didn’t expect this so I didn’t accurately measure the amount of time between the opening and closing. The wiper door was still closed after 19 minutes.
5) Prior to shutting down the engine I clamped the black/white vacuum hoses going the top of the wiper and headlight relay valves. After 18 minutes the wiper door and the headlight doors remained closed. When I unclamped the hose to the wiper relay the wiper door opened .25 inch without any “psst” sound (I wasn’t using the stethoscope). I then unclamped the hose to the headlight relay and the headlight doors remained closed.
6) I re-clamped the black/white hose to the headlight relay but left the hose to the wiper door relay open. After 12 minutes the wiper door opened .25 inch.

I then removed the wiper door relay from the car to test it following a Troubleshooting Guide and a vacuum gun as follows:
1) Applying vacuum to the small metal port at the top the valve moved freely up towards the diaphragm.
2) After releasing the vacuum from the small metal port the valve moved freely toward the filter end.
3) After applying vacuum to the small metal port and clamping that hose to maintain the vacuum, and blocking the Red port, I applied vacuum to the Yellow port – after 7 minutes the reading dropped from20.5 HG to 20.0 HG; after 9 more minutes it was still reading 20.0 HG.
4) Blocking off the Green port and applying vacuum to the Yellow port – After 9 minutes the reading dropped from 19 HG to 18.0 HG; after another 9 minutes it fell to 16.0 HG; after another 10 minutes it fell to 14.5 HG; after another 18 minutes it fell to 9.0 HG; after 9 hours (7:00 AM this morning) it was still at 9.0 HG .

I’m thinking the readings from the wiper relay test are okay, with maybe the exception of test number 4. Do you suppose that even though it ended up holding at 9.0 HG that it could be causing my problem?

Also, last night, with the wiper relay still removed from the car, I used the vacuum gun on the Red hose attached to the front of the wiper door actuator to completely close the wiper door. This morning the wiper door is still closed.

I’m still at a loss as to what might be causing my wiper door to open that .25 inch. And now I'm wondering why the headlight doors got involved in this. Any insight as to what’s going on and how to fix it, is appreciated.

Wally

 

Hi Wally;

Wow, you have certainly done your homework. If I was there to diagnose this my next move would be to use your vacuum gun and test the control circuit. This can be done easily.

1 - disconnect the small control hose at the check valve and connect the vacuum gun to it.
2 - pump up the circuit and observe the rate of vacuum "discharge".

I agree

By clamping off the control hose to the wiper door relay you have effectively transferred the issue to the headlight relay. The pathway for this is the 3 way split connector the control circuit connects through immediately inside the firewall. Follow a good vacuum schematic to see this.

 

Thanks for the reply Dave.

When I place the vacuum gun on the small hose going in through the firewall and try to apply vacuum I get nothing. What does that tell me, other than it won't hold a vacuum? Do I need to start checking each of the vacuum controls in and under the dash? If so, is there an order I should be following? Or is there one in partular control I should be checking?

Wally

 

Hi Wally,

Apologize for the delay in getting back to you, issues with my internet.

Wow, I didn't expect that result. First of all, I must tell you my experience is with '69 - '72 systems as the '68 is a little different. The basic components are the same, but I may miss something specific to the '68's.

Starting with the basics, I'm assuming the headlight switch is pushed in, and the override switches are pushed up. This should create a straight path for the vacuum signal to reach both relays. Each side of the control circuit is designed to have vacuum to the relay on startup, so the vacuum gun should duplicate that. We can now proceed to the next test.

Because the control circuit is split just inside the firewall through a 3 way, I like to eliminate one side at a time and test each independently. If you can find the 3 way under the dash, clamp off one side at a time and apply vacuum to the circuit. Test the headlight side, and then test the wiper door side. Hopefully you will find one side that is holding vacuum. That should make the hunt for the offending component easier to find on the leaking side.

The fact that your systems work when the engine is running tells me that you probably do not have a big leak. A small leak on the control side could be overcome by the volume of the engine's vacuum. The rather large vacuum created by the 8 cylinders dwarfs the very small capability of a vacuum gun. Sometimes even a small leak caused by a loose hose connection or small leak can overcome the vacuum gun's capabilities.

Hopefully you find one side holding vacuum. In that case, test each component on the leaking side including the hose connections. The worst offenders I have found are the headlight vacuum switch and the connection on the downstream side (90 degree) connection on the solenoid. If you need to test the solenoid, it does not need to be removed. I can give you a how to if needed.

Good luck, hope this helps. Let us know

 

Thanks Dave. We've got the grandsons for the weekend so I'll not be able to get back to this until Monday or Tuesday. I'll run the tests and let you know what I find.

 

Ha! I just got rid of mine, spoiled a little more and returned to the parents. That lil urchin in my sig pic is the first born one.

 

As luck would have it, both boys went down for their naps at the same time this afternoon so I was able to take a quick look at the vacuum system. In doing so I realized that I had not blocked off the black/white hose to the wiper relay since removing it for testing. Once I clamped it I was able to pull vacuum but it slowly goes from 20.0 HG back to 0.0 HG after about 5 or 6 seconds. If all is good, should that 20.0 HG hold? If so, my leak must be somewhere in the under dash area?

 

You are correct. In a perfect corvette vacuum world, the vacuum system would hold vacuum for quite a while if it had no leaks.

I would say the leak is probably under the dash. You have corrected the open vent, and tested the top of the relays, so that leaves the dash components. If it were me, I would follow my suggestion to block off each side separately at the "T" 3 way between the check valve, headlight switch, and the solenoid. Then test each side. That will give you which side the leak is on. Wiper door side, probably the solenoid is dirty inside, preventing a good seal. Headlight side, probably the vacuum valve on top of the switch is the culprit. These can all be tested in the car if you need to.

You can follow Willcox's schematic below. The "T" is located behind the speedometer high under the dash. It's normally reachable lying on one's back. I have also accessed the hoses by reaching up under the dash and following the hose through the firewall.

 

Hi Dave, I was able to get back to testing the under dash components and found the headlight manual override pull down under the dash leaking. Everything else checked out okay.

I couldn’t find the “T” where the vacuum source splits between the wipers and the headlights (I believe it is tucked up above the speedometer) so I blocked all the hoses under the dash to isolate the headlight switch for testing and pulled vacuum from the black/white hose at the top of the headlight relay between the two headlight actuators. With the parking lights on it held 20.0 HG and only dropped to 19.25 HG after 14 minutes. I’m guessing that’s good. With the headlights turned on it would not hold vacuum at all, which I’m guessing is how that should work as well.

Would you agree that the headlight manual pull down leaking could be the cause of my wiper door opening a bit after the engine has been shut down? I won’t know for sure until order a new one.

 

You are correct.

Yes I would agree. Any leak in the control circuit would do it. Refer to the result you got when you blocked off the control hose leading to the wiper door relay.

Not really. Do an easy test. Bypass the pull down by connecting the two hoses leading to it. Then do your vacuum test again. Observe any change in the vacuum reading result. You just might be surprised. You also might have another leak in the control circuit. This would expose anything else that needs attention.

Your situation is a classic example of a tight actuating circuit and a leaky control circuit.

 

I bypassed the headlight vacuum pull down switch by connecting the two hoses. Then started the engine to develop vacuum and shut the engine down. After about 15 minutes the wiper door opened about .25 inches, as before. I ran the test again by restarting the engine, and turned the w/s wipers and they worked fine. I then operated the electric wiper override switch to see if it worked and it did. I closed the wiper door and shut off the engine. After 10 minutes the wiper door opened .25 inches again.

When you suggested bypassing the headlight vacuum pull down switch, did you mean for me to “connect” the two hoses so vacuum could pass through the hoses, or to “block” both hoses to no vacuum could pass through?

 

Connect the two hoses. You will effectively eliminate the suspect component (headlight pull down) and allow the balance of the circuit to be tested. When the pull down is bypassed, test the control circuit with your vacuum gun again at the check valve to see if you get the same result as when you tested it before it was bypassed.

 

I connected the vacuum gun to the vacuum hose at the at the check valve that goes through the firewall to supply vacuum to the controls in the dash. I pulled 20.0 HG vacuum and in 7-8 minutes the wiper door opened. To close the door I used the vacuum gun on the “red” vacuum hose going to the front of the door actuator. I then went back to the supply line and again pulled 20.0 HG to repeat the testing of the control circuit. In 6 minutes it had dropped to 19.5 HG, after 13 minutes from the start it was at 19.0, after 29 minutes it had dropped to 18.0 HG, and after 51 minutes from the start it was at 17.0HG. That testing was done last night, and this morning at 7:45 AM it had settled at 6.0 HG and stopped dropping. Throughtout this entire testing period the wiper door never opened.

Later this morning, even though the wiper door was closed, I started the engine to build vacuum and after 8 minutes the wiper door opened .25 inches. When I pulled the vacuum source hose that provides vacuum to the control circuit from the check valve I could hear and feel vacuum escaping through that hose. So I’m thinking I should place an inline vacuum gauge on that hose to see if there is a drop in vacuum at the moment the wiper door is opening. If there is not, maybe that would tell me the problem is not in that system? Or to you think that because last night’s tests resulted in a continuous, although slow, vacuum loss the problem is in that system?

Thanks for all you help on this!