PeteZO6

After a couple of years of not having a fully functional wiper door – wiper system, I can finally say the cure is here, and the door works perfectly. As I understand it, the ’68 Corvettes don’t have the Interlock Valve, but ’69 through ’72 cars do.

The purpose of the Interlock Valve is to hold the wiper door open until the wipers park, then it lets vacuum through to the spring loaded diaphragm of the wiper Relay Valve. The Wiper Relay Valve is located on the passenger side inner front fender on early ‘69s and on the driver side firewall on all other cars. You can see the operation of the wiper door vacuum system in the diagram taken from the 1969 Chevrolet Chassis Service Manual below.


Mine failed a few years ago when I was caught in unexpected rain. When I turned off the wiper, the door slammed on the wiper arms.
A little testing showed the Interlock Valve was faulty, so I obtained a reproduction replacement. All was well again for awhile. Then that valve failed leaving the door open all the time. A little trouble shooting revealed the Interlock Valve was blocking vacuum to the relay, no matter where the wiper arms were – parked or operating. I got the door working again by removing the valve and bypassing it with a short piece of clear plastic tubing between the red and white hoses. Note: You have to operate the door with the override, or the door WILL close on the wiper arms.

That reproduction replacement valve not only didn’t function properly, but the excessive spring pressure had caused the plunger to break, rendering the valve totally useless – a rebuild wasn’t possible.

I tried to order another interlock valve, but all vendors were out. Seems somewhere along the line, a replacement manufacturer used a bad piece of information and the valves were faulty.

I don’t drive my Corvette much in the rain if it can be helped, so it wasn’t an emergency to get the valve replaced, but I wanted my car to be fully functional – everything working properly.

I was biding my time, reading the Forum regularly, and contributing what help I could to folks with vacuum problems as I have a fair understanding how the vacuum system works. I was contacted by a gentleman from the Forum, Dave, who had taken to making his own Vacuum Override Valves with an offer to send me one to try. He did indeed send me one of his beautifully made valves to test and report on. The valve is made completely differently than the original one-piece plastic casting, making it much more robust. The body is slightly larger diameter because there is a brass sleeve inside to give a good seal. Major dimensions are the same, which was proved to me when I compared it to a reproduction and when I installed it. Here are a couple of pictures of the valve.





The spring takes much less pressure to depress the plunger. This is a positive because it doesn’t need much spring pressure; the spring just has to be strong enough to push up against the wiper arm and allow the plunger to rise and close off vacuum to the relay when the wipers operate.

While I had no concerns about the valve working, I gave it a bench test just to ease my mind that it would work after installation. If you have a vacuum pump like a MityVac, it’s easy to test: Vacuum passes through the valve when the plunger is depressed. With the plunger out, it should hold vacuum on the middle (red) port. That’s important because you don’t want a vacuum leak when the wipers are operating.

I’ll recap how the valve operates and how to connect the hoses. The new valve isn’t color coded, but don’t worry, it’s easy. The hard part is physically accessing the valve for removal and installation.

First remove the grill between the hood and the wiper door.
Then remove the rubber plug on the actuator so you can manually open and close the door. With the door open, you can just get a 3/8 open end wrench on the two screws holding the valve to the bracket. It was easiest to put the wrench on the screws ‘end on’ and using a screwdriver in the other end of the wrench turn an eighth of a turn, then repeat until the screws are loose enough to hand turn.
Since the plunger is depressed when the wiper is parked, it would be difficult to depress the plunger against spring pressure and get the valve lined up so the bolts will start. I operated the wipers so the arms were straight up. This allowed me to open the door enough to access the hoses and valve, and not fight the plunger pushing up against the wiper arm.

Install the accordion boot on the valve, making sure it fits into the groves on the plunger and the valve. See photo.

There are three hoses that go on the valve; black, red, white. The black is just a vent and is only a few inches long. It goes on the top port.

Next is the red-stripe, on the middle port, and finally white-stripe on the bottom port. I put the hoses on before bolting the valve in place – it’s easier to do it when you can move the valve and the hose.

With the hoses on the valve, you can place it against the bracket and start one of the screws – OEM screws have pointed ends making getting them started in the captive nuts pretty easy.

When I had the valve completely installed, I took a screwdriver and pushed down on the plunger to be sure the wiper arm wasn’t applying excessive force on the valve, possibly damaging it. I have about 1/16” freeplay, or clearance, which is good. That "plate" behind the arm is part of the operating mechanism and is what the plunger rests on.



Once it’s together, push in the override, and normalize the control switch, under the steering column, if you have been using them to control the system. Start the engine, mist the windshield with water and turn on the wipers with the dash switch. The door should open, and the wipers wipe. Turn off the switch, the wipers will park, and THEN the door closes. That’s how mine operate now.
Well done Dave and Wilcox!
LINK TO VALVE AT WILCOX

Solid LT1

Great write up....glad someone in the USA is making these after dealing with Chinese JUNK for many years. I'm a bit confused about the statement that the valve is located on the passenger side inner fender on 1969 models only.....every C3 I have ever worked on has this control valve located just where your thread shows it.....under the wiper grille on the passengers side. Maybe I'm misreading what you said?

Bad Bird

Thanks for the write-up. I'll have to order one of these "good" valves to replace my repro garbage.

PeteZO6

Thanks! I meant the Relay Valve is located on the inner passenger fender on early '69s, firewall on all the others. You're right the interlock is always under the right wiper arm. I reworded my post to make it clear.

Pete

Solid LT1

Yes Chevy made several moves of the relay valve.....it is on the drivers side firewall on the 1970 too.....I think it was late 1971 when the metal vacuum lines were introduced that the relay valve returned to the passenger side inner fender well

This is probably the most complex system on the chrome bumper C3s but, one of my favorite features. My wife's 70 LT-1 just developed a strange trait after winter storage this year where the wiper door would slightly rise on startup then settle but, I could hear a faint vacuum leak after a little diagnosis I was pleased to discover it was just a failed override switch below the steering column. Job finished in under an hour

Bad Bird

I don't understand all the hate for the wiper door system. It is a unique early-C3 thing, and I love it.