This means nothing but fun!!

The vacuum has 2 routes to get to the wiper door. The problem is either with the control loop or the actuation loop.

The control loop goes as follows:
The vacuum check valve under the hood has 2 outlets. The smaller one goes into the car and splits off. One goes to the headlamps, the other the wiper door. The wiper door side routs through the override valve and the solenoid valve. The check valve is the manual one and the solenoid is the electric one that shuts the vacuum off when you turn the wiper switch on.

The line proceeds out to the valve under the door that the left wiper arm presses against when its down (this is the 'ok to close' switch) and then its on to the relay valve located under the right fender (not in all cars though).

The way this loop works is that there is always vacuum going all the way to the relay valve EXCEPT when you want the door open. All 3 valves in the line can send the 'open door' command to the relay valve. If the door ISN'T opening, this isn't your problem.

Now for the actuation loop. The other check valve output goes to the vacuum reservoir. Out of the reservoir it tees off. One side going up to the headlamps, the other going across the car to the wiper door relay valve. There has to be vacuum in this line for the door to open.

So what does the relay valve do?

The relay valve has 1 big hose going in, 2 big hoses going out and a small hose goign in. The out hoses connect to either side of the door actuator. Inside the relay valve is a sliding 'switch'. This switch routes the 'incoming vacuum' to the proper side of the actuator.

When vacuum is applied via the small hose (the control loop) the switch is in the 'door closed' position.

When this vacuum signal is broken (by opening one of the control valves mentioned above) an intwernal spring in the relay valve pushes the switch so that the incoming vacuum is routed to the 'open' side of the actuator.

Now that you hopefully understand how the system works you're probably still asking what's wrong with yours? The chances are that the relay valve is frozen or leaking. If this is the case, the vacuum isn't doing any good. Or you have a misrouted hose.

Start with the vacuum canister side of the system and follow the fat hoses to make sure they go to the right spot. The big hose should go INTO the relay valve in the middle. Check with a vacuum gage to make sure ou have vacuum getting that far. Then just systematicall check things to see what's working or not.

Thanks again.

Don

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~Juliet ...overlooking Mill Creek on the Chesapeake Bay...
Loaded Bridgehampton Blue on Blue '70 350/300Hp TH400 with a White Ragtop

NEW!! 1970 Corvette Registry Online:
http://www.annapolis.net/members/julepage/1970Registry.html

John http://vetteworks.tripod.com/ http://vetteworks.tripod.com/techdocuments/Vaccum.html

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GTR
Wallingford,CT
'69 350,4 spd,3:36 conv.
'72 350,TH400,3:08 coupe.

Many thanks for all your help. It does appear to be a frozen relay. Now to find the part (hopefully GM still has some kicking around).

Later,