Packard Forum Index » Post-War (1946-54)

Should the vacuum wiper motor hold a vacuum when off?

Not sure exactly what you mean but nothing in the motor holds vacuum when it is off. The slider valve on top of the motor operated by the wire cable prevents vacuum from entering from the engine hose connection on the passenger side of the motor when it is turned off. On the other side of the valve the rest of the motor is open to air. The sliding valve covers different port openings but when off, only the park position port gets vacuum. Valve and paddle in park position should both be sealing tightly preventing a vacuum leak to any of the other ports or to the outside.

When you turn motor on the valve slides. It closes the park port to vacuum then uncovers the main passage into the motor. Vacuum is allowed to enter motor where it is immediately passed to a second internal valve which directs the vacuum to one side of the paddle. Paddle then starts moving. This second valve also allows the other side of the paddle to be open to air so it can be free to move as vacuum pulls it to one side. As the paddle nears the end of its travel in the first direction a mechanism operated by the paddle switches the internal valve. Vacuum is redirected to the formerly open side of the paddle and it can start moving the opposite direction. At the same time the internal valve switched vacuum sides it opened the first side to air so the paddle could move. This back and forth action repeats as long as the motor is on. The main port the slider valve uncovers when you turn the motor on is actually a thin slot and motor speed is somewhat controlled by how much of the slot the slider valve uncovers so more or less vacuum can enter. When the valve slides to off again the main port closes and park port connects to vacuum so paddle is forced past its normal operating sweep range to pull the blades down to the chrome molding. When the paddle reaches park position it closes the port which should prevent any leaks as the motor sits.

On motors used from around 48 to 53 there is a second hose connection located on the drivers side of the motor. That connection will be just sitting empty unless the optional windshield washer is installed. When the operator knob is turned opposite the direction needed to turn the wiper motor on there will be spring tension felt. Turning that direction and holding the knob against the spring tension causes the sliding valve to move a small amount to the other side of the off position in order to uncover another port and allow vacuum to reach that second hose connection to operate the washer.

There is no vacuum chamber on the wiper motor so the short answer is no, it does not hold vacuum without the engine running.

In my car, i found the connection to the washer motor didn't fully close/leaked. I capped that off with one of those little rubber caps, and I got a lot better vacuum overall.

A cap is a good solution for motors without washers attached but if you need the port active the most likely reason for a leak only at the washer hose port is the wire cable working the slide valve is slightly mis adjusted.

For general leaks around or in the valve, the moving plastic portion of the slide valve has a flat spring under the sheet metal retainer which is supposed to keep it pushed tight to the machined slide surface on the pot metal motor cover. It is possible that spring is weak or since the sliding part of the valve is plastic, that could have warped. The machined surface on the valve or cover could be dirty or scratched too.

Any of those old motors could have dried out or just be tired and worn. A good many are probably long past due for a rebuild.

I will reformulate my question. If I use a manual vacuum pump at the vacuum hose connection should the wiper motor hold vacuum when the operating valve is closed? Come to think of it, if I substitute a vacuum gauge for the wiper motor, what should the vacuum be at idle? If I disconnect the vacuum line below the carburetor and connect the vacuum gauge I'm getting about 17.5 at slightly below 400 RPM curb idle. I'd think the vacuum at the wiper motor should be significantly higher.

Not really. The wiper motor is almost always working off manifold vacuum so what a gauge says at the manifold will almost always be what is read at the wiper motor. There may be a small contribution from the vacuum pump but unless manifold vacuum is abnormally low that is not the usual case.

On cars with vacuum wipers made prior to 55 the vacuum source starts at the manifold, goes to the top or vacuum section of fuel pump, thru pump, and on to wiper motor. Since the valve springs in the pump are relatively weak and manifold vacuum and flow volume is almost all the time greater than what can be produced by the pump, when you turn on the wiper motor the larger flow of the manifold vacuum just holds the valve springs open and pump simply becomes a flow thru portion of the line.

When engine is under a large load or the throttle is almost fully open the manifold vacuum and volume drops to a point low enough the springs in the pump can start working to close the valves in the pump. The pump then starts to work and becomes the stronger source to supply a limited amount of vacuum to the motor. While the strength may be similar to what is produced by the manifold, the volume is considerably lower. The pump is able to keep the motor running but without the volume, cannot keep it running at anything much more than slow speed. Typically is is just enough to keep the wipers from stopping completely.

A typical manual hand pump such as the Mityvac may be strong enough but not have the volume to work the motor -- and that is assuming there are no leaks. You should be able to see motor move slightly but it will probably take many squeezes to make it move an appreciable amount.

I guess I should try to better describe what I'm doing.

I've had the motor apart and I've seen how it works. I'm not trying to operate the motor with the manual pump. I wanted to see if it was leaking. It is.

I just now tested the vacuum at the hose to the motor and I was surprised to see that it was about the same on average while oscillating +/- about 1 lb.