justacog

Since the question came up recently about the vacuum lines on 1955 Oldsmobile Super 88's, I thought I'd do a survey of my stock, unmodified, 1954 Super 88 with power brakes. Some of these lines are obvious in their purpose but there are some questions in my head about these connections, so I thought I'd offer this up to see if I can better document how the vacuum system works on the wipers and brakes. I think the distributor connection is clear in it's connection but the automatic choke connection to the manifold is suspect.

- There are 2 lines coming off the top of the dual-action fuel pump.

• One of the lines goes to the top fitting on the wiper motor and nothing else (there is an open/unplugged line on the bottom of the wiper motor).
• The other line goes to the back of the carburetor and is attached to a brass T-fitting that is threaded into the base of the carburetor. There is another line off of that T-fitting that goes to the brake booster and a solenoid activated "storage tank" that's located under the driver's side of the car. As a note, when the brake pedal is pressed, there is an audible "click" from that solenoid.

- There is a vacuum line coming off the front RH side of the base of the carburetor that goes to the distributor vacuum advance.
- There looks to be a vacuum line coming off of the manifold going to the automatic choke housing. However, I don't think this is a vacuum line at all. It is in fact the choke "heater tube". I found in the service manual (page130) that there is a tube inserted all the way through the manifold (through a pipe plug hole below that hard choke tube line going to the carb) that comes out on the LH bottom side of the intake manifold that is open to ambient air pressure. That tube coming off of the manifold to the choke housing is just a head fake as far as being a vacuum line (though I suppose it's another potential source of vacuum leaks if this is true).

I think that's it.

So here are the things that are bother me in their function:

1. The two lines coming off of the top of the fuel pump. It's clear that one has to be vacuum, but why two lines? It would seem that the pump would have to have an air outlet which would logically be that second line, so one line is vacuum and the other is pressure. If that's true than the line going to the base of the back of the carburetor would be the vacuum line. However if it's the vacuum line helping power the brakes, why is it attached to the base of the carburetor which would effect the manifold vacuum of the engine...unless that "brass T-fitting" is also an integrated check valve.
2. That leads me to the second line off of the pump. If it is the "outlet" from the vacuum pump, creating pressure, then that would suggest that one of the connections on the wiper motor operates with vacuum (the connection on the bottom that isn't hooked up on my car) and the top connector (which my car connects to) operates the wipers with air pressure. There is a cut-away in the manual of the vacuum pump side, showing that appears to indicate that this is the case...but it's not entirely clear from that diagram.

I have no idea if any of the above is correct, but that's the only thing my little engineer's brain could deduce at this point. There are no schematics or anything I could find from GM or other forums that have any detail on this. I did rebuild my old fuel pump (it's now a spare) and going back to the rebuild I'm remembering one or two check valves up in that section above the diaphragm.

This is not a big deal for me, as everything works and is connected properly on my car, but I thought having a better, complete, understanding of this vacuum system would be good to document for myself and other owners for things like tracking down vacuum leaks.

Vintage Chief

I don't own the service manual, but from what I have garnered from other posts/diagrams, as well as the nature of how the automatic choke operates, the hard metal line leading from the intake manifold plenum to the choke is most likely not a source of intake manifold vacuum but instead as you have described. It supplies nothing more than filtered (hot/cold) air for choke operation.

madmax442.com

I can take pictures of my 1955 that I went thru the same thing. I can tell you this, the vacuum assist off the fuel pump obtains vacuum from manifold vacuum it does not generate vacuum. It keeps the vacuum going for the wipers in the event of a low vacuum situation like going up a hill in the rain. I made a drawing when I was going thru mine. Also, that bottom fitting on the wiper motor is just a vent hole. I will repost when I get home tonight.

justacog

Thanks!

Tedd Thompson

I believe, on the fuel pump, the vent side of the pump pressurizes the windshield wiper fluid jug, so you have pressure to feed the washers, the vacuum side supplements the carb lack of vacuum on uphill pull in the rain. I'm too lazy to go outside and look, besides Its cold out there, and I have since converted to the electric New Port system to avoid this problem... So should you...Tedd

justacog

Now the windshield washer system I know intimately. I restored that to working condition sometime back. I've got a video that I'll be posting on YouTube that describes in detail it's operation and how to repair it if needed.

The washer system is actually powered by vacuum. When pressing the button on the wiper control, vacuum is allowed to go to a main piston that is sucked up to the top of a chamber which has a 140mm long spring, which pull the cleaning fluid into a second smaller piston area (two check valves are in that one). When the button is released (and vacuum removed), the main spring pushes the piston down and fluid is pushed out of the secondary piston area to the squirter. Simple but incredibly ingenous. There are some other nuances to the system including a small vacuum release valve at the top of the system that are involved at some point if you hold the button long enough.

Some parts for that washer are available, but not all. The main "power spring" is in short supply (no longer made but some original stock available). I was able to find a stainless steel spring that can work as a replacement but the OEM spring is recommended if you can find one. All of the plastic parts are no longer made. You'd have to buy a used unit to get those, or I suppose someone could 3D print the parts.

Anyway, that's more than you probably wanted to know about the washer system .

madmax442.com

This may assist as well...from a 1955 Shop Manual.

justacog

Thanks for that MadMax. It's going to take me a while to digest that mentally. That part about "The diaphragm movement compresses the diaphragm spring and exhausts air from the upper chamber to the intake manifold" doesn't seem to make sense. That would imply that that that air from the pump is being pumped into the intake manifold? Wouldn't that effect how the engine runs? However, I'm also looking at that last paragraph that indicates that the pump diaphragm doesn't move when engine vacuum is sufficient and somehow doesn't effect manifold vacuum (even though its pumping air into it?) and only "adds" vacuum when needed. That is incredibly clever. Just looking at the plumbing and that description is making my brain hurt right now.

madmax442.com

Ok, here is my diagram. I can't imagine anything being different on a 1954 power brake car.

justacog

That's a great diagram to start with! The other fitting on the pump that you've labeled "out" is connected to the carburetor at that fitting where the black line begins going to the brakes, on my 54. Based on the info from the service manual, I think that port on the pump is more like "ambient pressure" for the top side of the pump diaphragm. The service manual says that the pump diaphragm stays relatively stationary if manifold vacuum is greater than what the diaphragm would produce (basically I think it's a balance between the pump diaphragm spring and the manifold vacuum). It would be standard air pressure if not connected to anything or intake manifold vacuum pressure. Being hooked to manifold vacuum pressure, the pump becomes dormant, so to speak, if the manifold vacuum is greater than the force of the diaphragm spring. However, when the manifold vacuum drops the diaphragm begins to operate provided the "boost" to the wiper motor. So even though the port you've marked "out" in the diagram is hooked directly to the t-fitting on the back of the carburetor, the intake vacuum is still sealed off from the boost pump, so to speak, by the pump diaphragm. That line is just providing the "ambient pressure/vacuum" for the diaphragm to start providing boost or not. There's a few more things involved with this process in both the top and bottom side of that diaphragm with the check valves.

That now makes an incredible amount of sense (at this moment in my brain). If that's the theory of operation for how that boost pump works, that's as brilliant as how the squirters work (even more so)!

Thoughts?

madmax442.com

I think you have it nailed. When I bought my car they had it all turned around and even had that vent port on the wiper motor connected to vaccum, I bought some of the lines from Fusick then made up the one from the pump to carb. As far as the washer pump, when you press the button on the wiper switch, vacuum pulls up the plunger against a spring, so when you release it, the squirters shoot washer fluid. Primative but brilliant. Thats what I love about old cars, particually Olds and Cadillac as they were more upscale.