How do Vacuum motors (wiper) work?

As you may suspect, searching for vacuum motor brings up lots about vacuum cleaners and searching for ‘vacuum motor’ wiper brings up plenty of parts suppliers (and the very interesting Jeep CJ3B page, which includes all the jeep models since the prototypes, including the M151 Mutt…But I digress).
So, very simply, how do (or did) vacuum motors for wipers physically work (meaning no “Poorly when you accelerate” answers, please).
Also, how did fuel pumps which also supplied ‘vacuum’ work (I have a vague idea from one of the sites, but more detail is always better).

I haven’t worked on one of the systems, but I believe I know the basics. The wipers are connected to a linkage attached to two diaphrams. The two diaphrams are each enclosed into a canister on the other side. The cansiter has a vacuum fitting and is air tight. The diaphrams are arranged on the linkage so that when one diaphram contracts the wipers move up, (vac applied) when the other diaphram contracts (vac applied) the wipers move back down.
A mechanical switch allows one diaphram to be connected to vacuum while the other is open to the air. At the top and bottom of the stroke the mechanical switch switches over.
There may also have been some that had a single chamber that overcame spring pressure, but I am not sure about that.

As far as your question about mechanical fuel pumps goes, I have never heard of a combo fuel pump/vac pump. Doing this would be fairly simple. A second diaphram and a one way valve on an existing mechanical pump would do just fine…

Yeah, Derek has a nice page. I used it as a model for The CJ2A Page and he helped me quite a bit.

How does the vaccum motor work? Well, I just pull out the little knob and if the windscreen is wet enough… :smiley:

Rick, thanks for the description of vacuum motors.

From the aforementioned CJ3B site, there is a brief description of combo fuel/vac pumps:

It then goes into a brief discussion of how this works, but for some reason I cannot follow the description of the operation very well (perhaps the lack of diagrams is confounding me)
CJ3B Vacuum Motors for Wipers

Now, as for you, LA…

:dubious: OK, this answer kinda fits in under:

don’t you think?

Sorry, SirRay. I really couldn’t resist! :stuck_out_tongue:

A vacuum wiper motor has a semi-circular trough. A pivoting paddle swipes back and forth in the trough; the edges of the paddle fit closely to the sides and bottom of the trough and have leather seals. The shaft of the paddle is hooked to the wiper arm and as the paddle swipes back and forth, the wiper arm moves. At each end of the paddle’s stroke, a cam on the shaft nudges a valve which applies vacuum to the othe side of the paddle, moving it back the other way.

The classic mechanical fuel pump has a rocking arm which bears on the camshaft on the engine end and on a diaphragm inside the fuel pump; as the arm rocks, the diaphragm moves up and down; fuel is sucked in through one check valve as the diaphragm moves up and squirted out through another check valve as the diaphragm moves down.

On Ramblers, there’s another diaphragm above the first which does the same for air, creating a vacuum, which is ported to the wipers. This gives the vacuum wipers additional vacuum under conditions in which you ordinarily wouldn’t have much, i.e, driving uphill. It’s pretty much a kloodge, but hey.