General questions about vacuum door lock operations
 

There are some principles that I may not understand about the vacuum door locking system. I hope you don't mind if I supply some drawings and ask some simplistic questions...

The images are a diagram of a theoretical vacuum door locking mechanism. At first I thought the diagram should have a sprung diaphragm in the middle, but I've realized that idea must be incorrect; the arm that extends from the vacuum mechanism must be mounted on a cylinder which moves up and down, or an unsprung diaphragm. I came to this conclusion because I realized that the mechanism do not return to some default state when vacuum is not applied; the arm remains in its current position when there is no vacuum present.

The first diagram shows a vacuum unit where vacuum has been applied to the unlock side of the unit. The diaphragm (or cylinder) moves toward the side where the vacuum is applied because the atmospheric pressure presses the opposite side. If we assume an atmospheric pressure of 14.7 psi and we apply a 20 psi vacuum to the unlock side of the unit, there there will be about 34.7 psi of pressure forcing the diaphragm to the unlock side on the unit.

The second diagram shows the same vacuum unit with 20 psi of vacuum applied to the lock side of the unit. Again, the diaphragm will move to the lock side because of the difference in air pressure on the -20 psi side and the +14.7 psi side of the diaphragm.

It is important to remember that for either condition, if the vacuum is lost, the unit does not attempt to change its state. If a locked vacuum unit loses all vacuum on the lock side, it stays locked; and conversely for the unlocked state.

So my first questions:

1. Do the vacuum units fundamentally work in the described fashion?
2. Is it true that what moves the diaphragm from one position to the other is the pressure differential on the two sides of the diaphragm?

1) yes
2) yes but the pressure differential would be 5.3 psi, not 34.7.

1) It is true for the 300D W123 chassis. The pod has 2 inlets like your diagram. W126, W201 or W124 chassis use push and pull, i.e. it pulls vacuum to lock or pressurize to unlock. The pod only has 1 inlet.

2) True but the pressure would never be greater than atmospheric pressure, i.e. 14.7psi.

This is my understanding.

You cannot have 20 psi of vacuum in a 14.7 psi atmosphere. Maybe you are thinking of 20 in Hg vacuum, which is approx 9.8 psi (differential.)


You might want to put a little more thought into that concept.

the way vacuum works is it removes atmospheric pressure from an area... so there is no way to get more vacuum "pressure" than the atmosphere can push against.
absolute (totally impossible to achieve) vacuum on earth at sea level maximum is 29.9ish inches of mercury vacuum... witch would equal 14.7 -psi or 0a
now on Jupiter or Venus where the atmospheric pressures are substantially greater, a higher negative pressure is attainable, but not here on Earth.

Thanks ah-kay, I had not begun researching the vacuum system on my "new" W126 yet. For the moment I want to keep this discussion about the W123 chassis vacuum operations.

I knew my knowledge in this area of pressure/vacuum and measurement would be deficit; so let's ignore actual amounts for the moment and just agree that there's a pressure differential created by the vacuum which causes movement in the unit towards the side with the vacuum applied.



Now for the next step; given the second original drawing (vacuum applied to the lock side of the unit), what is required to transition this unit to the unlocked state? It appears to me that you would have to apply vacuum to the unlock side of the unit--but that change alone would be insufficient; you would also have to reduce or remove the vacuum already present on the lock side. If you did not reduce the vacuum on the lock side, the unit would not transition because you would have equal vacuum on both sides of the movable diaphragm. Equal pressure (or vacuum) on both sides of the diaphragm will not cause the mechanism to move. This means that at some point you must "bleed off" the vacuum on the lock side of the unit.

(Someone correct me if I am wrong about that previous assertion. And please explain why its not the case.)

Now for my next assumption. I assume that the driver's door vacuum switch (let's call it the master switch) directs vacuum to one side of the system or the other. Assuming that the engine is off, moving the driver's door lock from the locked to the unlocked position causes the master vacuum switch to supply vacuum from the vacuum reservoir to the unlock circuit of the system. But I have already asserted that additionally the vacuum present in the lock side needs to be released. How is this done:

1. Does moving the master switch vent the lock circuit to the atmosphere?
2. Is the circuit expected to bleed down over time?
3. Is there a momentary cross-connect of the circuits, which would cause the two circuits to equalize before the vacuum is applied to the unlock side? I don't see how this is possible; eventually the system would completely equalize.

The master switch simultaneously applys vacuum to one circuit and vents the opposing circuit.


The reservoir vacuum bleeds down each time the door lock configuration is changed.

Thank you for that information. The implications then have become:

1. A sitting car will eventually lose all of the vacuum in the reservoir by simply switching the door lock system from lock to unlock and unlock to lock repeatedly (I believe I have seen this mentioned before).
2. A single circuit is not expected to hold vacuum over a long duration; there is no need for it.

This second point is critical; to be fully functional a single circuit only needs to hold vacuum long enough that the entire circuit can switch from one state to the other.

MAYBE...

It suddenly occurred to me that it is possible that the master switch, when placed into a position (either lock or unlock) continuously supplies vacuum to the appropriate circuit. Is this true, or does the switch only supply vacuum for a short period of time in order to change states?

Please do not complicate things.

The concept is really simple and your understanding is almost correct. W123 uses vacuum reservoir ( it is finite in size ) to hold vacuum and will lose vacuum over time or with repetitive lock/unlock actions. The pressure applied to the pod's diaphragm will NEVER be greater than atmospheric pressure, i.e. 14.7 psi. The pod will remain in that state once switched by the switching valve at the door. The job of the mechanical switch valve is to route vacuum between lock/unlock line, no more and no less. That is all to the W123.

Considering that one circuit is always connected to the reservoir, any lack of vacuum integrity in that circuit will cause a depletion of "stored" vacuum.

Thanks, but there's the current question. When the master switch is in a particular position, let's say the lock position, is it continuously supplying vacuum from the reservoir, or is the vacuum isolated in the lock circuit?

This question is important:

• If the master switch provides constant vacuum from the reservoir, then it is critical that each circuit provides vacuum-holding capabilities, both instantaneously and over time.
• If the master switch provides vacuum to the circuit momentarily and then isolates the circuit, the circuit only needs to be good enough to hold vacuum long enough to switch the mechanisms.

What this relates to is the leak-down requirements of the individual elements and the lines. If they lose vacuum but at a slow rate, then they don't need to be replaced.

So this question is critical to the replacement requirements for individual elements. Since the circuit loses all vacuum when it is switched, that loss is acceptable. But what about when the car is just sitting in an unlocked state?

Does the fact that an element may lose all of the vacuum in the circuit in a few hours matter? In a few minutes? In a few seconds?

the vacuum circuit is ALWAYS connected between, the pump, the reservoir, and the driver's lock actuator, the actuator is simply a two way valve, open to one circuit or the other. if there is a leak it will deplete the reservoir.

Not exactly. Check valves isolate the lock system from the pump when the pump isn't producing vacuum.

It's not quite that simple. A door lock actuator is always "open" to both the lock and unlock circuit.

vstech is correct.

In an ideal world, the vacuum will be held forever if there is absolutely no leak, i.e. the car can be sitting in lock or unlock position for a long long time and can be commanded to go the other state if requested. In practice, it will lose vacuum after a day, may be less. What is your vacuum problem? I do not get it still.

good point, but it's still connected to the pump... a leak in the check valve would be an issue as well...