This THREAD is about the "Tranny-Shifter Vacuum Control System"
 

For the Readers of this THREAD,
I am trying to help Charles from Texas [" Cr from Texas " ] OFF-FORUM with a multitude of projects and problems with his MBZ that are not directly related to the topic of this THREAD.

Please try to NOT to "glum" onto this already lengthy THREAD and discuss other topics... for this complicates those who SEARCH MercedesShop.Com's WebSite for answers to problems they have. IF the subjects/topics discussed on this THREAD drift too much and too often, it can cause readers NOT to find what they need... a way to come to grips with what I see as one of the most common and frustrating problems with the automatic transmission equipped diesel powered W123 and W126 cars... and to a lesser degree other model diesels as well.
Thank you for you cooperation,

Is the vacuum supposed to taper in a linear fashion from high vacuum at idle to 0 or near 0 vacuum at some speed?

Or is it more digital in nature, such as 8" at idle through 2Krpm, then 0" at 2+

Interesting "handle" there "12MPGHWY" !
 

"12MPGHWY" - In general I understand what is meant by " linear " -vs- non-linear and in this case, I must assume that you mean the " bleed " that the VCV performs upon this control system... namely that it would produce an equal amount of vacuum drop [expressed in inches of mercury] for say each 10 degrees of rotation as the throttle linkage rotates the VCV's actuator shaft... and to this presumed question I would say NO. This is based strictly upon my knowledge of the internal components and workings of the VCV. IF you were to instrument and analyze how the VCV performs it's task, I would expect it to be at least slightly non-linear on a graph of "Inches of Hg -vs- degrees of rotation of the VCV shaft.

As to what you mean by " Digital in nature " as applied to this "Tranny-Shifter Vacuum Control System" and/or the VCV's variable bleeding action it has upon this "Tranny-Shifter" control system... well I'm sorry but I have NO idea as to what you mean!? I would expect more often to see "Digital" -vs- "analog" to be the comparison made and this is most often in all kinds of electronics... e.g. Digital TV broadcasts !

Maybe IF you give us a description of your situation and/or problem and/or symptoms, we can understand better!??
Regards,

Here is another take on "12MPGHWY's " question...
 

"12MPGHWY" - Here is another way I would like to answer "around your question" for I'm still NOT certain I understand it.

I think the " latitude " of what vacuum pressure you start at in the line leading down to the tranny's modulator and what you end up with at higher sustained throttle positions going down the highway progressing finally to 4th gear... I think this " latitude " or "range" of acceptable start/finish vacuum readings is variable in large part depending upon the age and internal conditions of the transmission and maybe to a lesser degree some of the main components in the Tranny-Shifter Vacuum System... e.g. VIVO, and modulator. This is why I am very interested to try hand pumping the vacuum the tranny's modulator sees to test and find out what specific vacuum the car needs at whatever speed, gear change, and/or load condition I'm interested in resolving a given objectionable shift problem. Given this manually created vacuum reading that seems to solve the problem or make it less objectionable... given this, then I feel I would be far better equipped to help owners with older baby-boomer aged transmission problems.

On one final vane, I think you could have taken these cars off the MB new car assembly line those many years ago and then run them through a line of the best and brightest MB engineers and I think these cars [ especially the pre 1983 models]... I think these cars would shift a bit rough compared to the 1985 models with all their add-on components that refine the vacuum shifting control system. Contrary to what some people have expressed at times [ myself included at one time ] I think additional components on the 1985 W126 300SD's [ and the W123 models that has the same external shifter system]... I think these cars do a far better job at shifting smoothly. I say this more readily after I have looked at a side-by-side comparison between these two "Tranny-Shifter Vacuum Control" diagrams:
Compare the simplicity of this system which did not even have an EGR on all of the applicable models: http://www.peterschmid.com/vacuum/1980/123_1.jpg
with the complexity of this: http://www.peterschmid.com/vacuum/1977_1985/617_95/1984_1985.jpg
and yes, I have purposely excluded the 1985 California edition at: http://www.peterschmid.com/vacuum/1977_1985/617_95/1985_cal.jpg
which is the granddaddy of complexity having I believe ~5 additional components including the dreaded "CAT" [ catalytic converter ] !
This car [ and my in-laws have one for me to try and help maintain] was the epitome of what can go wrong when you try to apply technology before it is ready for the consuming public... and the mechanics who try to maintain them.

Personally I like our two 1980 240D & 300D Tranny-Shifter Vacuum control systems for I have learned to communicate with the system and much more precisely control my shifts by letting off and reapplying the throttle... but my Wife will never be able to do this you see so for many the later models are better. And I have little doubt that my quasi-manual foot shifting technique will extend the life of the tranny! Oh yes, I did effectively add vacuum dampening "dashpots" to these cars and this helps to soften the 1-to-2 shifts some.

Sorry IF I went off on more than one tangent here!
Regards,

You answered my question.

I mean digital in the literal sense as in on or off, the polar opposite of linear.

What I was thinking of was using either a 0-5V proportional valve, or a 12v PWM valve.

The 0-5 volt valve would be easiest, and required on a very basic circuit.

A PWM would work the best, be the most tunable and be more complex.

either way the valve would take place of the complex OEM method of varying the vacuum. It would go between the vacuum pump and trans bypassing the "variable leak" system.

The system would be easy and cheap to design, what I would need to know is what parameters to base the variance on, such as throttle position, rpm, manifold pressure, road speed, or a combination of multiple inputs.

In fact, the combination of a few off the shelf components would do it. Some electronic boost controllers use a simple 2D rpm to duty cycle variance, with several adjustment points that interpolate.

... digital tranny-shifter valve...........!?
 

Well you are out of my league when it comes to modifying these old vintage diesels. Again, I don't think there are precise input parameters by which you can then convert them into a signal that would be used to vary the vacuum to the tranny modulator. As I previously stated above, I think the varible vacuum needed for each of these old trannies is also a variable depending upon the condition of the tranny.

It might help IF we knew what YR/MODEL you are talking about and complexities of your particular "Trannyj-Shifter Vacuum Control System"

Right now I'm installing a combination digital ammeter / Analog Volt-meter that fits into the space of the ashtray & cigarette lighter... and IF you ever build one of these babies you are speaking of, I'll be happy to test it for ya ! I'm not at all sure that such a project is worth the effort and expense. Most modern transmissions might very well have such shifting but they have multiple internal solenoid/ hydraulic valves.
Regards,
Sam

PWM explained here better than I can explain.

http://en.wikipedia.org/wiki/Pulse-width_modulation

Sensors would need to be added most likely.

Your probably right, its probably better to just work with unit as it comes stock.

Much tweaking and still not right
 

Well here I am back again after spending quite a bit of time and experimentation tweaking the vac system on the 300TD and still dont have a satifactory result, I am starting to wonder if my VCV could be at fault, roughly how much vacuum should the VCV be bleeding off, Mine as Brian pointed out is the one used on later turbo diesels and I am trying to use it on an 81 tranny, perhaps I would be better off fitting the plastic type from the earlier models, Is it right that these are adjustable to increase/decrease the amount of bleed, could anyone furnish me with a part number for this valve? thanks

Felix

Where are you installing the orifice to limit the vacuum to the VCV? It's got to be upstream of the VCV.

What's the diameter of the orifice?

With a proper orifice, the vacuum should fall rapidly when the rack is opened.

You'll need to T into the modulator line and route the gauge into the vehicle to monitor the gauge while you drive it.

have tried four different orafices, not sure of the exact diameter, the colours were red, yellow, blue, and green, I tried them in the line directly downstream from the main tee in the brake booster hose, will try teeing into the modulator line and reading the guage whilst driving and will report back, thanks for your quick response

Felix

Umm I need to clarify what is meant by upstream and downstream here, starting from the vac line at the main brake booster hose heading for the VCV is this heading down or up, pardon my question ?

The orifice must be inserted in the line that goes to the VCV. The vacuum at idle should be approx. 10" or so and it needs to drop from there as the rack is opened. If the orifices don't affect the results, the VCV may be non-functional.

The main vacuum hose is upstream of the VCV. The orifice must go ahead (upstream) of the VCV.........between the main vacuum hose and the VCV.

Yes I thought so. I have tried this and the orafice does not affect the high reading the only measure I came up with to reduce the vaccum reading is to wind in the throttle cable partially depressing the 3:2 valve on the cam cover thereby bleeding off all the time this kind of produces the desired effect but only on warm engine as soon as I start again from cold I am back to square one i.e very late shifts, I did try fitting a ballafix valve ( on off water valve) on the modulator line to act as a further restriction but this did not really achieve the lower reading consistantly either, as you suggest I will tee into the modulator line and read the guage whilst driving and report back

If your problem is late shifts.........the vacuum level most likely will not help with this.

See if the vehicle has a Bowden cable. This cable is attached to the throttle linkage and is tightened as the throttle is opened. It leads down to the transmission and is usually on the right side of the engine.

This cable affects the timing of the shifts and it can be adjusted if you can find it.