Don't ask me how but while "browsing" for something totally unrelated, I found myself seeing that there is a WebSite/Page known as the:

"Dictionary of Automotive Terms”

So on a lark I asked for the definition for our "dashpot", you know that loveable little inexpensive green thingy usually mounted just above our Vacuum Control Valve [VCV] that is mounted on our Injection Pump [IP]... and low and behold I found two related definitions:

dashpot:
A unit using a cylinder and piston or a cylinder and diaphragm with a small vent hole, to retard or slow down the movement of some part.

dash-pot:
(DP) a diaphragm that controls the rate at which the throttle closes

These definitions have peaked my curiosity such that I might just buy one of these [< than $5] just to see what’s inside. Somehow at that price, I don’t think there is either a “piston” or “diaphragm”... and now that I think about it the green color probably coincides with the green colored orifice which has the smallest I.D. of the six "orifi" MBZ shows for these SYSTEMS… 0.7mm.

I’ll bet ya there is at least an orifice inside this “thingy” that is 0.7mm! In any event I think these were initially added to certain engine/tranny combos that needed to delay one or more of the shifts. For instance what if the tranny shifted well from 1-2, 2-3 but then 3-4 was too quick or as some say “ too soft ”. I can envision such a chamber with small restricted orifice acting to delay the eventual decline of the vacuum the CVC is bleeding off... and this might just correct such a 3-4 shift while not necessarily causing the others to change significantly. My Son's 300D with brand new tranny has such a problem.

Q1 – Has any of your dissected one of these green “ thingy(s) ”?
Q2 – Any other thoughts or ideas from anyone?
Q3 - Anyone want to wager on one of these fixing my 3-4 shift problem?

Just clearing a brain _art,
Sam

Went and put the mighty-vac on the line to the modulator. Got 13 in. at idle and went smoothly to 0 when the lever on the vacuum valve hit its full load stop. This is with a blue orifice and the EGR lines removed. So this seems fine. I am not going to worry about what was wrong before.

__________________
Jackson

1984 300d 223k
1994 Jeep Cherokee 2.5L 88k

"She may not look like much, but she's got it where it counts, kid. I've added some special modifications myself." -- Han Solo

"Would it help if I got out and pushed?" -- Princess Leia

Thanks for the posts.
I've been lost in the dark and scarry world of past owner's son selling the Becket, control panel including wood ('79 300 SD) and this thread has shed some light on rebuilding the vacuum system. I now have defrost and heat, blower stays on high, but I've lived through worse in WI winters. Vents would be nice.
Trany shifts fine untill 18-20 MPH downshift which ... clunk.
Thanks again. Please keep this thread alive.

When I started this THREAD I intended it to mainly cover the vacuum control system for the automatic transmissions on these cars. So when a couple of response POSTs strayed to and included how you had experienced what I will call “interactions” I was not prepared to respond… so I didn’t. This is where a completely different vacuum system causes another to react or not function properly… e.g. someone’s climate control causing the transmission to shift. Well now I have a greater appreciation for such “interactions” and wanted to kind of wrap up this THREAD with these comments about vacuum system “interactions”.

Since I started this THREAD, I had need to hunt down vacuum leaks in the door LOCK/unLOCK system. [ See my THREAD on this at: W123 Vacuum InterLock Diagram which is now in the “General Information” FORUM ].

What I’m leading up to is the fact that between the vacuum pump and the vacuum brake booster reservoir there are typically two “T”s and I think that most [ if not all ] of these incorporate a restricted orifice in them. Over coffee with my MBZ friend at lunch one day a realization came to me and so I quizzed my friend [40 years of work on these cars] as to what the “other” vacuum users were in these cars. Well guess which one he forgot to mention… YES, the most IMPORTANT… the power brakes!

As a retired safety engineer it had come to me that these “T”s branching off the large, main, vacuum line were restricted because the designers wanted to give priority to the brake system such that you could suffer a failure in any of these multitude of other systems and still have much of the vacuum available for the brakes.

These same restricted orifices also serve to minimize interactions between systems… e.g. the A/C causing an unwanted shift of the transmission. So along come mechanics and owners who correctly clean out the “guck” and crud that ends up clogging these “T”(s) and inline orifices and in some cases they also drill out these “T”(s). The white “T”(s) on the two 1980 cars I show below are about .8-to-.9mm in size and I have seen on this FORUM people recommending they be drilled out to 1/16 inch which is ~1.6mm… and such a dramatic change in size of these IMPORTANT restricted orifices clearly can upset the balance that was designed into these cars… balance that gives the BRAKES their needed priority over other vacuum users… and balance that prevents interaction between unrelated vacuum using systems… the tranny and the A/C.

Just my closing thoughts for this THREAD!

Sam

The purpose of the restricted orifices is to isolate the components downstream of the restriction from the brake system. If there is a failure in a vacuum pod for the climate control or a failure in the EGR valve, the restriction prevents the main vacuum line from dropping too low in vacuum. Most of these consumers use very little airflow and a .9mm orifice is sufficient provided one is willing to wait 15 seconds or so for complete actuation. The time is not a factor for these devices so the system works as designed.

If you were to remove the orifices completely from the system, the system would still function perfectly well, with no interactions between the various systems, provided that there were no leaks in the system.

The sole purpose of the restriction is to prevent a leak from dumping vacuum from the brake system. The size of this restriction is not magical. It depends on the capability of the vacuum pump. If you open a .8mm restriction to 1.6mm, you get 4X the flow, however, this flow is still insignificant when compared to the 6mm?? I.D. of the main vacuum line. I seriously doubt that a wide open 1.6mm restriction would drop the main line vacuum more than 1" of vacuum......or so.

A quick test of a wide open 1.6mm restriction on the main line would confirm or deny this theory.

but as a former safety professional [ now retired ] let me try to make sense using an analogy of what in my former profession is called "Safety Factor".

Way back when steam boilers were blowing asunder, the first design safety factors were developed and these were originally 6… which is to say that the pressure components were designed to hold at least 6 times the maximum operating pressure of these vessels. In more recent times our laws have allowed the lowering of such design safety factors… now to as low as 4 and obviously we have not returned to the days of trains and heating boilers blowing up and killing hundreds of people as was the case in the early days of steam engineering design.

However, IMHO I would not think of serving as a professional witness to explain how it was OK to play around with whatever the safety factor the German engineers designed into what I see as a critical system [because of the BRAKES]. Why do you think by 1985 these same German engineers added a completely separate electric vacuum pump and system just to supply those notoriously leaky door LOCK/unLOCK vacuum elements and lines? IMHO there is a good chance they did this because they had learned of the problems we are still today dealing with... and the designers wanted to remove any possibility that this leaking system could compromise the brakes in their later models. If this had been seen as an emminent problem, today it might have been the subject of a recall.

Yes, I have given professional testimony several times in courts but personally I would not even think about doing such on this subject matter.

I fully appreciate your comments about how most of the vacuum using components... how they are largely "static" and use the vacuum for short periods. The vacuum control valve on the IP that actively bleeds off the vacuum is the only actively "dynamic" vacuum component that I have discovered [thus far]... so yes, most of these components do not use much vacuum air... but in the safety field you must assume systems fail and then try and figure out what will be the results of such failures.

For the same reason they put the two tail lights on separate fuses they [the German design engineers] have been trying to make this main vacuum system that supplies the brakes... trying to make this system as reliable as practicably possible and IHMO these orifices are are a CRITICAL part of their design efforts. Why do you think they built these orifices into the "T"(s) rather than putting them "in-line" separate from the "T" as the other orifices were done... IMHO THEY PURPOSELY TRIED TO MAKE IT HARDER FOR US TO DEFEAT THEIR SAFETY DESIGN.

IMHO the #1 reason these "T"(s) are being drilled out is to clear the dirt from the orifice to restore the system to operation... so it is the need for maintenance and repair that is the initial problem. What I have done is to add an in-line air filter onto the under-the-dash vent... this to filter out dirt from where I see most of the dirt enters the vacuum system. This I learned both from this FORUM and from my recent friendship with a 40-year independent MBZ mechanic.

So IMHO I canNOT recommend to others to go where my professional SAFETY experience tells me not to go... DON't drill out the orifices in the main vacuum line that runs between the vacuum pump and the brake booster reservoir !

Best regards,
Sam

who upon reading this THREAD a short while ago related his experience as to what can happen when mechanics play around with brake booster vacuum supply lines:

"I have to go with your thinking Sam... I have had a massive booster vacuum hose failure at highway speed on one of our Saturn station wagons... It tested my heart rate thoroughly... I heard a backfire and then the engine went full throttle because of the massive unthrottleable air intake into the intake manifold (gas engine)coupled with Little or NO BRAKES ! because of power booster vacuum gone... lucky me was able to coast to shoulder... upon inspection the stupid mechanic shop thought the big booster line would be a good place to put the Vacuum Tee for the cruise control (no clamps either) just pushed onto the hoses... the backfire in the intake manifold blew the tee right out of the booster hose!!... I called the shop that installed it and they said they have hundreds out there right now with no problem...
I asked if he had a good lawyer on retainer....."

So if any of you don't agree to my cautious approach in the interest of safety, then let's agree to disagree and move on to other subjects...
for I would rather be giving back to this FORUM than beating a dead horse!

Sam

There really is nothing wrong with your cautious approach to the subject. However, as an engineer, I don't accept your conclusion because you don't provide a single piece of data to support it. In reality, you do not even have the data that shows the size of the orifice for every Mercedes vehicle that is currently operating.

My SD may already have a 1.6mm orifice in the T. If that's the situation, then drilling out the orifice to the existing 1.6mm would present a result that is no different than the factory intended.

So, while anyone is free to make an opinion on this forum, I'd certainly appreciate it if you toned down the rhetoric and the underlined bold letters because your opinions are not based upon any data and they might just mislead others and cause them unnecessary expense. People on this forum have been drilling those orifices for many years prior to your arrival and not a single individual has reported an unsavory incident due to that procedure.

For the record, I'll do some tests and determine the exact size of the factory orifices on the SD and will subsequently post them on this thread.

I may even drill the orifice to 1.6mm.........if it's not already there.........and measure the vacuum loss with an open line.

Brian,
IMHO I have not attacked you so why the strongly worded POST(s) when I express my opinion?... and why suggest that my free speech should not include bold face and underlining? I noticed you used some boldfacing above. Is it my rhetoric that needs toning down?... for I must say that it is your rhetoric that has touched a raw nerve at this end and I am not intimidated by such. If I'm breaking one of the FORUM's rules by using logic and reason to press home a point for SAFETY's sake, please show me the rule and I'll muzzle my "tone"!
Sam

You've broken no rules and I've made a simple request to prevent you from misleading others who may wish to drill those orifices and save the cost of a new main vacuum line.

As I told you above, folks have been drilling them for years prior to your arrival without any unsatisfactory results. You have ignored this bit of data.

You're free to express your opinion as you see fit. There is absolutely no reason to use boldface and underlining to scream said opinion to the forum.

However, again, as I mentioned above, although you remain adamant that such a drilling procedure should not be done, you've accomplished no testing to prove the unsafe nature of the practice.

This was an excellent discussion and answered a lot of questions for me. I just bought an '84 300D that shifts hard into 2, a little better into 3 and very smooth into 4. To get started, I have a few questions:

1. What kind of vacuum guage should I purchase to get started.
2. I printed a schematic diagram of my vacuum system from the peterschmid web site (an excellent resource) for my car but the color codes are too fuzzy to read. Are there any other sources out there for diagrams?

Thanks again for the discussion and any further help.

Richard

1. If you don't have a Mityvac, that tool will be the best purchase you can make. It can test for vacuum or supply vacuum, depending on your needs. The cost of the Mityvac is just slightly more than a vacuum gauge.

2. Here is your diagram:

Thanks, Brian. You guys on this forum are quite an asset.

Richard

Since you have the proper diagram, do yourself a favor and eliminate all the vacuum lines the come out of the #71 plug. Just remove the lines and plug the two T's (or remove them as well). You'll eliminate most of your vacuum problems by getting rid of the vacuum supply to the 3/2 switches in the black box.

OK. If I am reading this diagram correctly, it looks like I am eliminating a vacuum signal to the EGR system. I read references to the 3/2 switches before. Is this referring to a switch that signals a shift from 3rd to 2nd gear or am I way off. Does it all have to do with emissions controls?

I also read references to BB's. I am assumig this means literally BB's that you use in a BB gun because they are the right size.