That plastic cover looks to be part of the asssembly, but, in reality, you can coax it off with a utility knife or a very tiny screwdriver. The adjustment screw should be beneath it.

Probably won't allow it to hold any more vacuum, but, worth a try. Remember, you only turn the adjustment screw about 5-10 degrees in either direction.........more has no effect and may further damage the valve.

DieselDan44 - ... and so let me let you know this right up front and explain what I mean.

Back when I faced what you are now facing, I was lucky enough to have an Indy MBz shop friend who had a couple of old/used VCVs lying around and so I bench tested just as you are now about to do and finally found one that held a vacuum reasonably well with what appeared to be a controlled rate of bleed. From that point on, I was able to get the VCV to properly perform its function by changing the small vacuum orifices that are in the system further restricting how strong a vacuum was being fed to the VCV and eventually on to the tranny/modulator.

Thus it was this level of experience that my original premise for this THREAD was based... and why my title was what it is. Unfortunately I have yet figured out how to change the title of the #1 POST in a THREAD. IF ANY ONE KNOWS, PLEASE TELL ME THE SECRET!

Luckily it was NOT necessary for me to adjust my acceptably functional, used, VCV although the one I finally ended up with was adjustable!

Because of my limited experience with such, from this point on I will mostly leave you in Brian's trusty hands as to how you might successfully adjust a VCV.

I do know that the official MB procedure for testing VCVs is included in a 7-page long instruction but it also requires specialized test equipment. You might read through this test instruction and gleen something worthwhile! Good luck on that.

Here is the MB's VCV Test instruction I speak of, available at : http://www.peterschmid.com/trans_only.htm

DieselDan44 - Finally let me know if you would like for me to see if I can obtain a used VCV, bench test it for you BEFORE we try and negotiate a used parts price for it!
Regards,

... for I researched several OnLine new parts sources and found that the same VCV was apparently used on all the 1977 to 1985 240/300 Ds and 300SDs... but beginning in 1986 it appears that the 2.5L 190D and most likely the SDLs have a different VCV.
Brian - do you know an OnLine source that would have VCVs for the SDLs?
Regards,

Went on a junkyard mission this morning for a used VCV. Tested 5, and they all 'failed' what I think to be the right test. So I have to suspect my testing methods...

To confirm (and to add this ifnormation to the thread) what I think is right:

To test a VCV:
1.) Pull both input and output vacuum lines out of the VCV (can be still attached to the IP or on a bench).
2.) Attach 4mm id vac hose and Mity vac to the side where the dashpot was connected.
3.) Close rack/throttle all the way (min).
4.) Pump MityVac. With rack closed, the VCV should hold vacuum.
5.) Advance rack throttle, and watch gauge. Vacuum should go down slowly.
6.) Move throttle around a couple times and re-test (especially if its a junkyard part.

Am I doing this right? I have yet to encounter a good one so I don't know how a good one will measure and hold/bleed vacuum at different rack positions.

Thanks,
dd

__________________
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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

You are doing it correctly. Just to be sure........you mention that you connected the hose to the side where the dashpot is connected.

The dashpot should be connected to the top of the unit. The side port is the vent.

Pls advise.....

DD,
A little voice in the back of my head tells me that maybe, just maybe, you are thinking backwards about what position of the throttle linkage lever on the VCV is " closed "... so I am attaching [I hope] a photo that shows our 300D's VCV... and as you can see it actually appears "open" IF you think of the metal "tang" [there are two that serve as minimum and maximum "stops"] on the lever contacting metal pins protruding from the plastic body of the VCV to be the closed position. Try both extremes with the throttle linkage attachment lever rotated all the way left and all the way right.
When closed, it is my belief that the VCV will still bleed off the vacuum and how quickly depends upon where the VCV's adjustment is set... and yes, as you continue turning the VCV's throttle linkage lever, you should see an increased bleeding affect.
I hope this helps,

Attached Thumbnails

 

Connecting to the top of the unit where the dashpot connects to - that is what I meant. I will correct it in my original post.

Sam- thank you for the picture. Turns out I am doing it right, but someone could easily go wrong there.

All of the VCV's I have seen so far all bleed vac *very* fast even when closed - they go up to about 15mmHg and its back down to 0 in less than a second. It is slower than no vac at all. My VCV had no adjustment screw when I dissected it. Am I missing something here?

So I really was 0 for 5. Shucks. Going to sneak off from work during lunch to another yard and try 3 more...Ill report back.

-dd

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

... with those odds, you need a little help while on your junkyard forays!
Regards,

...im now .25 for 9...

I found a VCV that kind of holds some vacuum off a 240. It bleeds down about 1 mmHg every 2 seconds. Maybe this is good enough. I got it for two whole bucks.

i still can't find this adjustment screw talked about. maybe tweaking that will improve things to the point where it holds total vac.

How high of a vacuum should I look for the VCV to hold? 15mmHg or so? (equivalent to the max vac on the tranny line at no throttle?

-dd

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

DD - You know that the several VCV(s) that you have found bleeding too much... well some of these might have been only misadjusted [food for thought]. Brian [ who is more conversant with how to adjust these things]... remember he said that just a small adjustment was all that would be needed... and that means they might be easily misadjusted in the hands of someone unknowledgeable of the component's adjustment sensitivity.

But anyway you have one that looks promising so have you removed the small plastic dome? If you do it, take a "macro" closeup picture for us of what it looks like underneath. Remember that some [ I think the earlier ones ] do NOT have an adjustment... and IF you have one of these, your only way to adjust the bleeding action would be to use restricted orifices... the very thing that inspired me up to start this THREAD!

So send us a picture of "under the dome"!!!
Regards,

Sam had mentioned that both of his units also bleed vacuum at all times to some degree.

The later unit on the SD does not bleed any vacuum below the setpoint.

It does seem somewhat inconceivable that all nine units are NG. It may be possible that the older VCV's do bleed vacuum at all times. If so, you'll need a proper vacuum pump to test them. The size of the orifice and the amount of the bleed would work in tandem to provide the transmission with the proper signal.

We need some additional data from fully functional VCV's of earlier vintage ('78-'81).

... I said essentially the same thing and this is the kind of data that guys at MBz Meets might go for providing IF they are trusting enough to all a fellow dieselHead play with their cars! I might be going to my first Meet here in the S.F. Bay area in January IF it doesn't conflict with my nieces wedding in Miami!

Until then DD, give us a picture of what's under your "Dome"
Regards,

ill post pics when i get home tonight. i need to stay at work late so i can actually get some work done, since I'm junkyard diving by day

So, the performance of my original VCV wasn't really any different from almost of the ones I tested out in the yards. Maybe I now have two that are good? Woo hoo.

Maybe my dealer has one in stock theyd be kind enough to let me test.

Pics on the way...
dd

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Maybe... IF you take your vacuum pump with you and do your testing right there at the counter in front of them!! IF you are so brash to actually do this, ask them the loaded question "Are these new VCV(s) adjustable"?
I would bet the new ones are adjustable and the old ones were not... and that they don't even make the old style any more! My OnLine parts SEARH thus far has revealed only one part number from the 70(s) through to the 1985.
Regards,

yes, there seems to only be one PN for the whole line. they all had the same PN when I looked at them in the lots today. 240's 300's, SD's turbos, non turbos. i saw as early as a 77 and as late as an 84.

As far as making a scene at the local MB parts counter, I get a kick out of that kind of thing. Besides, my local MB service guys are great. Now I cant afford to bring my car to them, ever, but theyll talk forever about the 123's...they know em by heart.

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car