rebuild a VCV?
 

Has anyone tried to rebuild the no longer available VCV on top of the injection pump of an 82 300D?



The valve leaks through the valve seat. Also through the opening around the actuating shaft that the spring is attached to inside the valve, although I don't think that's a problem.





The valve proper has a metal base which is crimped over the plastic top where the vacuum line attaches to it. I should be able to cut the crimp off and access the inside of the valve to repair the valve seat. I'm not sure how I'd put it back together, but I think a little silicone sealant should do it since there is a plastic ring that snaps over the valve to hold it in place.





Anyone find a workaround besides converting to a manual transmission? :D

So I did verify that the VCV is the offending part. I took the valve apart and did a smoke test. You can see the holes in the diaphragm and where the smoke is leaking out.

This is the bottom of the valve proper. As you apply the accelerator, a linkage comes up and moves the arm on the outside of the valve body up, which turns the shaft inside the valve body. That raises a lever that pushes up on that brass part with the spring crimped inside it, which somehow unseats the valve and lets the vacuum out. Looks like something inside pushes down on that ball.

I'm not sure what that material is. Some kind of rubber I'm sure, but it's Mercedes voodoo rubber, so I'm wondering if there might be something better to seal it with. And, what material would make the best diaphragm.

If there was a dealer nearby I'd swing by and see if I couldn't get someone in the service department interested, they might have some secret Mercedes sealant that would plug the hole up for another 38 years.

I really think the best thing is to open it up and see about replacing the diaphragm.

Any thoughts?

Thanks for posting this. Eventually this will become an issue for everyone.

I’ve heard of people using that liquid electrical tape from Home Depot on motorcycle carburetor diaphragms. I’m not personally familiar with its durability.

Too bad there isn’t a way to disassemble the entire thing and replace the diaphragm with a sheet of viton.

For that little bit of a hole spray it off with brake cleaner and let is sit for 2 hours to be sure it all evaporates out of the rubber. Take a bit of silicon sealant the same type as you would use on your engine and fill in the hole with it and wipe it so it is nice and flush as it would have been if the real rubber was there. Then let it sit overnight and cure and the next day see if you think the results will work.

Silcone sealant usually gules stuff together if the areas have been degreased.

If you have any black wind shied sealant or stuff for sticking the rubber seals on doors that might also work.

Well, it wasn't a little bit of a hole! That thing is wore out! :) I think I will try this though, maybe with some door gasket adhesive. That seems like it would retain maximum flexibility. I tried sealing the holes with some RTV Blue, but it's difficult to get a nice thin coat, and I think I need to retain that feature in the middle of the gasket, that hump there, and the diaphragm is only 0.3mm, the rtv cured wasn't as flexible as the diaphragm itself.

Obviously, the thing to do is replace the diaphragm. I don't think I can do that and retain the hump in the middle of the diaphragm though. So whatever I use is going to have to stretch frequently.


Having trouble locating Viton sheet in small sizes and .3mm thickness. I did locate some nitrile coated nylon. Do you think that would suit? I'm wondering if the nylon will stretch. I guess I'll order some and find out while I'm looking for Viton.

Thanks for the suggestions!

Looks something like a motorcycle carb diaphragm, but I suspect most of them are too large in diameter.

https://www.carburetordiaphragms.com

But if you found one that works, you would have lots of grateful admirers.

Peter

I.P. VCV "Fun"
 

This is a gear thread, thank you ! .

So, I've made some progress. I thought I took more pic's than I did. Reassembly got a little sketchy, I realized I had to make some bushings for the glue up after applied the RTV and wasn't sure how long it would stay soft!
I put the diaphragm in an ultrasonic cleaner with clock cleaning fluid which is some ammonia, Dawn, and acetone.

It was extremely worn. I used a small paint brush and applied some RTV blue to it.

If you look at the parts stackup, or if you happen to be familiar with these, the large spring goes between the top of the valve, and the bottom of where the vacuum hose goes into the top of the valve, where there is a post that pushes the ball off the valve seat. The spring was pushing the diaphragm off the metal bottom of the whole assembly, so I glued it in there with some more RTV blue.

The diaphragm riveted back into the valve is shown in the last pic with the rtv spread on it right before I clamped it all back together. It's not really a rivet, the top of the valve body was just peened over to keep those two metal cups together with the diaphragm clamped between them. I had to cut that off to disassemble it, so I turned a little brass insert and pressed it in there to keep everything together. Then the plastic top goes into the metal base and the edge of that is peened over to hold that all together.

That is probably a little confusing. If anyone has a known bad one they want to send me I could do a better tutorial with pic's inserted.

Anyway, with the valve reassembled entirely, I now have 15" vacuum just before the VCV. Now I just have to try to adjust it. And, see how long the RTV lasts.
But, it's at least possible to take it apart, repair it, and put it back together.
1st step complete!

We're going to need a more permanent solution to this. IF enough people do these 3 steps, MB will remanufacture based on demand:

Mercedes has launched a demand based request tool to remanufacture NLA parts. Three simple steps:

1) Click this link: https://partssearch.mercedes-benz-classic.com/parts/demand

2) Enter your part number. For example, enter A1158350207 for the W115 heater blower motor

3) Hit submit

Old Timer Parts
 

Sounds reasonable to me ~ Mercedes recently began making W123 wagon hatch latches again and God knows, many more W123's need the vacuum valve .

FWIW, I am ever so pleased with my new factory hatch latch and inside handle .

:D

It would seem that making such a part would be child's play for a 3D printing service. The only remaining question would be price. Perhaps a deal breaker for a one-off or even a minimum production run even if you could get enough people to sign on for a group order.

Remanufacturing Obsolete Parts
 

It's easier and cheaper than that ~.

The "new" W123 wagon hatch latch is the same old one with the last digit in the part # over struck .

And, it's only $12 list price so they can make and sell low volume parts for a profit *if* they want to .

That VCV rubber diaphragm, however, is a bit more complicated situation. It is not (has never been) an actual existing replacement part. That would be the entire VCV unit. Much more involved materials and construction wise than perhaps,say, a latch.

I have been thinking about the VCV.

My understanding is it’s a programmed “leak” between the vacuum supply and the transmission.

Does it also regulate the upstream vacuum?

I wondering if this thing can be reverse engineered with an Arduino microcontroller board, throttle position sensor and a dc proportional valve. It doesn’t look like rocket science. You measure the throttle position and program the digital controller to give you a flow rate at the valve. The relationship is stored in a little table made from real input to output data from an old VCV. I’ve been thinking of this on and off for years.

It’s really a toss up between finding one at the JY, repairing the old one or making a new system.

Didn’t the older 300d cars (W115) lack these devices? I guess I’ve accidentally left mine unattached and felt the harsh shifts so we definitely need one.

I better do some research before I keep running my mouth. It may be harder than it looks.

Edit - ok, so if I wanted to do this I think the first step would be T off the line to the tranny modulator and run it to a vacuum gauge on my dash so I can see just what it is doing when. As a start seeing how many mmHg is needed at idle, half throttle and full throttle would be good. From there 1/4 and 3/4 throttle will give an indication of linearity.

IP VCV
 

How it works is fairly simple :

It reduces the amount of negative pressure (vacuum) to the transmission's modulator from a high of 15" to zero as you depress the accelerator .

It doesn't affect the upstream vacuum .